1. Don't plug it in and turn it on.

2. Do replace paper and electrolytic capacitors and selenium rectifiers.

3. Do check all resistors for value and replace if more than 20% high or low.

4. Do check the power transformer by applying power with all tubes removed from sockets.

Replace Capacitors and Selenium Rectifiers.

Capacitors are supposed to be a totally open circuit for DC but a paper capacitor conducts some DC current. This is referred to as leakage current and a capacitor that conducts is said to be leaky.

When you find these capacitors in radios or other old equipment replace them with modern plastic film capacitors. Don't bother testing, don't turn it on to see if it will work, don't hesitate, don't ask, don't tell, just replace them.

The photo below shows some capacitors that look nice but they are wolves in sheep's clothing. Or paper capacitors in plastic clothing.

The oddball on the right is disguised as a mica capacitor but it is nothing more than alternate stacks of foil and wax paper. True mica capacitors rarely have to be replaced but the paper ones are usually thicker than mica. True mica capacitors are not very thick.

The type of capacitor shown in Figure 9.2b is especially troublesome. In the mid 1960s, 50 years ago, I stocked up on these thinking they were good capacitors. They were at the time but time has been their enemy. They look really good. The ends of the plastic tube are filled with epoxy and they even test good on a digital RLC meter. The only clue that something is wrong is that the value reads out of tolerance high. I just finished going through them and they are 100% too leaky to be used in any circuit. Every last one out of more than 200 capacitors was bad and had to be thrown away.

Never, never, never, solder the diode across the terminals of the rectifier. It is going to short out some day and the results will certainly be catastrophic.

The section below was written by Mike (Mac) McCarty. Mac is a regular contributor to the Fun with Tubes email forum and Is highly knowledgeable in the areas of electricity, electronics, and mathematics. He has so many cogent thoughts on questions that are asked on the forum that I have prevailed on him to become a contributor to this book. I was going to write this section myself but when I remembered that Mac had written something along the same lines I read it over. I can't improve on what he said so why should I try to reinvent the wheel?

Mac is back on the fun with tubes forum after a long absents and his return is most welcome. He has written extensively on the subject of restoring antique electronics to operation and I recommend that you go there and read it over. Some of it is duplicated below but a lot more is material not found on this page.

I'm going to give you the instructions for locating the files before giving the link. Read the directions, then click the link.

The link will take you to the website of the "Vintage Radio and Phonograph Society". Once there click on the "Site Map" button.

Look down the list of links until you find "Technical Information Center". Click on it.

The two links of interest on this page are, "Introduction to Components by Mike McCarty" and "Introduction to Restoration by Mike McCarty". The first contains many photographs of old style components which you may have never seen before. The second is a much expanded and more detailed version of the section below. At the bottom of this page is a schematic diagram and parts list for the dim bulb tester. Now here is the link.

In a TRF all amplifiers up to the detector were tuned to the frequency of the received station. Early designers had not figured out how to make the circuits track. That is if the tuning capacitor were set to the same angle all tuned circuits would be tuned to the same frequency. When tracking was achieved all tuning capacitors could be on the same shaft and tuned by a single knob. The resonance phenomenon was not well understood in 1920 and what seems obvious to us now was not to them then.

In 1920 the top of the line for those who could afford the price was Atwater-Kent. Here are a couple of photographs of one that I had the pleasure of restoring and have the pleasure of owning.

A Case Study.

The Super Heterodyne Radio.

The TRF radio receivers have brought us up to about 1925. The principle of heterodyning had been developed by Edwin Armstrong in the 19 teens and patented by him in 1916. An early version was offered to the public as early as 1919 but it really didn't catch on in the consumer market until the late 1920s.

The Principle of Heterodyning

So What is a Mixer?

The Superhet Receiver.

The reason for this is that selectivity (or bandwidth) is a constant percentage of the center frequency. Suppose the Q of a tuned circuit is 40. At a frequency of 600 kHz the bandwidth is 15 kHz. That's about right for AM reception. On the other hand the bandwidth at 1500 kHz is 37.5 kHz. There is no way around this. We would have to repeal some of the laws of electricity and magnetism to keep this bandwidth change from happening.

If we could find a way to change the frequency of any given station to some predetermined frequency we could build a set of tuned circuits and amplifier stages that would always have the proper bandwidth and we wouldn't have to worry about tracking because the frequency of the circuits will never be changed. Calling up the station and asking them to change frequency doesn't seem to be a practical solution. We need some way to change the frequency after the station's signal enters our radio.

Are you ahead of me? Yes. The mixer is it. The frequency we generate to combine with the incoming station is produced within our own circuitry so it is local. It is called the "local oscillator". Let's say we have constructed a fixed tuned amplifier at a frequency of 455 kHz. (That's the almost universally used frequency for this amplifier. Car radios usually use 262 kHz. I don't know why.) So if we want to listen to a station on 780 kHz we must tune our local oscillator to 780 kHz + 455 kHz = 1235 kHz. The 1235 kHz local oscillator combines with the incoming station at 780 to produce two new frequencies at 455 kHz and 2015 kHz. The tuned circuits select the frequency at 455 kHz and reject all others. The frequency of 455 kHz is in-between the station's frequency and the audio frequencies so it is called the "Intermediate Frequency" or just IF for short.

Image Frequencies

The 1930s are often referred to as the golden age of radio. Most of the people who use this phrase are thinking of radio programming content. However it was also technically the golden age of radio. The difference between a 1930 model and a 1940 model is striking. Probably the most striking improvement to a nontechnical person would be sound quality. But there were also large improvements in reliability, stability, and ease of tuning.

Many consoles incorporated record players and a few even had the newest thing, FM radio. Many had push-pull output stages some boasting output power of 25 watts.

At the other extreme there were still many battery radios which were not intended as portables. These days they are called farm radios because they were intended to be used on farms where there was no electricity at all or if there was electricity it was 6 or 32 volts DC from a gasoline powered generator and a system of storage batteries.

Types of farm radios are as follows.

1. Dry battery only. The source of power was a large cardboard box containing A and B batteries that sat in a compartment in the bottom of the radio. When the batteries ran down the user had to buy a new one. I suspect that when the battery pack went dead that many farms did without radio during the depression.

2. Dry B battery and storage battery for A battery. If a means existed to charge batteries this was a good solution. A single B battery was less expensive than an A-B pack. In some radios the A battery was a 2 volt single cell lead acid battery. In others it was a 6 volt battery of the same type.

3. Vibrator derived B+ from a 6 volt storage battery. Many farms had wind powered 6 volt generators that kept one or more 6 volt lead acid batteries charged. A 6 volt radio could operate well from such a system without the ongoing expense of buying replacement B batteries.

4. 32 volt vibrator radios. Other farms, such as the one I grew up on, had a Delco gas powered 32 volt DC generator system. A bank of five 6 volt batteries were charged daily by the generator. The trick was to manage the power usage so there would be enough juice left in the batteries the next morning to start the generator. In addition to making electricity the generator made enough RFI to make radio listening impossible for all but the strongest station which was WHO in Des Moines.

Troubleshooting.

One component found in two of the types is the vibrator. Because it is an electro-mechanical device it is the least reliable part in the radio. If you are asked to repair one of these sets you may find that the vibrator can't be made to function. There are companies you can find with a Google search that will provide vibrator replacements that use transistors as the switching elements. You really don't have any choice in replacing this part. Nobody is making mechanical vibrators anymore. In addition the owner will likely want you to build a suitable DC power supply to run the radio. This should not be difficult in this age of power transistors and IC voltage regulators. Remember that the original battery gave a wide range of voltages from fully charged to near run down. Any power supply does not have to be regulated within 1 millivolt or even 1 percent.

AC Powered Radios.

Some Common Problems.

If after the radio is working you hear hum in the speaker you may need to add a filter choke in series with the resistor. 5 henrys will usually be enough. The filter choke may not be required if the filter capacitors which were quite small in the original were replaced with larger values. For example it was not uncommon to find filter capacitors of 2 or 4 microfarads particularly in radios of the early 30s. If you replace these with 22 or 47 uf caps the resistor will usually provide sufficient filtering. Leave the can capacitors standing on top of the chassis and install the modern ones under the chassis out of sight. Don't forget to disconnect the old capacitors.

This radio circuit represents a triumph of the bottom line over quality and safety. It was the radio that the Atwater-Kent company went out of business rather then make. Every feature had one purpose, to reduce the cost of manufacture. After World War II there were millions sold under hundreds of brand names from Admiral to Zenith.

Much scorn has been heaped on the little AA5 by myself and others but the truth is it's a wonder of design and simplicity. The word is starting to get around and local antique dealers are bringing me their AA5s to put into operating condition. I have had the opportunity to see a great many different radios. The marvel is how well they work considering their construction. There's no such thing as a wiring harness. Wires go every which way, crossing over, twisted around, power next to audio, audio next to RF, power next to RF. Terminal strips are seldom used or used in great moderation. That sometimes means that wires are just brought together in the middle of the air and soldered together. Yet still they keep on ticken'. The design has been so well refined that it seems impossible to build one that won't work.

It is called the All American Five because all brands were made in America. This was long before the mass exodus of the electronics industry. The five comes from the fact that it used 5 tubes. There were variations using 4 tubes which didn't perform very well and 6 tubes which cost more. The average person who didn't stay up late to see how many distant stations they could hear (known as D X ing) didn't care about the improvements provided by 6 tubes. Consequently the All American Five became a post war standard which did not fade away until replaced by the All Japanese Six, the six transistor radio made in Japan.

The most prominent feature of the AA5 was that it had no power transformer. The result was that the circuit common was connected to one side of the power line. Because there was no polarized plug there was a 50 - 50 chance that the radio's chassis would be connected to the hot side of the power line. Very late models from the sixties were carefully designed not to have any metal parts on the outside even going so far as to recess screws in deep holes. In the 40s and 50s there was no such concern on the part of manufacturers. It was common to see chassis mounting screws exposed on the bottom of the plastic case. These radios could be, and sometimes were, lethal. All it would take was for an unwary person to complete the circuit between a metal part on the radio to a kitchen sink. I once owned one that was in a metal case. My friends and I called these radios "suicide boxes".

Most people who owned and used them were blithely unaware of the danger. I'm sure it must have cost some of them their lives. One side of the power line connects to the chassis. Depending on how it happens to be plugged in the chassis could be "hot" 120 volts above power line ground. Most of them have the switch in the chassis side of the line which means that even if you have it plugged in correctly, when you turn the switch off the chassis will be electrically hot. With these units there is no right way to plug them in.

A Change For Safety.

It should be noted that not all radios used the metal chassis as the circuit common. In some radios all connections that went to common connected together with wire usually at a terminal lug. From this point there was a resistor and capacitor in parallel to the chassis. Commonly used values for these were 220 k ohms and 0.001 uf. When modifying such a radio the 0.001 uf capacitor should be replaced with one that is X rated. When the instructions below refer to the chassis the common point is meant.

I strongly urge modification of the radio circuit to make it somewhat safer. Here's the diagram.

Separate the two wires for a couple of inches, and strip about 1/4 inch of insulation off the ends of the wires. Twist the fine strands of wire tightly together and melt a small amount of solder over them to hold the strands together. This is known as tinning the wire.

Now remove the wire connected from one side of the switch to the chassis. Strip and tin the piece of wire connected to the rectifier tube socket. Connect this wire to the terminal of the switch you just removed the chassis grounding wire from. Solder the wire in place.

Remove the piece of old line cord from the other terminal of the switch. Solder the wire coming from the NARROW prong of the new line cord to that terminal of the switch.

Solder the wire from the WIDE prong of the new line cord to the chassis connection where the switch used to be grounded.

You may find after making this modification that the hum level has increased. The phrase "lead dress" was probably never spoken in an AA5 factory so the wires go every which way. The reason for putting the switch in the neutral lead was to minimize hum transfer from the off on switch to the volume control pot. Now that the switch is in the "hot" side of the line there may be more hum than desirable in the speaker. When installing the polarized cord make sure the hot leads run along the chassis and as far away as possible from the three terminals on the side of the control. It may be best to route the wires straight down to the chassis perpendicular to the shaft. Route the leads that carry audio along the chassis and away from the power line leads. You may have to solder new pieces of hookup wire in these leads to make this possible. Experience has shown that careful routing of the leads will reduce the hum to acceptable levels.

The switch modification will make the radio much safer to use and work on but the safest way to work on these sets is to obtain and use an isolation transformer. The minimum you need is a 30 watt transformer. A higher wattage unit would give you more flexibility. It may cost you some money but how much is your life worth. If you think you are immortal or just want to live dangerously there isn't much I can say to you. If you decide to work without an isolation transformer get one of those test lights or a small night light and permanently connect one side of it to the power ground and terminate the other in a test wire. Touch the wire to the radio chassis. If the light lights up reverse the plug. Test it with the switch off. If there is no right way, plug the radio into a plug strip with a switch and use that to turn the radio on and off. Temporarily solder a jumper across the radio's off on switch to prevent yourself from absent mindedly turning it off.

Jim sent me an email in which he makes the following recommendations.

"These are excellent suggestions. However, I would suggest taking these steps further, by equipping the radio with a ground fault interrupter (GFI) .

In my research, I find that many hair dryers are equipped with cords that have plugs containing the GFCI protective device. These look like wall warts on the end of the cord. I found used hair dryers in second hand stores selling for under $5 each. So, the plan is to purchase a used hair dryer with a good cord and GFCI plug, remove the cord and use it to power an AA5 radio, in one of two ways:

The simplest way to power the radio is to make the hair dryer cord into a GFCI extension cord. Purchase, from a hardware store, an outlet that installs on the end of a cord. Then, properly wire and attach the outlet to the hair dryer cord and you have a GFCI extension cord. Plug the radio into your GFCI extension cord and plug the GFCI extension cord into AC power. Secure the radio cord into the extension cord outlet, since you want the extension cord permanently connected.

The other way is to use the hair dryer cord to replace the cord that came with the radio. If you choose this method, note that the hair dryer cord is thicker than most AA5 radio cords. The entry hole into the chassis may need enlargement and the strain relief may require replacement to fit the larger cord.

Look for hair dryers with plugs that have two buttons labeled TEST and RESET. If an outlet is available in the second hand store, test for proper operation of the GFCI before buying the hair dryer.

Hair dryers have been manufactured with GFCI plugs for over 10 years now, but not all models have GFCI plugs. Therefore, check very carefully to be sure you purchase a hair dryer with a GFCI plug. GFCI plugs may also be found on hair curlers and other bathroom and kitchen electrics, so check those as well if you can't find a suitable hair dryer.

Jim also sent along this anecdote.

In my own case I repaired an AA5 radio which used a capacitor between the metal chassis and one side of the line cord. I found about 120 volts DC between the radio internal ground and chassis. I traced the problem to continuity between the audio output transformer's primary winding and its frame. The radio played well despite this problem. I remounted the transformer with insulated fasteners and spacers. GFI would have protected humans from electrical shock if I hadn't fixed the problem, or if the problem developed later.

Jim"

Testing tubes.

The cold tube into a hot radio goes like this. You have a complete set of AA5 tubes you know to be good and decide to substitute them in to a defective radio one at a time. You turn off the radio, remove one tube, replace it with one of the same type number and turn the radio back on. The cold heater won't take its share of the voltage for a few seconds. The other tubes will proportionally divide up the over voltage until the cold one warms up. If the one you plugged in cold is one of the 12 volt tubes the chances of doing serious damage are small but still exist. If the cold one is the 35 or 50, the other tubes are going to get quite a large over voltage shot and at least one of them may not survive.

The Dial Light.

If the light burns out the voltage across that part of the heater of the rectifier tube will go up to about 12 volts. That portion of the heater will over heat and will burn out within a few hours of operation. Since dial light bulbs are much cheaper than rectifier tubes, don't operate a radio with a burned out dial light. If you have a large collection of AA5s you should keep some spares on hand.

Troubleshooting Hints.

If you have a radio which lights up but has no sound try this. Touch the tip of a screwdriver to the center terminal on the volume control while touching the metal part of the screwdriver with one finger. There are two terminals on the back of the control, these are the on-off switch. You don't want to touch them! The terminals of the potentiometer are in a group of 3 and are on the side of the control. Turn the volume to middle or higher. Touch the screwdriver to the terminal and touch the metal shaft of the screwdriver with a finger. You may or may not hear a loud hum or buzz. If you do the audio section is working and the trouble is in the converter, IF or detector. If you don't hear anything the trouble is in one of the two audio tubes. Not necessarily the tubes themselves. As you learned earlier this is a form of half splitting.

Use a DC voltmeter to measure the voltages on plates and screen grids. A leaky capacitor can put a positive voltage on the control grid of the 50L6/50C5, but you already replaced that one didn't you?

If the audio section is working, try scratching on the grid terminal of the IF tube with the screwdriver. If you hear static the IF amplifier is OK and the trouble is most likely in the converter.

Check to see if the oscillator is running. The oscillator grid of the converter tube should have a negative voltage on it. A VOM won't work here. Use one of those cheap DMMs. Wind one lead of a 1 meg ohm resistor around the measurement probe. Touch the other lead to the oscillator grid terminal and read the voltage. The purpose of the resistor is to keep the input capacitance of the DMM from stopping the oscillator. You should read anything from about -2 volts to -50 volts. The reading will be about 10 % low but you're not going after precision, you just want to know if there is a negative voltage indicating that the oscillator is running.

If you are using an authentic VTVM you don't need to use the resistor. The DC probe on most VTVMs has a 1 meg ohm resistor built in and the meter calibration takes this into account.

Some AA5s might fool you because of a peculiar money saving modification. Look ahead to figure 9.11. Note the connections to grid 1 of the 1R5. You will note that there is no capacitor in the grid line and a coil that someone forgot to connect. There is virtually no inductive coupling between the tuned winding of the oscillator coil and the grid winding. The coupling is capacitive and replaces the typical 220 pf capacitor which is usually found here. I have worked on such an AA5 and I can say that it does work. It strikes me as going to great lengths to save a few cents on one capacitor per unit. It must have been economically feasible or it wouldn't have been done.

One more thing you are likely to encounter is a noisy volume control. Most pots have a small slit opening behind the terminals. Turn the radio so the terminals are up and use a medicine dropper to drop control cleaner at the slit. Rotate the control vigorously to work the cleaner into all of the rotating contacts.

Variations on a Familiar Theme.

In some radios, particularly AM/FM there wasn't enough heater voltage for the rectifier so the 35Z5/35W4 would be replaced by a selenium rectifier. If you get one of these go ahead and replace it with a silicon. The extra voltage won't hurt anything in this case.

In the AA4, 35 + 50, + 2 X 12 doesn't add up to the line voltage. A tube manual I have from 1946 lists 45Z3 and 45Z5 which could have been substituted for the 35Z5 in an AA4. I once owned one of these radios and it didn't use one of these 45 volt tubes. I assume that most other AA4s used a fixed resistor to drop the extra voltage as mine did.

AA6s on the other hand needed a lower voltage tube in one of the sockets. The 35L6 and 35C5 were made for this purpose. I have seen AA6s which had a 50L6 in the output tube socket while the schematic listed a 35L6. Either someone didn't know or didn't have a 35L6 in stock and decided to send the radio home with a 50L6 to get the radio working rather than wait to order the correct tube.

Millions of AA5s were made and thousands have been restored. There are still tens of thousands out there waiting to be restored. Have fun.

I remember that the AGC action was about the same as an AA5 but not as good as an AA6 wherein the AGC is applied to RF, converter, and IF stages. In keeping with small voltages approximately 25% of the derived AGC voltage is applied to the control grid of the RF stage. I assume that the tapped secondary of the RF plate coil and the resistor in series with grid 3 of the 1R5 is to improve performance under extremely strong signal conditions. I don't remember it ever being blocked even when riding in a car with it and driving past a radio station's tower.

The oscillator needs a special note as this method of coupling to grid 1 of the converter tube is used in many AA5s as well. What appears to be a link winding which has one end not connected is just that although there is probably very little in the way of magnetic coupling going on. Capacitance from the tuned section of the oscillator coil to the link replaces the mica capacitor which is usually found here. This strikes me as going to great lengths to eliminate one capacitor but the economics must have worked or it wouldn't have been done this way.

Note that local audio ground is established from the negative side, pin 1, of the filament of the 1U5. You may find the filament string a bit tough to tease out so here is a simplified diagram of the chain.

The two 1 k ohm resistors shown in this diagram are actually the 2 k ohm tapped resistor shown in the complete diagram above. This resistor became open while I owned the radio and before I had acquired sufficient skill to replace it myself. I suppose that just one of the sections opened up. The service shop where I took it made no attempt to obtain the original Motorola part. Instead they hung a 10 watt resistor by stiff wires coming through a hole drilled in the chassis. If I still had the radio I would do something about this.

Troubleshooting.

Remember that the selenium rectifier should be replaced with a silicon diode. Do not give in to temptation to leave it in place. When a selenium rectifier fails it gets hot and some of the selenium is oxidized. Selenium is in the same column of the periodic table as sulfur and shares some of its characteristics. Of interest here is its oxide. Sulfur dioxide and selenium dioxide are both rather poisonous. Also they both smell like rotten eggs. If you are working on a radio that still has a selenium rectifier in place and you smell rotten eggs, unplug the radio and open the windows. A selenium rectifier is a poison bomb waiting to go off. Physically remove the rectifier from the radio because some future owner might try to restore it to authenticity by reconnecting it. Use a shorter machine screw than was used to mount the rectifier and mount a terminal strip. Connect the wires that originally went to the rectifier and solder a silicon diode in place of the rectifier. The plus marked side of the selenium corresponds to the banded, cathode, end of the diode.

The value of the 150 ohm resistor will most likely have to be increased to keep the voltage at pin 7 of the 3V4 from exceeding 9 volts. Actually 9 or even 9.5 volts at this point wouldn't hurt any. If you are uneasy with the differing voltages along the filament string you can always do a little re-engineering by adding equalizing resistors.

If the 20 uf capacitor shorts it would most likely take out the top half of the 2 k resistor especially if the owner unwisely left the radio turned on even though it was silent.

If the 80 uf capacitor shorts out it will also silence the radio but it is unlikely that any additional damage will be done even to the 3V4. The 2 k resistor from the 105 volt supply is almost a constant current source and the 3V4 will not sustain any damage.

Elsewhere the circuitry is not unlike an AA6 so if the filaments have proper voltage but the radio remains silent all you need to do is round up the usual suspects.

If you or the owner wants battery capability you will have to construct a battery. If you are extremely lucky you will be able to find an AB pack that fits the radio. You might even find one still in it. It will be run down long ago but you can use the box and the connector socket. If not you will have to devise a method of connecting the battery that is easy to connect and disconnect.

The 90 volt B battery is easy. Just ten 9 volt transistor batteries will do perfectly. I have heard of people making up the B battery from AA cells but I view this as overkill. The current drawn from the B battery is less than that drawn by a 6 transistor radio so the 9 volt batteries should last a long time. In the original AB pack the A battery was made up of F cells which was probably overkill. After the B battery was run down I could get several months of experimentation out of the A battery. Modern D cells or even C cells should do the job nicely. It may be that the B battery was intended to run down so the user would stop using it before the A battery lost sufficient voltage to damage the filament's coating from being operated below design temperature.

One more thing you are likely to encounter is a noisy volume control. Most pots have a small slit opening behind the terminals. Turn the radio so the terminals are up and use a medicine dropper to drop control cleaner at the slit. Rotate the control vigorously to work the cleaner into all of the rotating contacts. If the radio has short wave bands the band switch will also need cleaning. I have seen many radios in which the switch was so dirty that the radio would not work. If the audio section is functioning but the radio is silent clean the band switch first.

Beginning after world war II the crystal phonograph became very popular. I owned one myself. These players used a Rochelle salt crystal cartridge which delivered about 6 volts into a 500 k ohm load. This high output meant that only one amplifier tube was required. The diagram is shown below.

If you find one of these phonographs your chances of ever getting it to operate in its original state are absolutely zero. The reason is the crystal pickup. Rochelle salt is a very hydroscopic material. That means it loves water and will grab onto any stray water molecules that come by. My original phonograph was bought and used for about 2 years in Iowa. Then we moved to Florida. In that humid climate it only lasted about 6 months. After having the cartridge replaced for a cost of 8 dollars it went out after another 6 months. 8 dollars was a considerable sum of money in 1952 and my mother said I would just have to do without rather than spend another 8 bucks on another crystal. The symptom is the sound grows progressively weaker and weaker until the volume control is up all the way and from that point it fades out to inaudibility. No doubt one of these cartridges would last for several years in Arizona but humid climates are death to them.

If you must get one of these phonographs restored to operation the only way I can see to do it is to install a ceramic or even a magnetic cartridge and install a transistor preamp to bring up the level to that required. It could easily be powered from the DC level at the cathode of the power tube and hidden in an electrolytic capacitor can.

A much better phonograph was the one with two stages of amplification. If you take away the 12SA7/12BE6 and the 12SK7/12BA6 from an AA5 and replace the heaters of the missing tubes with a 150 ohm 5 watt resistor you have the circuit of a ceramic phonograph. These were a little more expensive but if you lived in a humid climate the difference would be made up in a year. The reason that more gain was needed was that the ceramic cartridge output was less than 1 volt.

Other than the cartridge the filter capacitor was the part most likely to fail. In an old player this is one of the parts that should be replaced as a matter of routine.

The test for either a one stage or two stage phonograph amplifier is to lift up the arm with the unit turned on and the volume up all the way. Touch the cartridge pins one at a time while not touching anything else. If you hear a loud hum when touching one the amplifier is alright and the cartridge is dead. Even ceramic cartridges can go dead after 50 years. Although I do have a Silvertone wire recorder and phonograph that has a working ceramic cartridge in it. The sound is a little weak but it is still usable enough to record from a record to wire.

Record Changers.

The odds of either one of us encountering this exact model are small although I must say the movement was quite robust and as long as I kept things oiled it worked fine.

The three types you are most likely to encounter are the ones made by BIC, yes, that's the same as the pen company, VM (Voice of Music), and the RCA 45. The BIC name is not on every changer built by BIC. They made drop in changers for console units made by many different manufacturers. Most manufacturers put their own name on the changer. VM also made changers for consoles but you are apt to see the VM name plate on these changers. I suspect that VM wrote a clause in their contracts that required that the VM name remain on the changer.

The BIC and VM changers worked much the same. There are gear teeth cut into the bottom end of the turntable bushing. The change drive is a gear about 5 inches in diameter with a gap at one point. During play the gap is positioned around the gear on the turntable. When the cycle is triggered a small projection is moved forward which catches the teeth and turns the change drive gear just enough to engage its teeth in the turntable's gear. There are various grooves and projections on the change gear that move the arm, activate record size detectors and initiate record drop. The change cycle takes place over about 4 revolutions of the turntable.

The RCA 45 was another of those innovations that comes along about once in a generation. It and the records it played had been designed for each other. The entire change cycle took place in a single revolution of the turntable.

Troubleshooting.

Another cause of stalling is a frozen up bearing in the changer mechanism. The lighter hydrocarbons in the oil have evaporated leaving behind a substance that is more like tar than anything else. The bearings most likely turn with great difficulty if at all. The right way to treat this condition is to completely disassemble the mechanism and soak all parts in a degreaser, then relubricate with sewing machine oil and reassemble. I must admit that I have never felt up to such a time consuming procedure with its high risk of losing parts or forgetting how they go back together.

One way is to simply flood the bearings with new oil and wipe up the excess. The idea is to flush out the heavy hydrocarbons and replace what's in the bearings with good oil. In particularly stubborn cases WD-40 can be used to flush out the tar and then oil the moving parts. DO NOT LEAVE WD-40 IN THE BEARINGS. It is NOT a lubricant. You MUST replace the WD-40 residue with real oil or they will freeze up tighter than they were before.

You should use a very light oil on the parts of a record changer. The old hardware store standby 3 in 1 will do but it will tar up again in a few years. Sewing machine oil is a much better choice and you don't have to look very hard to find it.

Troubleshooting a record changer can be difficult because it is hard to see what's going on under there. Here is a way to get the inner workings into the open and still allow it to be operated.

Once the bearings are properly lubricated and running freely the changer will work most of the time. If it doesn't there may be a missing spring, misadjusted or misaligned part, or a worn part.

Ace hardware does stock a few helical springs and if you need one of those you may be able to find it. If a flat spring is called for it may be harder to find. Flat springs are found in old windup clocks. Here is a case where you may need to sacrifice one antique to restore another one.

Misalignment is sometimes the result of a missing screw. They do tend to vibrate out and get lost. Finding a new screw shouldn't be hard even if it is metric. Changer parts often pivot on nylon inserts. Do not oil these. If a previous owner did the insert has likely disintegrated and needs to be replaced. Such parts may be very hard or impossible to find. If you have some skills with files and other small hand tools you may be able to fabricate one out of aluminum or hardwood. If made of wood the friction surface should be heavily waxed, not oiled.

A previous owner may have bent a part for some reason known only to that person. Bending it back is probably the only solution but be cautious. Metal fatigue can set in very quickly particularly if the part is made of aluminum. If it breaks you are probably back to fabricating a new part.

If a part is just worn out the only option is to make a new one as indicated above.

Sometimes a changer will be just so worn out that it can't be restored. One option is to disable the change mechanism by removing the change cycle initiation part or parts and using it as a single play turntable. Another option is to say "it can't be fixed". If you are working on it for someone else and they insist that they want it to work you may have to say "take it to someone else".

These renamed consoles usually featured a 12 inch electro magnetic speaker, push-pull outputs and no negative feedback. The power tubes were most likely to be 6V6s although some had 6F6s or 6K6s. A few did have 6L6s. These were monophonic. The output transformer was usually mounted on the speaker. Although stereo was introduced in late 1957 it did not really catch on with the general public until after 1960.

Phase Inverters.

Sears introduced the first wire recorder in 1948 under its Silvertone brand. The selling point was that the recording media could be used over and over until performers and engineers got everything right. There were other makes of wire recorders most notably Webcor which made machines intended for office dictation. The recording wire was very fine and there was more than a mile of it on a spool. The wire speed was 22.5 inches per second which meant if something went wrong the wire would be in a hopeless tangle before the user could get to the stop control. Such a tangle could not be recovered and the tangled section had to be cut out and thrown away with the loss of anything recorded on it.

The wire recorder had a fairly short run and was replaced by the tape recorder in the early 50s. Tape had the advantage of running more slowly, 3 and 3/4 or 7 and 1/2 inches per second so a reel did not need to contain nearly as much of it. The tape was 1/4 inch wide which made tangling caused by a malfunction of the machine much less likely. It would take a kid or a cat playing with a reel to get the tape in a hopeless tangle.

Magnetic recording.

What you will get will be the biggest mess of distortion you have ever heard. Language will be incomprehensible, Musical instruments unrecognizable, and you won't want to listen to it for very long. The problem is the magnetization curve of almost any metal. Below is the graph showing amount of magnetization versus the magnetizing force.

If magnetizing force is altered from zero and then brought back to zero the remaining magnetism will depend on whether the magnetizing force was positive or negative. If the force goes up the red line to the tip of the curve in the first quadrant and then is reduced to zero the remaining magnetism will follow the right hand side of the curve back down to zero force but there will be some magnetism left. This is desirable but the problem is the nonlinearity of the curve.

If in each case we start with a piece of fully demagnetized material and take the magnetizing force up from zero to a specific value and then back to zero the remaining magnetism can be plotted on a graph. What results is the graph below.

If a direct current was superimposed on the audio signal to be recorded and the value of the DC places the magnetizing force at the center of the linear region as indicated by the dashed line recording can take place with a tolerable amount of distortion. I once owned a recorder that used a permanent magnet to erase the tape and DC bias for recording. It wasn't suitable for music but I used it for several years to exchange voice letters with members of my family.

What got magnetic recording off the ground was the use of AC bias. A frequency of approximately 10 times the highest frequency to be reproduced is added to the audio. Note, the audio does not alter the amplitude of the bias signal as in amplitude modulation. It is simple addition. That electro magnet I mentioned earlier is called the Record/Playback Head. A much oversimplified diagram of it is shown schematically in Figure 9.18.

If you are no stranger to tape recorders you have probably read that recording takes place at the trailing edge of the gap. I have, but it was never explained. Here is that long overdue explanation. As a single domain passes from the center to the edge of the gap the bias field is decreasing as for the erase head. But the field does not reduce to zero. The value it loops to is the value of the recorded signal at that moment. After the domain passes the gap it is left magnetized to the exact value of magnetism to correctly represent the recorded wave. When the tape is passed over the playback head the wave comes out accurately reproduced instead of being distorted.

What keeps the recorded signal from going to zero at the trailing edge of the gap is the frequency relative to the width of the gap. The bias frequency is much too high to be recorded because the head gap would have to be much narrower to make that possible. The bias signal makes recording linear without being recorded itself.

Troubleshooting.

Other than dirty heads the most common problem with a tape recorder is that it will not erase a recorded tape and when a new or bulk erased tape is tried the recorded sound is weak and badly distorted. Are you ahead of me on this one? The AC bias signal is generated by an oscillator and it is not running.

Many people record infrequently so the record/playback switch contacts have become corroded. Most times cleaning the contacts of the switch will restore the recorder to operation. This is also true for a recorder that has not been used in any way for several years or decades.

Because a tape recorder is a mechanical device it can have similar troubles to a record changer. Many have idler wheels that no longer grip. Cleaning of rubber surfaces and tightening springs will usually set things right.

Old recorders with rubber belts are another story. Some compounds of rubber turn to gue after 20 years. Those that don't, stretch and become too loose to transfer power. In many cases such belts can be replaced with O rings. They are available from auto and industrial supply houses.

The Silvertone Wire Recorder.

I have no solution for this in spite of giving it considerable thought. If anyone out there has found a successful fix I hope you will share it with me so I can share it with others.

I will tell you how I fixed mine for all the good it will do you. I knew that Pentron made tape recorders for sears and when I was buying Silvertone wire recorders I saw a Pentron and bought it on the off chance that that might have been true for wire recorders as well. The electronics were quite different but the transport turned out to be identical. Pentron had had the good sense to use time tested and proven rubber rimmed steel idler wheels. The paint job on the Pentron was different from the Silvertone I wanted to restore so I transferred the idler and drive wheels from the Pentron to the Silvertone. So I have a working wire recorder.

You might be fortunate enough to find phonograph idler wheels that will fit. If that fails I can't give you much hope. I have another rare portable Silvertone I would like to restore but so far I have not found any wheels to replace the plastic ones that were original equipment.