•  Repair and Restoration of Vacuum-Tube Radios

by

 Alfred Corbin

296 pages

An introduction to the art of repairing or restoring vintage radios.

This book is not a “Radio for Dummies” treatment, but can give  a good insight into how vacuum-tube circuits function and the failures they tend to exhibit.

The text is basic enough for a non-technically educated person to grasp, yet includes enough theory for a technician or engineer to learn more about this rapidly disappearing technology.  It focuses mainly on the practical aspects of failure modes in old radios and tracking down problems.  Woven in with the practical meat-and-potatoes of vacuum-tube radio servicing is just enough theory to give a novice a good understanding of the principles of radio receiver  operation.                        

Along with the practical and theoretical aspects, the reader is treated to a thorough indoctrination in the nature of various components found in old radios, shop practice, safety hints and lots of related information.  If you’re interested, check my website on Yahoo antiqueradiorepair.net.

Alfred Corbin

tslfwb@aol.com

Here’s an excerpt from the book:

             FIRST AID FOR AN OLD RADIO

Is there any sound or noise from the set?

No sound or noise.  Are all the tubes lighted?

In an AC/DC type of set,  if one tube filament fails, none of the tubes will light.  In a set which includes a power transformer, the tubes are hooked up to light independently, so a single tube might be burned out while the others are lit.  Test or substitute good tubes until all tubes are lit.

With all tubes lit,  any noise or sound?  If there is a loud 60 cycle hum, like a tuba note,  look for a failed filter condenser. Consider replacing that condenser if you are equipped with the necessary skill and have a suitable replacement part.

Is there background hiss, or noise like  that heard between stations, but no stations?  Can you change the volume of the noise by the volume control?  If so, look for a failed front end tube.  In the old sets, this tube is called a mixer, converter, oscillator/detector, or a first detector.  Replace that tube, if you can identify the tubes and their individual functions.  If not, read on.

If a station can be received,  is the audio distorted or growly?   Examine the speaker for physical damage  such as a warped or torn paper cone.  If the speaker is OK, try replacing the audio amplifier tube or the audio power output tube.  If no luck, there is an excellent chance the coupling capacitor between the audio amplifier plate and the audio output grid is leaky and should be replaced.  Again, you must have a suitable replacement part and the skill to install it.

Is the output of the radio unusually weak?  Try replacing the rectifier tube.  Ordinarily, a rectifier will fail outright, but sometimes a rectifier will develop high internal resistance and not provide sufficient DC voltage supply to the set.  Can you identify the rectifier tube?  Read on…

Use all of your senses.  A radio technician’s best test equipment is his own built-in sense of touch, sight, hearing, and smell.  Is there any sign of overheating in any component? Any strange odors?  Do any of the tubes show a faint blue internal glow, indicating gas or overheating?  Is the set especially sensitive to mechanical vibration?  

The above symptoms cover some of the most likely failures in most old radios.  Beyond this, read on for more detailed information on the theory and servicing of radio receivers.

****************************************************

COMPONENT FAILURE MODES

There are an infinite number of ways in which a radio, or any other electronic circuit may fail, but  the majority of failures of components are predictable and fairly easy to identify.

Capacitors:  After tubes, the next most common failure in most old radios is a capacitor breakdown.  Of all the capacitors in a typical old radio, the RF capacitors are the least likely to fail.  These are typically fairly low (microfarad) values and usually used at much lower voltages than their rated breakdown voltage.  Most small RF capacitors are constructed with a mica or high-grade plastic dielectric insulating material between plates, and are usually encased as  a well-sealed module.

At the other end of the reliability spectrum are the very common paper/foil capacitors.  For many years during the 1930s through the 1950s, practically every radio included at least a dozen of this kind of capacitor, typically in the range of .001 to about 0.5 microfarad, usually rated at a breakdown voltage of 100 to 600 volts.  The working element in this kind of capacitor is a rolled-up sandwich of two strips of thin metal foil separated by a thin paper barrier, with the paper usually impregnated with wax or oil to improve its insulating quality.  Over the years, the paper insulator may become contaminated with moisture, or develop a pinhole leak, inviting a breakdown of the voltage between the plates of foil.  The breakdown may show as a high-resistance leakage or as a catastrophic failure in the form of an electric arc.  Often an arc- type of breakdown will leave a carbonized trail, resulting in permanent leakage across the cap;  sometimes the cap will essentially heal itself after the breakdown.

• In an old radio, many of the paper-style caps may appear to be swollen, shrunken, or their ends may bulge.  These caps might be perfectly OK, but should be replaced.  Other paper caps may look OK, but might exhibit leakage when voltage is applied.  Usually, a cap with a leakage problem will show up because of the leakage affecting another component.  In the case of a coupling capacitor between the plate of a tube to the grid of the next tube, a tiny leakage is sufficient to cause a runaway current in the following tube, with very obvious results.  If a cap seems to be warmer than its surroundings, it should be replaced.

A capacitor suspected of leakage can be tested with an ohmmeter.  The cap must be disconnected from its external circuit components to avoid the measurement being confused by the resistance of other components.  On the highest ohms scale, a good capacitor should indicate at least 1 megohm resistance, preferably more.  This is a good test, but not foolproof.  Sometimes a leaky capacitor will leak only when a high DC voltage is applied across it.  If the application of such a cap is coupling between stages, remove the following tube, and check the DC voltage at the grid terminal of the removed tube.  If any positive DC voltage is indicated, the cap is definitely leaky and must be replaced.

In very rare instances a cap will open up, with the result that the circuit acts as if the cap is not there.  In the example of a coupling cap,  no signal will be coupled to the following stage.

Electrolytic caps,  universally used as power supply filter  caps,  are susceptible to failures through age and climate and temperature.  An electrolytic capacitor normally might have a measurable leakage resistance, perhaps on the order of 10,000 ohms or more, but, when failing, that resistance will drop down to a near short-circuit condition.  A failed electrolytic cap must be replaced.  One of the first indications of a broken-down filter capacitor is the rectifier plate, or plates, glowing red hot because of the excessive current shunted through the shorted cap.  Also,  there is the presence of unusually loud hum in the audio.

Finally, a variable capacitor such as used in radio tuning circuits, is practically immune to failure because of its simple, rugged mechanical construction; however, it is fairly common for a variable cap to become extremely noisy or non-functional if one of its plates has been bent and is contacting an adjacent plate.   Also, a common problem with variable caps is the presence of just a small speck of dirt or trash between the plates.   This usually shows as a high noise level as the capacitor is tuned through its range.

             RECAPPING (or not?)

Many collectors and antique radio fixers are of the opinion that any old radio should be re-capped before any other action is considered.  Recapping is defined as the wholesale replacement of all the capacitors in a radio. This is a  serious  move, and can lead to the near destruction of a good old set if handled carelessly.

Radios of the era of interest, about 1930 to 1960, used many capacitors as filters, tone controls, noise suppressors and couplers between stages.  With few exceptions, the filter capacitors were  the electrolytic type and the smaller caps were constructed of paper and foil, usually impregnated with wax to seal out moisture.  Aside from loudspeaker paper cones, capacitors are  the most likely  parts of an antique radio that may become degraded and fail with age.

In a capacitor used to couple a signal from the high-voltage plate of a tube to the sensitive grid of a following tube, a tiny amount of leakage current through the cap can distort a signal beyond recognition, and can cause slow destruction of the tube of the following stage.

In the case of a filter capacitor which has developed significant leakage, the first indication is usually an undesirable level of hum in the audio output; at worst, the end result of a failed filter cap can be an extremely messy explosion of the component, sometimes followed by a fire.

Many novices in the hobby of collecting and restoring antique radios have the impression that recapping  is a way to repair a radio which is not working.  This is a fallacy.  Replacing all the caps, at least all the paper and electrolytic caps in a set,  is just a way to lower the probability of future failures and to prolong the useful life of a cherished old radio.  Recapping a radio should be considered only after the set is brought up to good working condition.

An experienced repairer or restorer will follow a course of trying a set as is to determine the nature and extent of the repairs which must be made, applying the necessary repair, if fixing is practical, then assessing the condition of the capacitors in the set.

As a first step, any capacitor that looks bad should be replaced; that is, if the cap is swollen, shrunken, or bulging at its ends.  According to many oldtimers in the restoration game, there is one notable exception: many restorers are convinced that all “Black Beauty” caps should be replaced.  Sprague Black Beauty caps are of paper/foil construction with an outside casing of hard black plastic with a lot of yellow and orange code rings.   Because of their  yellow stripes they are often called “bumblebees”.  They have the appearance of excellent quality, but seem to have a bad reputation for failure.

A filter cap, often constructed in an aluminum can, showing any sign of leaking liquid or paste should be replaced.

A good initial assessment of the condition of a filter cap is an ohmmeter test of the leakage through the cap.  To make this test, it must first be determined that external circuit resistances are not included in the measurement.  If necessary,  disconnect any wire from the “high” side of the filter which may lead to a shunting resistance to ground.  With an ohmmeter on the highest ohms scale, the resistance measured across the cap should be off scale (infinite), or at least 10,000 ohms.  Keep in mind that an ohmmeter test applies only a few volts to the cap under test, and that the cap may exhibit much more tendency to leak when subjected to its normal environment of 100 volts DC or more.

The smaller paper/foil caps can also be tested for leakage by an ohmmeter, but usually an ohmmeter test is not sensitive enough to detect a tiny level of leakage which could impair the performance of the radio.  The ohmmeter test, however, will clearly identify a cap whose leakage is a near-short circuit condition. 

Re-capping a set, that is, replacing all of the capacitors is a big, serious job.  If you wish to be 100 per cent sure of the condition of your set, and provide some assurance for the future, it is advisable to re-cap, but there are certain rules to follow to avoid creating a monstrous mess.

First, make whatever repairs are necessary to bring the set up to good operating condition.

 REPLACE THE CAPACITORS ONE AT A TIME!

 After each cap has been replaced, turn the radio on and make sure it is still operating as it should.  Many beginners (and old-timers) have made the error of replacing all the caps at once, then spent hours or days troubleshooting to find where one of the caps has been misconnected, a solder short has been installed, or another component has been damaged in the process.

Selection of the replacement caps is a matter of personal choice.  It is aesthetically preferable to replace each cap with one of a style similar to the old one, but exact replacements are sometimes difficult to find.  Usually, a cap of the same general size and shape can be found.  Capacitor values are not extremely critical, and a paper cap can be replaced with one of a nominal value within about 20 per cent of the original value without affecting the set’s performance.  It is important, however, that the replacement cap have at least as high a voltage rating as the cap which is to be replaced.

In spite of one’s best efforts, it’s easy to lose track of exactly where the two leads of a replacement cap must be connected.  Although it is not cosmetically quite perfect, many experienced technicians find it practical to cut out the old cap leaving a stub of its leads in the circuit and soldering the new cap to the old lead stubs. Another good trick, not always possible, is to leave the old cap in place, connect the new cap across the old one, then cut out the old cap.  The process of recapping involves a fine balance between the quest for excellent performance and excellent, original, appearance.

In the case of electrolytic filter cap replacement, most filter caps are mounted in a vertical position on the chassis and ideally should be replaced with a cap of similar construction.  This may create a problem, especially in the older radios.  With a little magic and creativity, it is sometimes possible to remove all the guts of a failed old filter cap and install small modern replacement caps inside the can, to preserve the original appearance of the set.  Most of the more modern sets included three, sometimes four, separate electrolytic capacitors inside a single can.  Today, tiny electrolytic or tantalum capacitors are available that occupy a small fraction of the space that the early styles  used, so it is reasonably easy to fit several miniature caps of the necessary microfarad value and voltage rating in the available space.  When finished and installed, a filter cap made up of several small modern units will do the job without altering the appearance of your antique.

Although it’s a tedious job, some restorers make a practice of salvaging the outer tube of an old paper cap and stuffing it with a smaller modern cap, retaining the original appearance.  Most paper caps can be heated with a heat gun or hair dryer until the wax melts and the old guts can be removed without any damage to the hard cardboard shell.

When replacing filter caps, if the exact replacement value is not available, go to the next available higher value.  Voltage rating of the replacement cap can be any value equal to or higher than the original.

When re-capping a set, limit the process to only the electrolytic filters and the tubular paper and old plastic tubular caps.  Do not replace the mica caps which are commonly found in the front-end (RF and IF) circuits.  These small capacitors, usually packaged as flat squares about the size of a postage stamp, or tiny ceramic cylinders, are generally very reliable and replacement is not advised unless it is clearly indicated that one is faulty.

 $23.95 

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or by US Mail, from:

Alfred Corbin

261 Ventura Circle

Fort Walton Beach

Florida 32548

tslfwb@aol.com