Repair Your AC/DC Tube Radio

By Bill Jones. This article will allow you to repair a number of radios if you are inclined to follow the steps given here.  While the discussion will be centered mainly on the five and six tube AC/DC radio much of the discussion applies to the transformer radio. Included in this article will be the necessary minimum equipment required. It is necessary to find the parts needed, and this will be discussed.

The bare equipment required is a Volt-Ohm Meter (VOM) along with a good soldering iron.  The digital meter is great since it is accurate, and difficult to burn out. I use a Fluke meter and highly recommend it. I also have a good Volt Ohm meter. I find a Vacuum Tube Volt meter most useful.  You need means for getting tube data sheets.

You may require two irons if you intend to work on the transformer sets. One iron should be a 500 watt iron (for transformer radios), and another smaller iron. I would suggest a 40 watt iron.  The older AC sets often had components that were soldered to the chassis (ground), and it is difficult to solder or unsolder to the chassis of these older radios without a 500 watt iron. It is assumed that those interested have the tools required, and know how to use them. These tools being long nose pliers, diagonal or wire cutters, along with the proper size screwdrivers.

I live in a city that has an antique radio club. It is normally easy to find tubes and other common components from members of the club. If not available from a radio club most components are available on the web. I use eBay for tubes if they are not available from a member of the club.  You can get most components on eBay and other web sources such as Antique Radio Forums.

 I will not consider the battery radio - that is to say radios before about 1922. These radios often present the same repair problems as the later radios, but the components are usually more difficult to find. These early radios are of historic interest, and it may be unimportant as to their working condition. It is their appearance (condition) that is most important. If you do want to have a  working battery radio you may be faced with obtaining coils, transformers, tubes and other components that would require time, and effort to find, but they are available somewhere,  Some parts may even be reproductions.

This article will allow you to repair most radios – probably 80% or more of the radios that you may have.  This article is written with reference to the tube line up that would be similar to the 50L6, 35Z5, 12SQ7, 12SK7 and the 12SA7. It will then still be helpful with the radios using miniature tubes. The miniature tube sets had similar circuits. The miniature tubes had much the same operating characteristics as the older tubes. This article will be helpful when considering the ballast tube sets, and the resistive line cord sets. A number of these sets were TRF’s, but there were superhets that used the resistive line cords. Many of the sets used the 25Z5 for the rectifier. These sets often used the 6 volt tubes in the R.F. and I.F. and even the type 43 output tube was used before the 25L6 became available. I have restored some of the TRF’s that originally used the resistive line cord. Tt would be difficult to list all the various tube line ups that were manufactured, but this article would apply to most popular sets manufactured that used the 50L6, 35Z5,…. line up or the five tube miniature tube lineup. The circuitry was essentially the same.

I have had an interest in radios since I was 10 years old. I got a job at a radio shop when I was 16 years old. It was there that I was taught how to repair any radio that came into our shop. I had an expert mentor.  Ray Bingamton would help me with any radio that I was having difficulty with. He enjoyed it, and so did I. When I did go to work I knew how to use all the necessary tools, and was good at soldering.  I was familiar with all the radio components. I could also repair a few radios.

It was necessary to use substitute parts to repair radios at the time during WW2.  The exact replacement parts were usually available. I would estimate that I have repaired well over 8000 radios. These radios were AC/DC, or AC transformer sets, car radios, and portables. When I got into the repair of radios the sets were mostly Superhetrodynes (Superhets).   There were very few TRF’s (Tuned Radios Frequency) sets to be repaired.  The TRF was obsolete at that time. I would estimate that half of the radios were the five tube AC/DC sets. While my desire is to give you a quick start to begin with, there are many books that cover the repair of radios. Many of the books will go as deeply into the theory of communications as you desire. You will also find good discussions on the web of how to repair radios. There are also radio forums that are on the web, and these are helpful.  Reading about the theory is helpful. Reading about the repair is helpful, but to actually repair a radio is of most help. You can read all you wish, but you must be prepared to get your hands dirty.  Knowledge is very important, but experience is necessary.

This note is considered to be basic.  I will assume that we are discussing the repair of AC/DC superhets, and that you do understand the block diagram of these sets. For purpose of discussion we show the basic block diagram.

                                               

Superhet Diagram -Right Off the Web


You should be aware that a few of the early AC/DC sets had one side of the AC line cord soldered to the chassis. These sets are rare.  They are of some danger when the chassis is at 120 volts AC. When I work on this type of radio I like to have the AC plugged in such that the chassis is at the AC ground.   Injury did occur because of the AC being connected directly to chassis it became clear that change was necessary. Most AC/DC sets were then designed such that the ground was not directly connected to the chassis. While the AC is not directly connected to the chassis remember that the radio ground may still be at 120 volte AC depending on how it is plugged in.

It should be noted that there are modern radios that use the chassis as ground. These sets have protection built in so that protection from injury is excellent. For example, the radio knobs will not pull off.  The knobs remain with the cabinet as the chassis is removed. These sets disconnect the line cord when the back of the radio is removed. I have seen this protection means used in some European sets and late model radios.

While the discussion is centered on the AC/DC radio there will be some discussion on the transformer radio. There is a big difference between the two types of radios. This difference centers on the power supply, and the tube bias means. The transformer sets have the tube filaments in parallel. The AC/DC radio has the tube filaments in series. If one tube filament is open then none of the other tubes light up.

Don’t plug the radio in just yet. You will need to find the tube filament pin out, and what the voltages on the other pins of the tube should be. This tube pinout is shown in a tube specification sheet or data sheet.

I find that a tube manual is most helpful for the purpose of getting the tube pinout along with its operating voltages. However, you can also find the tube information on the web. If you suspect an open filament on a tube you can check the resistance of the tube filament to see if it has continuity. You therefore need the tube pinout to find the filament pins. You will also need the tube operating voltages for further tests.

Before plugging in the radio it is good to check the resistance at the AC plug of the radio.   This resistance is in the order of 50 ohms or more depending on the AC switch. The AC switch may have some corrosion if it has not been used for some time. Operating the switch a number of times may be required to remove some of the corrosion.  If there is an open circuit (no reading on the VOM) then the radio may have a bad tube, but the radio switch or the cord could be bad instead of a tube. If there is no resistance reading then check the switch which should be less than a few ohms. While it should be less than an ohm, there may be a couple of ohms in the test leads of the VOM. Check the AC line cord for continuity. If the cord and switch are good then a tube filament is probably open.  You then check each tube filament with your VOM to find the bad tube.

I like to check resistance at the filter capacitors before plugging the set in. This is not to check the filter capacitors necessarily, but to insure that there are no shorts on the B+.  The resistance across each filter capacitor should be well over 10,000 Ohms. You may have three filter capacitors. One could be for an audio cathode bypass.  The cathode bypass will read several hundred ohms. A schematic diagram of the set is helpful. The schematic is normally available on the web.  Of course, you need the make, and model number.

If you have a reading of a few hundred ohms at the high voltage filter capacitor (cathode of the rectifier) then you must find the reason for the low reading before plugging the radio line cord to an AC outlet. This low resistance could of course be the filter capacitors, or a shorted capacitor on the plate of the audio output tube. In any case the reason for the low resistance must be found before plugging the radio into the 120 Volt AC outlet. Otherwise you could burn out the rectifier tube.

Now that all checks are good – we may plug the radio to the AC outlet.  The tubes light up and you will hopefully hear some noise from the speaker. Most often, the filter capacitors are bad, and the radio will hum.  This is usual and encouraging since it means that the audio section is working. It is normally a loud hum.  The radio may receive stations, but it may not until the filter capacitors are replaced. It is usual to solder a new capacitor across the original capacitor, but better practice to remove the wires going to the old filter.  The new capacitors should be the same value (or larger) as the old capacitor. Soldering a new filter capacitor across the original capacitor has never been a problem on the AC/DC set, but I hesitate to do this with the transformer set. I have found on the high voltage transformer sets that a bad filter capacitor may have an internal resistance that will cause the original capacitor to overheat, and this heating could cause the capacitor to explode.  It could burn out the transformer or burn out the rectifier tube. I have never been around a capacitor explosion, but I understand it can cause a loud bang.  I suspect that it could be of some danger if you were close to the capacitor when it did explode.

The value of the filter capacitor on the AC/DC set is not critical, but the capacitance should be at least as large (larger, not smaller) as the original value. I normally use 33 or 40 micro-Farad filters, but some sets have filter capacitors that may go up to 80 microfarad’s.  Many of the old sets will use 20 mfd, but I usually replace them with a 33 micro-Farad.    Today, the new filter capacitors are physically much smaller than the original capacitors.

Remember that the filter capacitors have a polarity, and the negative side goes to the radio ground. There can be a minor problem with large capacitance filter capacitors. These capacitors may cause the pilot light to burn out when the set is turned off and then quickly turned back on.

I normally expect the AC/DC sets to have a permanent magnet speaker, but if it has a field coil then you should check to be sure that the field coil is not open. Some of the early 1930’s sets did have a field coil, and it was connected from B+ to ground.  If the field coil was bad I would replace with a permanent magnet speaker.

If you have no hum or other noise you should check the B+ voltage. This voltage should be around 90 to 120 Volts. However, the speaker may be bad plus the filter capacitors may also be bad, and if the filters are bad the B+ voltage will be low. Check the AC voltage at the second filter capacitor (the one connected to the screen of the output audio tube). This AC voltage should be less than five volts AC.  If it is in the order of 20 volts AC or so, then the filter capacitors are bad.   Replacement of the filter capacitors will usually bring the DC voltage back to the normal value. If it does not bring it up then the rectifier tube may be bad even if the filament is good.  Replacing the rectifier tube would then bring the B+ back to about a hundred Volts.  As you may note the B+ and output stage can be difficult.

If there is still no noise from the speaker then check the B+ on the plate of the output tube. It should be about 15 Volts less than the screen Voltage. The screen voltage is normally the B+ Voltage. If there is no plate voltage then the output transformer is usually bad – check the continuity.  The output transformer should be several hundred ohms. You will often find a bad output transformer when there is no audio output.

Turn the volume up full, and touch the grid of the output tube using a capacitor of around .01 microfarads (a safety matter).  Put your finger on one side of the capacitor, and the other end of the capacitor on the grid. This will cause a good functioning audio system to provide a slight hum. If there is still no noise output from the speaker then the output transformer or tube may be bad, but it is possible that the speaker itself is bad.  To check the speaker resistance it is necessary to unsolder one side of the output transformer leads that go to the speaker voice coil. You can then check the resistance of the speaker to see if it is open. It is not unusual for the cone to be frozen such that is cannot move. A frozen cone is usually caused by dirt, and this cone can be freed by removing the dirt – it takes care.  You may check for a frozen cone by slightly pressing on the center of the speaker cone, and it should move freely.  These old cones are fragile so you must be careful when you do this. You may use a 1.5 volt battery across the voice coil of a permanent magnet speaker, and a good speaker will produce a click caused by the battery voltage.  While I never expect a speaker cone to be open – it is possible, but highly unusual.

With the audio amplifier working check the grid voltage of the output tube. This voltage should be zero.  If it is slightly positive then the coupling capacitor to the grid of the output tube from the plate of the preamplifier may be bad. It is also possible that the output amplifier tube is gassy. Either condition will likely cause audio distortion. It is difficult to tell whether the capacitor is bad or the tube is gassy.  If you had an AC transformer set then you could tell by unplugging the audio output tube, and then if the grid voltage is still positive you know that the capacitor is bad.  With an AC/DC set I would go ahead, and replace the capacitor - if you do not have a known good tube available to try. If the grid is still positive the tube is bad.

Now that the output section is working you should take the .01 capacitor, and touch the grid of the preamplifier tube, and the hum should be much louder than it was at the output tube. Remember that the volume control is at full volume.  If the hum is not louder than the plate voltage of the preamp plate voltage may be too low. Check the voltage, and it should be around sixty volts or so. This voltage measurement does depend on the resistance of your VOM. The voltage would be higher when tested with a vacuum tube voltmeter. You could also just check the resistor value with your meter. If you do not have a schematic the value of the resistor is usually around 100k   (100,000 ohms). It is unusual, but the tube could be bad.  With the capacitor, touch the terminal of the volume control that is not connected to ground, and you should get the same hum as you did when touching the grid of the preamp. Remember that the ground could be at 120 volts AC depending on how the AC plug is plugged into the AC outlet.

I am suggesting that you use the .01 capacitor so that you do not accidentally get shocked. If you are aware of the voltages present around where you are working on the radio then this capacitor is not really necessary. I normally use a small screwdriver with my finger touching the screwdriver.
If you are new at radio repair then you should be reminded to be careful that your Volt-Ohm meter not be set to Ohms when you want to measure a voltage.  You only have to burn out one meter, and then you seldom forget again. If you plan to continue radio repair then I suggest that you get a digital meter or a Vacuum Tube Volt meter. They do not burn out when they are set to ohms, but try to measure voltage.

The next step is to test the I.F. section. The I.F. transformer that is connected to the detector plates is called the second I.F.  While most AC/DC sets have a first and second I.F. stage there are a few receivers that have only one I.F. can. Arvin made a number of sets with one I.F. can. There are two tube, and three tube sets, but I would have to look at the schematic before discussing them.  I would expect the two tube radio to be a TRF.

The second I.F. tube is ready to be checked, and this is done by checking the various voltages on the tube. Check the screen, plate, and grid voltages with your VOM.  If the plate voltage is zero then the I.F. transformer is usually bad, but be certain that there is voltage on the B+ side of the I.F. transformer.  Most sets use the B+ for the screen.   If the screen voltage is low or even zero then there may be a resistor from B+ to the screen with a bypass capacitor to radio ground.  This low voltage may be caused by a shorted screen bypass capacitor or by a bad resistor feeding the screen. Check the screen bypass for a short – that is, a few ohms.  It is not unusual to have the screen at the same operating voltage as the plate.  In the Transformer set it is normal to have a resistor with a bypass capacitor in the screen circuit.

The I.F. grid voltage should be checked, and with a set that is not receiving a signal, the voltage should be zero.  With a working set the grid will be negative when receiving a station, but you cannot test for a negative grid voltage with a working receiver because the large capacitance of the  VOM will stop the I.F. amplifier from working.  If the tube is working properly the cathode voltage will be several volts positive. If it is zero I would suspect a bad I.F. tube or one of the tube voltages. Repeat this procedure for the first I. F. tube.

The mixer or converter tube drives the first I.F. and this tube should be tested for proper voltages just as you tested the I.F. tubes. There is a slight difference.   These sets usually have a mixer tube that has an extra grid that is at a voltage similar to the screen voltage. You should have the specifications or the data sheet for this tube, and it will give the normal operating voltages for this tube. The oscillator grid voltage is negative when working properly but, you cannot test for a negative voltage with a VOM since it will stop the oscillator from working.  If the voltages on the mixer tube are proper, and the set appears to be working, but receives no signals when the variable capacitor is operated then it may have a bad mixer tube. This can happen because the oscillator may not oscillate, but the tube   will still amplify noise from the antenna. It is not unusual for these tubes to test good on a tube checker.  With a bad mixer tube the oscillator may work at low frequency’s but stop working at the high frequencies. The mixer tube may not have a cathode resistor. The cathode of the mixer tube is usually at ground – zero volts.

With a Vacuum Tube Volt meter you can test the oscillator grid voltage, which will be negative on the mixer tube grid.  A few sets will not be as described here. There are radios with separate oscillator tubes, and a there are several arrangements for the oscillator circuits. A  schematic would be helpful in order to be sure of the oscillator type. In any case the tube normal operating voltages will be given in the tube data sheet.

With a six tube set the sixth R.F. tube may not have a tuned circuit associated with the tube. This tube is called a broad band amplifier.  Check this R.F. tube for its proper operating voltages.  There are sets with two tuned R.F. stages. This is more desirable than the broad band stage since the tuned circuit makes the set more sensitive.  This means more gain and better selectivity.

While I do not remember a superhet receiver that does not have an automatic gain control I would expect that they do exist.  It is important that the automatic gain control resistor from the detector tube be within its proper resistance value as it can stop the set from working if it is open. Its value should be in the order of one to three megohms. The resistor feeds the grids of the I.F. tubes, and perhaps the R.F. tubes. It is also important that the bypass capacitor on the A.G.C. resistor be good because if it is not the set will not operate properly.  A bad capacitor can cause what is called motor boating or an audio oscillation.

The Transformer set has advantages. With a transformer the AC line voltage is isolated from the chassis, and this allows the chassis to be used as ground. I prefer this since it makes measurement easier. The ground for the AC/DC set is often difficult to get to without causing some sort of undesirable short between two contacts that may be at different voltages, and you may cause harm to the radio while making these measurements. Test instruments have a ground clip that must be attached to the radio ground, and if you happen to touch this to the  AC line while making a measurement it may injure the measuring device. I do not like to use my instruments on the AC/DC sets because of this problem.

The Transformer set is usually a better radio than the AC/DC set. It is designed with improved performance. It has a greater audio output with a larger speaker than the AC/DC radios.  It was generally a more expensive set. However, some of the later model AC/DC radios had excellent sound, sensitivity, and selectivity as good as or better than the older transformer radios.

The Transformer set has a high B+ voltage, often over three hundred volts.  This is desirable because it allows the use of more powerful output tubes. The speakers are larger. These sets may have push-pull outputs for more audio output.  The transformer sets may have a resistor at the center tap of the high voltage secondary of the transformer.  The center tap resistor goes to ground. This then provides a negative voltage at the resistor. This negative voltage is used as a bias for the tubes that require a negative voltage.  The bias is used for the audio output tubes, and often other reasons. Using this method requires the B+ high voltage filter capacitor ground to be returned to the negative center tap resistor. This means that the filter capacitor ground must be insulated from the chassis. This method adds some expense to the cost of manufacturing, and is actually unnecessary. Cathode bias is much simpler. It is straightforward, and less expensive. When replacing the high voltage filter capacitor use care such that you do not accidentally short the negative bias to ground.

There are a number of radio repair persons that may start a radio repair by replacing the old capacitors with new capacitors. This is called “re-capping” the set. I have never done this because it may on occasion cause unexpected problems by accidentally shorting components together.  It is also time consuming and difficult, and is unnecessary work.  The mica capacitors may be ignored since they seldom go bad, but they do go bad. They can short because of moisture, some mica capacitors are silver micas. Silver migrates with time and this may cause problems.  I much prefer to find out what is wrong with the set rather than starting by replacing capacitors that may be good.   I try to avoid those sets that have been recapped. I do not object to replacing certain capacitors on the older sets. It is good practice to replace the filter capacitors plus the coupling capacitor in the audio section of the output tube.  Even the AGC (automatic gain control) by pass could be considered  as a normal replacement.

These old paper capacitors normally will have a resistive value of several megohms because of moisture, but the moisture is driven out by heat when the radio is put back in use.  When recapping it is normal to use Mylar capacitors, but most any capacitor today is better than the older paper capacitors. I do use the Mylar, and think that there is none better. The paper capacitor will usually collect moisture with time.  It could be said, however, that if the paper capacitor has worked for fifty years then it will probably last for another fifty years.

If you have questions concerning your radio please send me an email, and I will try to help.