Revision Date Notes
1.1 Oct. 24, 94 Revised and reordered as part 5.
1.2 Dec. 12, 94 Minor edits, added new material on caps and tv
1.3 Jan. 8, 95 More stuff on caps.
1.4 March 3, '95 Include Dan Schoo's writeup on paper caps.
2.0 Nov. 19, '95 Move from part 5 to part 7
3.0 Dec 11, '99 Basic HTML added, and moved to Part 6, and safety hints on T.V. tubes added.
Part 7 - Radio and electronic phono technical questions.
WARNING: HIGH VOLTAGE. SHOCK and FIRE hazards.
Vacuum tube / valve electronics runs on much higher voltages than transistor or solid state devices. These sets were generally not provided with interlocks or power fuses. In certain designs, the power line may be connected directly to the chassis. Many home-entertainment electronic devices had 250 volts or higher as a standard operating voltages, and voltages as high as 750 or 800 volts, may be present in some circuits.
Fault conditions may cause HIGH VOLTAGE to be present ANYWHERE, even after the set it turned off and disconnected from the power line (mains). Use a grounding lead to assure that no voltage is present before working on a set. Be especially careful to ensure that 'reservoir' capacitors and the high-voltage line in general is discharged before touching anything in the set.
Be especially careful with old television sets; these used even higher voltages, and the picture tube requires several kilovolts as a minimum: the tubes' glass surface is often coated inside and outside to form a *very* good capacitor, so NEVER go and touch a tube without first ensuring, with a well-insulated shorting tool, that all this energy is discharged. Such a tool can be fabricated from a well-insulated screwdriver (rated for at least 10KV and kept clean) with a good wire bond to the blade, the other end of the wire ending in an alligator clip which is FIRST connected to the chassis or the grounding of the tube. The screwdriver tip is then carefully pressed into the anode connector at the side of the tube, such that you can see that there is contact. Move it around a little to make sure the contact has been made, and then remove the screwdriver. Remember, if you get a shock from the tube, it might make you jerk in anw which way, possible dropping the set or the tube, and there is then the danger of injury from the imploding glass tube if it breaks.
There is sufficient power in old radio and T.V. sets to overheat components to the point that they
will catch fire, and many components used in old electronics will
support combustion. While addition of a fuse can reduce fire hazards,
it is not a sure and complete protection against overloads which may be
adequate to overheat components, but inadequate to blow the fuse. In
addition, some soldering irons operate at temperatures of 400-500C (approx
700-900F), and are hot enough to ignite many flammable materials such as
paper and cloth.
You should have an appropriate fire extinguisher in your workshop or where you work on sets.
Several of the CHEMICALS and PROCESSES discussed in the newsgroup, and in this FAQ, present safety hazards of one type or another. Fire hazards are common, and many chemicals and processes require substantial ventilation as well. Read manufacturers' labels and follow all instructions for safe handling closely. Above all, do not store or use chemicals with food or food preparation items.
Small children (and some not so small)---if you have some of these around, take some precautions to make sure their inquisitiveness does not get them into something that will hurt them, or damage anything. Old electronic equipment is full of bright colors that will attract small fingers. The best thing to do with children is introduce them to radio. Don't just tell them "no, don't touch," etc. It's amazing how quickly, diligently, and thoroughly a child will learn mathematics and physics, with the help of an old radio and someone who will take the time to explain it to them. Noxious chemicals and children don't mix.
Do not attempt any process unless you know exactly what you are doing, have evaluated the risks, and have taken safety precautions. Many of the regular contributors to rec.antiques.radio+phono have been formally trained in chemistry and physics laboratory procedures, and use chemicals and processes professionally. They may discuss techniques that require substantial safety precautions without noting the hazards involved.
If there is the slightest doubt in your mind about the safety of any process or material, don't charge off and "just do it" because others say "it works." Ask questions. There is no substitute for learning under supervision. Many community colleges and high schools offer courses open to adults, including courses in laboratory sciences and shop practices.
Q. I've got a very nice Philco tombstone radio that is only a
decoration because it doesn't play. What can I do to get it to play?
A. This section of the FAQ addresses getting them to play as nicely as they look. While not intended to be a comprehensive primer, this section covers many questions that come up regularly. The topics discussed in this section of the FAQ presume that you have a working knowledge of vacuum tube circuits.
Q. Why does a 35Z5 or 35W4 rectifier have a number 40 or 47 bulb
connected across part of the heater?
A. The heater serves as a voltage divider. Resistance of cold filaments is much lower than when they are hot, and connecting a bulb in series will put almost the whole 110 VAC across it until the heaters warm up. The plate current flows through the bulb/heater to balance the current once the tubes are warmed up. Note that this also applies to ballast tube setups---the ballast resistance is designed to increase as the set warms up. It's a way of putting a cheap light bulb in a cheap radio. (Historical note: This is an interview question I used to use when interviewing engineering applicants in the fifties and sixties).
Q. I just found a (very old tube) radio in a (barn, attic, junk
sale, etc.). It's complete. Can I plug it in and see if it works?
A. If you didn't hear the radio playing, it would be very wise to do some resistance checking first.
a. What is the condition of the line cord? Replace it if it is frayed or the rubber is petrified.
b. Condition of filter capacitors. Wet electrolytics, which were used in the 1930's, should be replaced without question before applying any power. These are identifiable by the metal cans with vent holes on them. Dry electrolytics (which aren't really dry inside) can also lose their film and be low resistance. If DC resistance between the B+ line and circuit ground (this may not be chassis ground) is 500K or less, find out why. Make sure the speaker is included in this check if it has a field coil or has the output transformer mounted on it. With electrolytics and any voltage divider resistors out of the B+ circuit, DC resistance should be several meghohms.
c. If it's an AC-DC set, check to see if one side of the line is wired to the chassis. Many of them were. If so, keep the set away from any metal objects to avoid shock hazard. Some of the early AC-DC sets would hum like crazy if they were plugged in with the chassis "hot."
d. If it's an AC set, consider installing a fuse in the line circuit. 2 amps 250 volts for sets with 80/5Y3, 4 amps 250 volts for sets with 5Z3/5U4.
e. Do a cosmetic inspection. You'll want to vacuum off any old dust, dirt, cobwebs, etc. first. Look for things like charred resistors, melted wax from capacitors, coils, and transformers, and any indications that the radio go put in the (barn, attic, etc.) because something was wrong with it.
f. Take a look at the bias circuit for the power output stage. See below for discussion of typical bias circuits. If there is an electrolytic in the circuit, make sure it isn't "low ohms." If your output stage is 6L6's, or if it is filament tubes like 2A3, 6A3, 45, or 47, take a very good look at things.
g. Condition of old wiring is important. Don't fool around with petrified insulation that is breaking off the wires.
A few hours spent doing a good visual inspection and some ohmmeter checks can pay off handsomely. If you've got to replace a charred resistor, find out what burned it out and fix that too, before applying power to the set.
Remember that 99% of vacuum tube failures are due to open heaters or filaments. The other 1% are due to gas or interelectrode shorts. This leaves the item that tube testers have a big BAD-?-GOOD meter to measure, emission, down in the mud as a tube fault that makes a radio play poorly. Except for rectifier tubes that have been "sucked dry" by a gassy output tube or shorted filter cap, most of the tubes I have diagnosed as causing problems because of low emission would not exhibit that low emission in a tube tester. Example: a 6SQ7 diode that quit conducting after 15-20 minutes of playing. Diagnosis was confirmed by soldering a 1N34 diode across the terminals.
If the getter material (you can see it on glass tubes) is white instead of silver, the tube is probably gassy----most common on power output and rectifier tubes. A few sets used gas-filled rectifiers. The 0Z4 is most common in auto radios, but you may find and old set with an 82 or 83 mercury vapor rectifier.
Also remember that with tube equipment, DANGER, HIGH VOLTAGE. applies. In home entertainment transformer sets, we are talking about as much as 500 volts, and most smaller transformer sets used somewhere between 250 and 350 volts as the main B+ voltage. The transformerless sets generally provide 135 volts, and have the mains power (to use the British term) hooked directly to various circuits and often the chassis as well.
Q. The chassis of my radio is covered with a thick layer of dust, fine
dirt, and underneath is a film of brown crud. How can I clean this
thing up without damaging it?
A. This particular topic gets a lot of discussion and advice, some of it very bad. Your radio has some irreplaceable components, and if you use the wrong methods, you can make a junker out of a restorable set in a hurry. There are some things to keep in mind:
a. The chassis is probably cadmium-plated steel. Some radios were made with nickel-plated steel (looks green when corroded), copper-plated steel, or chromium-plated steel. A few chassis were made of aluminum. If it is a dull silver color, check with a magnet. An aluminum chassis is non-magnetic, all of the steel chassis are magnetic.
b. The dial face may be a water-soluble paint or a decal.
c. Colored knob markings (lines and dots, as well as letters filled with color) may be water-soluble.
d. Any silk-screened surface markings may come right off. These include tube layout information on the chassis, inspector's marks, and other printing.
e. The tuning mechanism may be stiff because of petrified lubricant in various shafts and rotating elements.
f. Coils, IF transformers, and tuning condensers may be difficult or impossible to replace if you damage one.
g. If the radio is complete, tubes in place, the crud and dirt is on top of everything, not in the electronics. You want to get it off the radio, not melt it down so that it flows into the working parts.
You can remove the tubes. Make sure that the tubes are clearly marked as to tube type, and make sure you have an accurate diagram so that you can replace the tubes in the same sockets you removed them from. Get a pencil and piece of paper and make notes about things you move, disconnect, or take apart, so that you can get everything back together the way it was originally. Begin by vacuum cleaning the set, and use a soft brush to loosen dirt while keeping the vacuum nozzle near the brush so that it will pick up loosened dirt. If you find mouse droppings, be prepared to examine the set closely for damage from mouse pee. Gently brush off the tuning condenser, being careful not to bend the plates. Once the surface dirt is off, you can begin to consider how best to remove the crud, and how far to go with the cleanup.
There are two things that are very poor to use around electronics: steel wool and soap-type detergents. Steel wool will shed little particles and raise havoc. Soaps and liquid detergents leave residues that can be hard to remove. Liquid detergents also do a fabulous job of softening and removing silk screen inks, water soluble dial markings, and tube markings, even those that may be safely soaked in water for a few minutes.
Start on the chassis crud by using a damp rag moistened with plain water. Don't slosh water onto things. Most tap water is safe to use around electronics, and is an excellent solvent. I note that I have refurbished electronics that have been immersed for days in fresh water after they have been allowed to dry out, and found very little damage, mostly to capacitors. If the crud comes off with water alone, continue with the damp cloth treatment. It may be slow, but it will leave a clean surface with little residue. Finish the job with moistened Q-tips to get into various nooks and crannies. Be careful that you don't remove marking inks and paints.
A stronger alkiline solvent is clear household ammonia. This also evaporates without leaving a residue. If water is not melting the crud, try a little ammonia on a Q-tip. Use the ammonia straight, and if it gets results, use it on a damp rag to moisten the chassis. Generally, once ammonia-sensitive crud has been melted, it will come right off using a rag dampened with water. Be careful not to get ammonia on a shellac wood finish---it will cut the shellac and leave marks.
If this doesn't get results, try a mild acid---clear cider vinegar. Use the same methods as with ammonia, finishing with a rag dampened with water.
By this time, you should have most of the removable crud off the
chassis. Some other solvents to try---only in small areas with Q-tips:
Isopropyl alcohol. This dissolves a great many things, including flux rosin, some marking inks, etc.
Trichloroethane (GC Electronics "Chloro-Kleen"). Also dissolves many things. Don't use on plastics until you have checked to make sure it is safe. Chloro-Kleen works very well on phenolic and ceramic-mounted switches such as bandswitches and pushbutton switches.
Lacquer thinner. This is a "court of the last resort." It is a powerful solvent that will damage many plastics, remove a lot of marking inks in a jiffy, and generally raise merry hell if you get it in the wrong place. Use on metal parts only.
Also pay attention to the various warnings about flammability and use only in well-ventilated areas.
Corrosion on cadmium-plated chassis generally does not respond very well to anything. You can use Naval Jelly to improve the situation, particularly if there is visible rust. Light fingerprints often will respond to automobile polish (Dupont No. 7 is good). This treatment (followed by an application of Simoniz paste wax) will make many lightly-scratched plastics look like new.
The best solvent for use with petrified lubricants in tuning mechanisms is diesel fuel. If there are separately-mounted shafts or gear mechanisms, you can often take them off----just make sure you can get them back on again, and positioned properly. Watch for spring-loaded double gears in gear mechanisms that need to be preloaded when you assemble them. Shafts should be relubricated with a light grease like white Lubriplate---use only enough to leave a film on the parts needing lubrication, and wipe off the rest. Gear trains generally work well with a little 3-in-1 oil on axle pivots and a film of lubriplate on the gear teeth. A stiff volume or tone control will generally respond to a drop of 3-in-1 at the end of the bushing---use only a drop, and wipe it off after about 5 minutes.
Tube washing gets a lot of attention. Keep in mind that washing most tubes won't make them work any better. Before you start, make sure that the tubes are clearly marked as to what they are. While there is no mistaking a 6A7, a T-9 beam power pentode with no markings may be a a 6W6, a 25L6, a 35L6, a 60L6, or a 6V6. A 50L6 plugged into a 25L6 or 35L6 socket can produce interesting symptoms that can be very hard to diagnose. Contrary to popular opinion, tube markings on glass will come off, some more easily than others. During the 1950's and 60's, tubes were specifically marked with easily removable markings in an attempt to thwart a grey market in used tubes being washed, reboxed, and sold as new. Generally, just holding the tube under flowing water will rinse off most of the dirt--- a little help from rubbing the surface with a thumb where it is not marked generally gets fine results. Use a china marker to marked the type on any tube that isn't clearly identified, and let them dry thoroughly before reinstalling. DO NOT let water get into the mica or plastic base of 'octal' type (and similar) tubes - it will take a very long time indeed to evaporate and can provide a very destructive circuit path when the tube is put back into the set and switched on. Tubes that are loose in their base, or have a loose top cap, respond to squirting a little superglue into the gap. Make sure, in the case of a loose base, that the leads aren't twisted (and/or shorted).
Q. What about AF power amplifier bias circuits?
A. You can do a little inspecting to see what your radio uses.
a. By far, the most common circuit is to use a cathode resistor with an electrolytic capacitor for AC bypass. This is what you will find in all of the transformerless sets. AC bypass is less critical in push-pull output stages, although most of them operate class AB (i.e, both tubes biassed near cutoff). If the capacitor is shorted, the output tubes will over-dissipate and their plates will glow red in a few minutes. If the capacitor is open, audio output will be low and distorted.
b. Back bias. I was somewhat surprised in checking Terman "Radio and Electronics Engineering" 4th edition (1955) not to find this circuit. It uses a power resistor in the B- return to develop a bias voltage, typically 10-30 volts, and may be used in conjunction with the cathode resistor self-bias circuit. The center tap of the power transformer will be connected to one end of the power resistor and B- circuits will be connected to the other end. On sets using filament power tubes, the filament supply may be connected here, and the power tube grids returned to the power transformer center tap. Most of the bias voltage is developed by output tube plate current. If there is a leaky electrolytic here, it will generally overstress this resistor and burn it out.
c. Separate "C" bias supply. In this case, the set will have second rectifier tube, filter, etc. These are not common in home entertainment equipment, much more likely to be found in theater and public address amplifiers.
Q. OK, I've checked that the tube heaters are continuous, that the
filters are OK, and generally walked through and done the visual and
ohmmeter inspection. I want to plug it in. What do I look for?
A. This is the moment of truth, even for an old grey-hairs. Fortunately, tubes will take abuse that transistors won't tolerate. But you want to have your eyes and ears wide open, and be prepared to shut the thing back off instantly. Some people like bringing them up on a Variac, which is an expensive piece of equipment unless you are in the restoration business. So I'll assume you are going to plug the thing into the 100 volt line, turn it on, and see what happens. Make sure you have some sort of antenna connected on sets without a built-in loop.
a. On AC-DC sets, turn it on. The tubes should light up, and in 10-15
seconds (when the rectifier and power tube heaters warm up) you should
hear 60 cycle hum in the loudspeaker. Indeed, hum is a built-in feature
of these sets. If it is overwhelming, you've got a bad filter cap.
Check for smoke signals and signs of overheating. If you can tune in a
station, you are probably in business. On 35Z5/35W4-type radios, if the
pilot lamp burns out after the set warms up, you've got a short in B+
somewhere---probably a shorted filter cap. Turn the set off and find
the problem---if you've got a short, the rectifier heater will take the
load and burn out after a while.
b. On transformer sets, I like to connected a 600 volt DC meter across B+, preferably in the supply to the IF screen grid or plate. If the rectifier is a filament type (80, 5Y3, 5U4, etc.) you'll see full B+ a couple or three seconds after turning the set on, and it should drop to about 100 volts on the IF screen when the cathode tubes warm up (around 10 seconds). Check for smoke signals, burning, and that all the heaters glow. A low level of 120 cycle hum is to be expected, though a really fancy set will give almost no hum at all. Once again, if you can tune in a station, you are probably in business.
Watch in particular for a violent purple glow in tubes, particularly the power output and rectifier, plates beginning to glow red, and other signs that there is a short circuit. If the radio doesn't play, keep a close watch on things, although if you have good B+, no gassy tubes, and no red plates, and things are OK after five or ten minutes, you are probably safe in continuing on to do trouble-shooting. A few sets use tubes with mercury vapor in them, which normally glow purple between the elements. Typical are the 83 (not 83-V) and 0Z4 rectifiers, and the gas-discharge VR (voltage-regulator) tubes (0A2,0C3, etc.).
Trouble-shooting. If all the tubes light up, you've got B+, and no smoke signals, you can begin your walk through the radio. If the radio is completely dead---no stations, no static---try rocking the bandswitch if the radio has one. Also, the volume control, any tone controls, etc. I've found that on ancient sets, it's a good idea to walk right through and do voltage checks everywhere, no matter how well the radio seems to play. If you have a schematic with voltages marked on it, so much the better, although some of the voltages given by manufacturers can disagree rather markedly from actuals that can be figured by reverse-engineering the design.
a. Power output stage: Check screen and plate voltages. These should
be close to B+ at the rectifier. Check for positive bias voltage at the
cathode on self-bias circuits or negative voltage at the grids if
b. Audio amplifier. Usually a triode. If the 6SQ7 diode-amplifier type, the only thing to check is plate voltage, which should show a drop across the plate resistor. On resistance-coupled output circuits, make sure the coupling cap is not leaking current to the output tube grid circuit, which will pull up the grid voltage and make the output tube plate(s) glow red. Probing the AF amplifier grid generally shouldn't show any voltage, but should make plenty of noise in the speaker.
c. IF amplifier. Check for screen voltage. If you don't have any, you've got a shorted bypass cap and a dead radio. Plate voltage should be near the supply voltage (generally fed by a blue wire to the 2nd IF transformer). Cathode should show some bias being developed (i.e., plate current through the tube). The grid will generally show the AVC voltage, though your meter will shunt a lot of it, unless it is a high-impedance type, such as a VTVM.
d. Mixer. If the pentagrid type, tetrode, or pentode, check screen voltage. Check for proper bias voltage on the cathode.
On voltage checks: if you have a schematic and voltages, these can be a general guide to the voltages you should see on the tube elements. If you don't have voltage measurement data, most of the tube manuals give standard values for circuit DC levels as "typical operation." Most designers used these "typical operation" values in circuit design.
Q. I have a nice old fifteen tube radio. It's got problems with
insulation falling of the wiring, and a couple of repairs that were
badly done. There is a lot of dirt in the coil boxes and bandswitch,
and I can't get at them to clean them up. All of the paper capacitors I
checked were leaking electrically, and several resistors have drifted
way out of tolerance. What can I do with this set to get it working
A. There is a point where the best thing to do with an old radio is to take the thing completely apart, clean up everything, and build it up as a new radio. While this may seem like a lot of work, it actually is easier than a major piecemeal restoration. For one thing, taking major components off the chassis will open up areas and make the rest of the set easier to work on. You will need an accurate schematic for the set, and you will need to make copious notes as you take it apart. Note how the bandswitch and other tap switches are wired, and identify the connections on the schematic. Make notes on what components are where, the hardware used to mount them. You will want a bunch of containers, typically one for each type, to keep parts in. Mark the containers---don't rely on memory for anything. Your notes are going to be "kit building instructions" for putting it back together. Clean off all the old solder to make it easy to install components.
The results can be little short of astounding. You can start with a dog that has parasitics, won't align properly, and has been butchered by hackers who fixed everything except what was wrong, and end up with a brand new radio with superb performance. Work? Yes. But take it one tube circuit at a time, one subassembly at a time, etc., and you'll be surprised and pleased with the results.
Q. What about electrolytic capacitors? Can they be re-formed?
A. Roy Morgan sent in a drill for re-forming old caps. Keep in mind that some caps won't come back to life. The "wets" from the early thirties generally have internal problems and corrosion, and a lot of the axials have dried out internally. Note that a "dry" electrolytic has a moist gauze with electrolyte inside---what makes them "dry" is that the electrolyte doesn't slosh around. "Dry" can types, like the Mallory FP series, often will come back to life with a re-forming. I used the procedure that follows on a 20/20/15 mike 450V Mallory FP with a date code of June 1945 that probably last saw power in the 1960's, and the cap came back to usable condition. Here's Roy's procedure:
To Re-form electrolytic capacitors:
With the "patient" set off, set the external supply at the rated voltage of the cap(s), and feed the old set at the input to it's B+ filter through a 100K, 2W resistor. The old caps will slowly come up to voltage as their elecrolytic layer re-forms after long storage. You may want to unhook bleeders or screen voltage dividers if present in order to get no dc load other than the caps. Once re-formed up to nearly the cap rating, increase the external supply voltage to the point where increased voltage only increases the current drawn (the electrolytics begin to "leak".) You can vary the series resistor depending on the voltage of the cap you're trying to reform.
If the final cap(s) voltage is high enough, it doesn't need to be replaced. If it's too low, put new one(s) in (leave any original cans in place for appearance, and substitute new axial lead ones under the chassis.)
Some caps take only a few minutes to re-form. Some take a day or so! Be patient. Your Adjusta-Volt or Variac can be well-used for this if your external supply is solid state, or has a separate hv supply transformer. I have one good for 900 volts no-load having 5R4's and separate filament transformers. This lets me re-form 500 volt electrolytics if I need to.
With a 500 volt supply, and a number of 100k or 200k resistors, you can re-form a number of caps all at once. Measure the voltage on the caps as time goes on with a high-input-resistance meter (VTVM or solid state DVM). Allowing an electrolytic to idle with a small leakage current of 1 to 5 ma won't hurt it, so if the thing re-forms to it's limit during the night after you've left it on the re-former, no harm is done.
Most electrolytics in good health will leak at a voltage from 125 to 200 percent of the continuous rating. If the leakage voltage is only a little above the needed circuit voltage, or is below about 110 percent of the cap's rating, then you can excpect it to not live too long. New axial lead caps are fairly cheap, and are good peace of mind in my opinion.
(I didn't have a separate power supply. What I did was disconnect B+ from the caps and feed the rectifier output through 100K resistors to each section. With a 670VCT plate winding, and only a few ma. current draw, an 80 will come very close to delivering 500 volts peak (1.41*370 is a little over 500. Once the caps settled down, I put 20K's in the circuit to pull them up even further---they had about 480 volts on them at the end).
Q. What about testing other caps?
A. This is also from Roy Morgan.
PAPER COUPLING CAPS:
Test interstage coupling caps (e.g. from an audio driver tube to the grid of the output amp tube) by measuring the dc voltage at the grid (across the grid resistor if it's not going to ground). Use a high-impedance voltmeter like a VTVM or DMM. If it's above zero, you need a new cap! The vast majority of paper caps from the 30's through the 60's are at least moderately leaky now. Your tubes will thank you with long life for replacing these caps. Ceramic caps have indefinite life expectancy, as do good quality modern film caps.
You can do this kind of testing while you are re-forming the filter caps in-circuit. The tubes are off, and will not be harmed by excessive plate current while you find all those leaky paper caps. The voltages across them will be higher than normal running conditions, because the driving stage is not drawing any plate current.
SCREEN BYPASS CAPS:
With B+ applied and the tube pulled or set off, the voltage at the screen, again measured with a high-impedance voltmeter, should be the full B+ or value at the other end of the dropping resistor. If not, the cap is leaking.
Set your high-impedance voltmeter to a high-enough range and clip one end of the cap to the DC probe and connect (carefully) the other end to a B+ supply corresponding to the rating of the cap. The meter will jump up briefly and then settle down toward zero. Analog meters (VTVM's) are good for this because you can watch the movement of the needle. Once the reading settles, any indication much above zero indicates leakage. A quick ohms-law estimate with the input resistance of you meter will give you a value for the leakage. DVM's are often 10 megohms.
Q. I looked under my radio and there are a lot of parts with several
color markings on them but no printing. What does this mean?
A. There has been a color code for marking part values since the early 1930s. The numbers are always the same:
Black = 0 Brown = 1 Red = 2 Orange = 3 Yellow = 4 Green = 5 Blue = 6 Violet = 7 Grey = 8 White = 9
Mica and molded paper capacitors, in little rectangular plastic packages, used the same color values, but there were about twenty different schemes for locating the dots, and most of them use six dots, with three or four giving the value. These can be a nightmare to decipher. Generally, either the first or second dot in the top row is the first significant figure, and either the rightmost dot in the top row or the rightmost dot in the bottom row is the multiplier. The size of the capacitor (bigger values are physically bigger) and the circuit application will give a clue as to the approximate value. The left bottom dot generally gives the voltage rating in 100s of volts (red is 200; green, 500), and the center bottom dot generally gives the temperature characteristic. The left top dot may be a significant figure or may be a type indicator. Some types have six dot positions, but one or more with no marking, which may mean "not used" or "brown."
Knowing the series of standard values for resistors and capacitors can help in deciphering color codes. These were changed during WW II. Prewar 20% resistors (no tolerance color) were 1000, 1500, 2000, 2500 ohms, etc. Postwar were 1200, 1800, 2700, 3300, 3900, etc., replacing the old 0/5 scheme with approximately 20% jumps in value. Mica capacitors in old radios were generally 50, 100, 150, 200 "micromicrofarads" (picofarads---term did not come into use until the early 1960's in the US). Molded paper capacitors are generally in the 1000 pf. (0.001 microfarad) to 10000 pf. range, with 0/5 as second figures. Postwar production switched to 12, 18, 22, 27, 33, 39 as significant figures, although the old scheme was still commonly used.
Wattage ratings of resistors in different package sizes have been revised several times, always increasing the rating for a given package size. When replacing resistors, modern 1-watt metal film resistors generally are about the right physical dimensions for older 1/4, 1/3, and 1/2 watt resistors. Values should be derated 50%; that is a 1 watt resistor should calculate to a dissipation of 1/2 watt or less in a circuit.
An overstressed resistor will overheat, and discolor its color bands, sometimes very deceptively. In particular, the red and orange multipliers may look brown, and it may require inspection with a magnifier to see that the center of the resistor is charred. Any resistor that looks as though it has been heated to the point of charring or discoloring its markings should be replaced . Also, some compositions used for composition resistors were unstable over time, and a resistor that looks perfectly good and is in a circuit location where overstress is nearly impossible may be wildly out of tolerance. Use an ohmmeter to check, but check your ohmmeter against some known-good new resistors of similar value.
Typical resistor failures:
240 ohm 1 watt cathode resistor for a 7C5---looks like it might have
gotten warm, colors still normal, actually is 150 ohms. Inspection
with a magnifying glass after removal found more signs of overheating
on a side that was not visible with the resistor soldered in place.
Failing "low" like this is not common, and generally comes from using a
resistor with too low a wattage rating for the application. The
coupling capacitor to the 7C5 grid was leaking, pulling the grid up
enough to over-dissipate the resistor. Oddly enough, the tube survived.
240 ohm 1/2 watt screen resistor for a 6K7. This was found on visual inspection, connected to a replacement bypass capacitor in a suspicious-looking repair. Ohmmeter showed about 10K ohms, and the circuit location should have a 2K ohm resistor. Closer inspection after removal disclosed a charred center which had turned the red multiplier brown. This resistor was originally 2400 ohms, used to replace a 2K.
33K 2 watt screen resistor for a 6BA6. The screen bypass capacitor was shorted, "killing" the set. Ohmmeter showed about 250K. This resistor showed no signs of distress. A shorted bypass capacitor often takes out the resistor in the circuit, but a further check in this radio showed about 2/3rds of the resistors over 20% high, some as much as twice the value, even though they were not discolored. It got 100% resistor replacement.
A resistor that is physically broken generally has been subjected to a short circuit condition that overheated it until it exploded. Look for a hard short in the circuit.
Q. My old radio has a lot of tubes covered with wax, and some of the wax
has melted out and is on the bottom of the cabinet. What should I do
A. These are inexpensive wax-impregnated paper-dielectric capacitors. They were notorious, even when fairly new, for developing opens, shorts, intermittents, high dissipation, and tend to be rather fragile as well, particularly when soldering around them. Melted-out wax is common, and may be only the result of heat developed under a chassis in normal operation. From reliability and other engineering points of view, replacing all of them with newer capacitors of other types is part of a refurbishment/overhaul.
Some collectors feel that 40-60 year old capacitors are "survivors," that wholesale replacement is unwarranted. Also, there are two schools of thought on replacing components with others that are very dissimilar-looking, even in areas that are not normally visible when a radio is installed in its cabinet. A few restorers go so far as to melt the wax out of old capacitors, remove the foil-paper "innards," install a new capacitor, and refill the body with wax.
Other restorers feel just as strongly that consistent appearance is more important, and that 100% replacement with no attempt to disguise the appearance of new components is to be preferred. Alfred Ghirardi, in "Radio Physics Course," has a lengthy discussion of failure modes of these capacitors, and states an expected service life of 10,000 operating hours.
Whether to do a wholesale replacement or not is a decision
you'll have to make yourself, and whether to use modern radial-lead
components or to try to find lookalike replacements or disguise the new
ones, also has no uniform consensus. Your radio may not give you much
choice about wholesale replacement. If you find more than one or two
bad ones, or if the set has mysterious ills, parasitics, or poor
performance, or is intermittent, 100% replacement is indicated. If the
item you are repairing is "blue collar" or "high tech," 100% replacement
with obviously new good-quality components seems to be preferable. By
"blue collar," I refer to test equipment and items such as Hammond
organs and studio equipment that worked for a living. By "high tech," I
mean good communications receivers and genuine high-fidelity equipment.
Many of these items used higher quality components originally.
One item that has complete consensus is quality of workmanship. You will want to learn how to remove component leads completely, clean up old terminals, and make neat new solder joints.
Q. I found an RCA model 630 ten inch TV set at a flea market. The
power cord is shot, and when I pulled the chassis out, I found the wires
to the switch appeared to have had the insulation burned off. I found
that the 5U4 plates were melted together. I put in a new 5U4 and
plugged the set in, but it doesn't do anything---no picture, no sound.
What should I do now?
A. First of all, a TV set draws substantially more power than a radio. Do yourself a favor and install a fuse in the primary power wiring to the switch. Use a slow-blow fuse rated at about 150-200% of the set's power consumption. For a set drawing 250 watts, a 4 amp should give reasonable protection.
On a 630, there is a black box mounted on the left rear of the set, with some power resistors inside. Open the box and check the resistors. These are back-bias resistors, in the B- circuit. If they are open, check all the filter caps. Replace the resistors, if necessary.
Bringing up an old TV takes some care, and the order in which you check things out is important. As with all old electronics, assume that it has several things wrong with it. Check that the CRT heater is continuous (ohmmeter)---you should be able to see it glow when you turn the set on. The first thing to fix is the power supply. Once you have good B+, and all the tubes are lit up, do you have a raster? If not, check the horizontal oscillator and amplifier. Note that the horizontal amplifier has very high voltages in it, and that some faults may cause these high voltages to appear where they shouldn't be. Don't go probing around in the horizontal circuit with the set turned on. Horizontal amplifiers on magnetic deflection sets ran with voltage and current levels appropriate for a transmitter, and several postwar sets continued to use the 807 beam tetrode as a horizontal amplifier tube, rather than one of the purpose-built tubes. Shut the set off, connect your probes, then turn the set on, take your readings, then shut the set back off again. Don't touch anything in the set without first assuring that it is shut off, then touch an insulated probe connected through a 1K resistor to ground to all of the terminals in the circuit to assure that there isn't a high voltage charge somewhere. If the horizontal circuits are OK, the 1B3 high voltage rectifier filament will glow. Make sure that the high voltage cable isn't shorted somewhere, and that there isn't a lot of dust or crud to bleed off the high voltage---problems here are usually pretty obvious in the dark, where you can see corona discharges, arcing, and other leakage problems.
Unless you have equipment of measuring 10KV, you can't measure the high voltage directly, but if the 1B3 filament is lighting, and the flyback plate winding to the 1B3 is not open, you probably have high voltage. If you have high voltage, and the tube does not show any light (this may be a spot or a line, rather than a raster), check the CRT grid-cathode bias voltage---once again, keeping hands completely away from the CRT socket unless the set is turned off and you've grounded terminals through 1K. The brightness control should be able to swing the voltage back and forth from about -20 to -60 volts. Check grid 2 voltage---should be around 250.
If you have a horizontal line on the CRT, you are not getting vertical deflection. Check that the oscillator is oscillating, that the output stage is operating.
Once you have a raster, then you can start debugging any
problems in the video and audio circuits. Prewar and early postwar TV
sets trapped the audio right behind the tuner and used separate IF
strips for video and audio. Later sets use "intercarrier" IF's, with
one IF strip and a sound trap at the end of the IF chain. In either
case, "raster, no picture, no sound" means that the problem is between
the tuner and the sound trap. "Picture, no sound," or "sound, no
picture" means the problem is after the sound trap. Don't fuss with the
tweaks on the IF strip (strips) unless you have the proper equipment and
instructions for doing an alignment. Unlike most radios, these are
stagger-tuned, and you don't just "tweak them up" for best performance.
The video comes from a conventional AM detector and a "high fidelity" voltage amplifier, connected to the CRT cathode. Note that the bandpass of the video amplifier is very wide, and the term "video amplifier" has become a generic term from a wideband untuned amplifier. The audio is through a conventional ratio detector and single-ended audio amplifier to a (incredibly cheap setup for something that cost $400 in '46) small speaker.
One fairly standard complaint is loss of raster sync. If the tubes are OK, this is generally the paper capacitor bugaboo at work. Loss of both horizontal and vertical means that the coupling out of the video amp has a problem. Horizontal sync comes from differentiating the video signal, and vertical sync from integrating the double-speed interlace "trick" pulses that ride on the "pedestal" portion of the video signal (the vertical sync portion).
These are some basic things about forties TV sets. Note that
the CRT's on early magnetic deflection sets had offset guns and "ion
trap" magnets. This was to prevent burning a spot in the center of the
CRT. Around 1948, the aluminized phosphor coating, which was impervious
to ion burns, went into production, eliminating the need for offset
guns. If the ion trap is misadjusted, the electron beam won't be aimed
at the phosphor screen properly, so the raster will be dim or
nonexistent, or have "neck shadows" at the edges. This, like the IF
tweaks, is another case of "if it's working, don't fix it." Electrostatic
deflection sets that used tubes like the 7JP4, did not have ion burn
problems, so are mounted with nothing on their necks. These sets also
did not require transmitter-like power for horizontal deflection, so did
not have high voltage derived from the horizontal circuit. Instead, a
separate RF oscillator was used. CRT circuits in electrostatic
deflection sets are quite similar to oscilloscope CRT circuits.
There are several books on servicing television sets that generally apply to forties sets, although they are generally oriented toward later sets. Compared to later sets, most forties TV sets were powered through transformer supplies, did not have any tricks like B+ boost.
Q. My radio plays, but the audio is distorted. Announcers sound like
mush-mouths, and music sounds as though gravel is rattling in the
instruments. I checked it with another speaker, and it sounds just as
A. The most common causes of distortion are in the power amplifier circuit. (Note that in the following I am assuming class A or AB1 operation, where tubes do not draw grid current. If the grids of your power amplifier tubes are driven by power tubes, such as a 6N7 or 6V6's, most of the following applies to operation at low output):-
1. Check that the coupling cap (or caps, in the case of push-pull) are not leaking DC from the preceding stage and pulling the output tube grid high. Most circuits use a 180K to 500K grid leak to ground and a .05 or .1 microfarad coupling cap. At low-moderate audio output, there should be no measureable DC voltage across the grid leak resistor. Check the grid leaks themselves for proper value and good connections (typically to ground).
Wax paper coupling caps here are notorious for giving problems, and are candidates for replacement even if they appear to be good. The tube itself may be developing excess gas current in the grid circuit. Disconnect the coupling cap, turn the set on, and make sure there is no voltage developed across the grid leak. If there is, replace the tube. Note that most tube testers won't disclose this problem. With larger tubes (6V6, 6L6), replacement tubes made after the mid-70's often had poor gas current characteristics, and some designs were built with higher-value grid leak resistors than specified by the manufacturers on the assumption that replacements would "never be that bad." Most beam tubes specify a maximum impedance in the grid circuit of 500K for cathode bias, 100K for fixed bias operation.
2. Check the value of the cathode resistor. Be careful here, because a resistor that has overheated may not only have changed value, but have charred the color bands so that they look like a very different value resistor. If the circuit uses a cathode bypass capacitor (usually an electrolytic, 20 mfd. 25 volt typical), check that it isn't leaking current, and check that it has capacitance.
3. Check grid bias with the set running. Proper bias for various tubes can be estimated from tabular data in tube manuals, and ranges from around -7.5 volts for a small high-gain beam pentode like a 50L6 to around -60 volts for a large low-gain triode like a 2A3.
4. On a push-pull output stage, check that both sides are operating. An easy check is to jumper across the grid leak resistor with a clip lead, and see if things change. If jumpering one input kills the audio, the other side is inoperative. Prime things to suspect if one side is dead are the power tube on that side, open transformer plate winding (no B+ on that tube), open coupling cap, or problems in the voltage amplifier ahead of the output stage.
5. If you haven't found the problem yet, check the quality of the audio coming out of the preamplifier stages.
6. DC imbalance can cause problems in push-pull circuits. Most old radios don't have any place to measure this. You can wire 100 ohm resistors into the plate circuits, in series with the output transformer, and measure the quiescent DC voltage across them. For most old radios, a 20% imbalance is tolerable. Keep in mind that the voltage developed across a cathode resistor is total cathode current, both screen and plate, and that a common cathode resistor in a push-pull circuit is looking at the effects of two tubes simultaneously.
Q. I've got an "All American Five" 50L6 radio that has new filter caps,
but the hum that comes out of the speaker is really out of sight. I can
hear it in the next room when the volume is so low I can't really hear
the station it's tuned to. I know these sets hum, but should it be that
bad? All the tubes test good on a mutual conductance tube tester.
A. No---you've probably got a very common tube fault that a tube tester doesn't detect, heater-cathode leakage, probably in the 50L6. In these sets, the low end of the 50L6 heater is about 38 VAC above ground, and the high end, up at 88 volts. What you are getting is AC on the cathode, and the only real solution is a 50L6 that doesn't have heater-cathode leakage. 12SQ7's can also have this problem, although they are always wired at the ground end of the heater string. The only real diagnostic is to scope the cathodes of both tubes.
One item that aggravates this situation is that many "All
American Five" sets had no bypass capacitor across the power amplifier
cathode bias resistor. Hanging a 50 mfd. 50 volt cap here often will
improve set performance and reduce hum, although it won't solve a
serious case of leakage.
Before trying to diagnose hum problems, particularly in a series string set, try turning the plug to the wall socket around the other way, to reverse the polarity of the chassis. Many of the older 300 ma. series string sets were very sensitive to primary power polarity, and would have very loud hum if the power plug were connected the wrong way.
Q. What sort of tools and test equipment do I need?
A. A 20,000 ohms/volt multimeter is indispensible. They are relatively inexpensive, and modern multimeters have protection circuits in them. You can trouble-shoot and fix almost anything discussed in this newsgroup with a multimeter and some knowledge of circuit theory. Many prefer an analog meter with a needle over digital. You can watch the needle move and see what's happening.
While not "test equipment," tools for unsoldering and soldering components and wire are also mandatory. Soldering is discussed in another FAQ question.
Other small hand tools include screwdrivers, allen wrenches (for knobs with setscrews), nut drivers, and small diagonal cutters and needle-nosed pliers. There is only one kind of tool, a good quality tool. Buy the best. They'll last forever, and do their jobs well. Don't buy cheap knucklebusters. They are hard to use, will make scratches, bend, and break, and scar up the work. Buy the best---many of the good tool manufactures have sold the same tools for over fifty years, and many of us use tools that old today.
Beyond the basics are the following:
a. Oscilloscope. This has become the primary instrument for use in electronics work of all sorts. While they were not commonly used for radio repair in the 1930's and '40's. There are a great variety of scopes, ranging from the old relaxation oscillator sweep type used in the thirties (and sold by Heath as late as the 70's) to the very latest solid state scopes with triggered delaying sweep and multiple trace vertical inputs. Almost any scope that works is fine for working on old radios and vacuum tube amplifiers. While you can get old vacuum tube scopes for very low prices, keep in mind that you may find yourself trouble-shooting and fixing it.
b. RF signal generator. Once again, these come in many sizes and shapes. These are used for aligning tuned circuits (RF and IF amplifiers). For an AM-shortwave radio, you need 100 kc. to around 15-20 mc, with AM modulation capability, and for FM, you should have 88-108 capability as well. A sweep signal generator (i.e., able to swing the frequency back-and-forth over a small range electronically, with a voltage output to drive an oscilloscope horizontal amplifier) and a suitable scope are very nice to have but not mandatory.
c. Tube tester. The value of tube testers as a primary diagnostic tool tends to be overrated, but a good mutual conductance tester (Hickock made several) can be of value if it is used appropriately. Cheap "tube checkers" will test filaments (an ohmmeter will do as well) and whether the tube conducts or not, and may detect hard short circuits (these do happen). A Tektronix 570 curve tracer (a specialty oscilloscope that gives graphic displays of tube characteristics) is the ultimate in test devices. However, the ultimate "tube tester" is the equipment in which the tube is used. The function of tube testers, more than anything else, was to sell replacement vacuum tubes. And many really nasty tube-related problems will only show up in the socket in the equipment where they are supposed to function properly.
If you have a good scope, multimeter, and signal generator, and know how
to use them, you have all the tools you need for radio work. Here are
some other items, some of which were popular as radio shop tools, and
some of which aren't primarily test equipment:-
d. Signal analyzer, signal tracer. These were very popular in radio shops. They are an AF amplifier, small speaker, and a diode detector that can be switched in and out of the probe circuit---in essence, a small radio without any tuned circuits. If signal is getting into the antenna, you can probe each stage and hear it, and quickly locate a "dead" or "distorted" stage.
e. Condenser tester. Also "radio shop" stuff from the 1930-50 era. An inexpensive L-R-C bridge with an electronic oscillator. Used properly, it can be a handy tool. I use the term "condensor" because it was the standard term for a "capacitor" in the US until the late 1950's.
f. VTVM (stands for "Vacuum Tube Voltmeter"). The virtue of these is the high input impedance (generally megohms) and their ability to measure resistances into the megohms range. Largely supplanted by oscilloscopes, which draw a picture of the signal, but of value today for their ability to measure high resistance.
g. Grid dip meter. This is a small oscillator that comes with a set of plug-in oscillator coils that can be poked into tuned circuits. They rely on the fact that a resonant circuit near the oscillator coil will cause the grid current of the oscillator tube to drop, hence "grid-dip." A very simple and handy little device, though generally used with things like transmitters that have to be tuned before power is applied. Since they oscillate, they are also a fine "poor man's signal generator."
There were several specialty houses in the US in the 1930-50 era that
built very good measurement equipment. I'll mention them by name:-
Boonton Radio, built Q-meters and R-X bridges. These measure the inductance and other characteristics of RF coils and tuned circuits. Generally used to support coil design efforts. The British Marconi Q-meters are excellent as well.
Measurements Corp. Built very nice signal generators, much higher quality than those from repair equipment manufacturers like Hickock.
General Radio (Cambridge, Mass.). This company moved to the suburbs in the late 1950's and is now known as Genrad. Their 650 impedance bridge was the general use DC/400 cps L-R-C bridge. It used a small battery and a 400 cps "hummer" (a small vibrator) to generate AC for measuring impedance of things like audio transformers. Over the years, General Radio built a broad line of devices, primarily for engineering use, only some of which are applicable to radio electronics.
Guildline of Canada. I mention them because they built some of the very best calibration standards. Their potentiometers and other products are not only "not test equipment" but can easily be damaged if used for testing things. The proper use of such equipment is calibration of working equipment, and the appropriate place for it is a calibration shop.
While I mention equipment common in the US, I am familiar with products of Marconi in England, who built engineering support products similar to the Boonton, Measurements, and General Radio products. I believe that Telefunken, Phillips, and Thompson-CSF (spelling?---French company) also built and sold similar equipment. The US stuff often shows up at things like ham swapfests, and is bought and sold by several companies, notably Tucker, of Dallas, Texas.
Q. My radio is supposed to have 295 volts on the screen of the 6L6
amplifiers. I read 303.5 on my digital voltmeter. Is something wrong?
A. Yes, both your expectation that the screens are supposed to read 295 volts, not 295 +/- 20%, and that your DVM is precise just because it gives you a lot of digits. When was that DVM last calibrated (or was it ever calibrated) against a known standard of some sort?
Most shop test equipment is wildly inaccurate to begin with, and has had enough use and abuse (and time) since last checked that you can't trust the readings at all. At best, they will tell you "around 300 volts" or "around 455 Khz" unless you have some way to check against standards. Don't trust anything to be telling you other than "approximately" unless you have had it checked against standards recently, know what accuracies you can expect, and things that can affect accuracy. Most major cities have services which have standards against which to check test equipment, and if you have something like a GR 650 bridge that is working properly, it may be worth the tariff to have it's calibration checked by one of these shops.
When selecting test equipment, keep in mind that that nice old Tek scope may have 35 or 30 tubes and 50 adjustments, and pose much more of a maintenance problem than any radio.
Q. I don't trust the calibration of my instruments? What can I use to
A. There is a good frequency standard available for free: WWV, which broadcasts on 5, 10, and 15 Mhz. If you have a signal generator with a crystal calibration oscillator, you can tune in WWV on a shortwave receiver, tweak the crystal tank circuit, and have a fairly good reference to WWV for other frequencies----though it's a long stretch from 5Mhz to 455 Khz. Fresh dry batteries generally are fairly close to their nominal voltages, and an automobile battery that is fully charged is a first cut "standard" 12.6 volts. Accurate voltages above that are hard to find in the basement workshop. Ohmmeters tend to be wildly inaccurate, but you can measure a bunch of resistors of different values to get "somewhere near."
(Faq editor note [still outstanding]: other countries have frequency-standard time stations; if someone familiar with them could E-mail me the information, I will include it here).
The rule of thumb is that two-figure accuracy is readily achievable, and more than what is needed for service work. However, if you are using flea-market test equipment, it may have been discarded or surplussed because it could not be calibrated, or may not have been checked and calibrated for thirty or forty years.
Q. I tried to use a Tek scope to trouble-shoot my AC-DC set, but when I
connected the probe ground, I got sparks and burned out the wire.
A. US AC-DC sets typically have one side of the line connected directly to chassis ground. Some European sets may also have a direct connection between one side of the supply mains and the chassis. Virtually all US test equipment built over the last 40 years uses a three-prong plug with a direct connection between the ground prong and the test equipment chassis. What happened here is that the radio was plugged in with the high side of the line connected to its ground, and you connected the ground strap across the line voltage. While in US power distribution systems, the "neutral" wire is connected to earth ground at the distribution panel, grounding the line neutral at the radio may cause currents to circulate in the neutral-ground circuits (ground loop).
The best way to avoid a shock hazard with an AC-DC set is to use an isolation transformer. It is possible, but not recommended, to "float" the test equipment ground by using a two-prong "cheater," but this may cause other problems. Plugging the set in so that the grounded side is neutral may also work, particularly if you use a .01 mfd or larger cap in the ground circuit to the scope. However, with any method other than an isolation transformer, the scope and the radio may have some voltage between them, posing a shock hazard as well as problems making measurements.
AC-only sets were often connected with a .02 mfd cap from each side of the AC line to the chassis to provide an AC reference ground between the chassis and the AC line. If either of these capacitors is shorted, the chassis is directly connected to one side of the line. Find these caps and check them before doing any trouble-shooting.
Q. I want to fix my old radio myself, and have never used a soldering
iron before. What do I need to do?
A. Soldering equipment for radio work is discussed in section 7 of the FAQ.
Q. Replacing all those capacitors is a lot of work. Somebody told me
that I could just clip the leads and solder new caps to the old leads.
That sounds a lot easier. Should I do that?
A. Going back to Frye's "Mac's Service Shop," a column that appeared in the old "Radio News" in the 1940's, a proper repair is to make the radio "like new," using the methods that were used to build it originally. The Yiddish term, "schlock," was invented for folks who do things like clip out old parts and solder new ones to the leads. Yes, removing solder from terminals and prying the ends of tightly-wrapped leads open so that you can remove an old part is hard work, and it will take a while to learn to do it with any ease. Take those old components completely out, clean off the terminals, and install the new components neatly. In many cases, particularly if you are replacing wax paper capacitors with axial-lead mylars, you will find the old leads bent around quite tightly to connect one end of the capacitor to the nearest ground that could be reached. The new capacitors are much smaller, and may install much more neatly, particularly if an appropriate ground point is nearby for bypass caps.
If you take pride in good workmanship, you'll end up with a set that works well, isn't a fire hazard, and doesn't have mysterious squawks and squeals. Sloppy workmanship is a red flag to anyone who looks at the radio---it says that there is probably extra trouble installed by whoever did the poor work. And, most of the time, investigation shows miswires, wrong-value components, and a host of other problems.
Q. I have an early 1930's radio and want to replace the wax paper
capacitors, but want to keep the chassis looking original. Where can I
get look-alike wax paper caps?
A. The manufacturers who made these discontinued them years ago. You will probably find repairs from the 1946-70 era in old radios using paper capacitors molded in plastic, and even these are difficult to find nowadays. While recent axial lead caps with mylar and other plastic dielectrics work well, as long as the voltage rating is adequate. Generally, 400 volt caps will work, but 600 or 630 volt caps are safe in any set with an 80 or 5Z3 rectifier. However, they don't look anything like the old axial lead capacitors.
It is possible to melt the wax out of old capacitors, salvage the cardboard sleeves, and install new axial lead caps inside. While axial-lead caps are somewhat hard to find, and tend to be expensive, they are still made, and are usually small enough to fit in the old cardboard sleeve.
Dan Schoo, who does this type of restoration regularly, kindly wrote up a procedure for salvaging the old sleeves and putting new caps inside, and made it available for inclusion in the faq. Here it is:
Rebuilding Wax Filled Paper Capacitors
by Daniel Schoo
From: email@example.com (Dans Cockatoo Ranch)
A paper capacitor is a type of capacitor that was used
extensively in radios from the thirties through the fifties. They
are made of wax impregnated kraft paper and two thin metal foils
cut into long narrow strips. The foils were placed one on each
side of the kraft paper and rolled up along the long dimension
into a rod shaped assembly. The foils were skewed such that they
extended a little past the paper at the ends of the rod, one on
each end. This provided an electrical connection point to each
foil over it's entire length.
The voltage rating of the capacitor was controlled by the thickness of the paper. Thicker paper could hold off a higher voltage. The capacity was controlled by the surface area of the foils. Longer wider foil wraps would have higher capacity. This is why higher voltage and/or higher capacitance values would require a larger size for the capacitor.
The lead wires were attached to the foils extending out from the ends of the rod. The entire assembly was then slipped into a cardboard sleeve and the sleeve was filled with wax. Later types were molded into plastic shells and had paper labels attached or were printed with colored bands or text to indicate the values.
The black band around one end of the sleeve and the words 'outside foil' indicate the lead that is attached to the foil strip wound on the outside of the kraft paper. This is important in some applications and tells the assembler which lead to use during construction. Paper capacitors were used for higher voltages at medium to small capacities. The voltage ranges are usually from 100 to 600 volts and from .0001 to 1 microfarad in capacity. They typically fail by becoming leaky and allowing DC current to pass.
The purpose of rebuilding an old type paper capacitor is for appearance only. When restoring an old radio to operable condition, some owners desire to keep the appearance of the components under the chassis as close to original as possible. When certain components fail such as capacitors, it is not possible or even desirable to replace them with original types. To keep the original appearance, the old component is taken apart and a new one is hidden inside the old shell.
After you have determined that a capacitor is bad, or if you just
want to replace one because you have a basic dislike for them,
remove it from the radio. Begin by melting out the wax potting.
Wear eye protection and use a heat gun, blow drier or small torch
with a hot air attachment like a Master Ultratorch. Do not use a
torch or other open flame on the capacitor as this will apply too
much heat in a small area and probably cause it to burn. Not much
heat is required to melt the wax but it has to be steady and even
to heat up the entire body of the capacitor. Hold the capacitor
sleeve with a long nose pliers and heat it slowly until all the
wax has dripped out. Discard the old wax.
Some paper capacitors have cardboard end disks. For these, the ends of the sleeve are rolled inward to retain the disks. Unroll the end crimps and smooth them out. Remove the end disks with a small screwdriver. After the end disks are out continue to heat the capacitor until the rest of the wax is out. After most of the wax has run out, hold the capacitor with an insulated pad, grab a lead with a pliers and pull the insides out. If the wires come off, push the insides out with a small screwdriver. If the insides are stuck in the cardboard sleeve, it may be necessary to drill them out. Pull out the wire leads and drill a small pilot hole down through the center of the capacitor. Drill another larger hole about half the diameter of the sleeve. You should be able to dig out the rest of the insides with a small screwdriver. Be careful not to puncture the sleeve.
Once the sleeve is cleaned out you can install the new modern capacitor. The most difficult part of this is to find a suitable capacitor that will fit into the paper shell. You can substitute a new capacitor with an equal or higher voltage rating than the old one but try to get as close as possible to the original capacity. Fortunately many of the modern capacitors are much more compact than the old paper ones. Modern capacitors use plastic films like polycarbonate, polypropylene, polystyrene, and polyester which is also known as Mylar. The most common is Mylar and is suitable for many replacement applications. These do not degrade with time like paper does and should give years of good service. The popular Sprague "Orange Drop" is a Mylar capacitor. These are not suitable for use in this application because they are designed for printed circuit mounting and the leads are radial. This means that they extend out the side of the capacitor at a right angle. You must use an axial lead capacitor with the leads extending out in line with the capacitor body.
Slide the new capacitor inside the old sleeve and center it. If
there is a lot of space around the new capacitor such that it is
loose you can wrap a few turns of plastic tape around it to build
it up. Slit the tape down to about a quarter of an inch wide and
wrap it in a band around the middle of the capacitor until it
fits snugly in the sleeve. After the new capacitor is centered in
the sleeve you can fill the ends with wax. You can get beeswax at
any well stocked hardware store. It comes in small tan blocks
about three inches square and one inch thick. Cut off a small
chunk and place it in a small metal can. Prepare the can by
bending a pour spout into the top edge and make sure it is clean
and dry. Wear proper eye and skin protection when heating the wax
just in case it spatters.
Heat up the wax slowly with a heat gun, a hair dryer or small torch. Remove the heat when all of the wax has melted and be careful not to overheat it. If it begins to smoke remove the heat immediately. Support the capacitor in a vice or tape it to the edge of a table top. When the wax has melted thoroughly, pour it slowly into the end of the sleeve just up to the edge. If the capacitor had end caps leave enough room to reinstall them. When the wax has cooled sufficiently, flip over the capacitor and fill the other end. Allow the capacitor to cool completely before installing it in the radio.
V Daniel Schoo (o o) Electronics Design Engineer ( V ) Fermilab, Batavia, Illinois, USA .......m.m......Dan's Cockatoo Ranch vvv
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