The Stovebolt Page   Tech Tips	Tube-type Radio Repairs
From Fred "Truckernix" Nixon and the Radio Clinic folks

Life after death?

It just sits there in the dash, on a shelf or in a box -- like some pre-historic artifact dug up from the Ulduvai Gorge (and about as lifelike). Make it work again? What are you nuts??? That confusing box of wires, tubes, dust balls, mouse turds and things too fierce to contemplate?  Actually, once you take a few quiet moments to ruminate on the facts and theories of how these old tube type radios work, you may decide you can bring the dead to life and make that mute thing in your dash sing some old tunes again!

Here are some repair hints to help you repair your radio. Hopefully, this radio FAQ will lift the shroud of mystery surrounding tube radio repair and you will feel more confident in your efforts at repair. We can offer more help and advice in diagnosing and repairing faults in these old tube radios at the Radio Clinic forum.

Q.) What kind of setup is needed to check out [and maybe repair] an old tube type truck radio?

1. A power source capable of supplying 6 or 12V, and the necessary current level for the radio - usually 3-6 amps. If a battery is used, it should have a fuse of 10-15 amps max, but a trickle charger is preferable because of it's built in load limit.
2. A simple Volt-Ohm-Amp meter.
3. The usual small screwdrivers, wire cutters, and long nose pliers are necessary, and a small soldering iron if you plan on doing any component replacement.
4. A smooth, well lit work bench will aid in keeping track of any small bits that jump out unexpectedly.
5. To actually test run the radio, you'll need a simple speaker [usually 4 to 8 ohms], and some type of antenna - a length of insulated wire will do, with just enough bare to make contact in the proper socket.
6. A circuit diagram, or schematic, will be useful for trouble shooting, and some may be found at the links below.
7. You also need to remember RULE ONE for working with electronic equipment, which can generate voltages in the 250-350V range: Always Keep One Hand In Your Pocket, or at least NEVER put both bare hands into the powered up radio at once - THE VOLTAGE CONDUCTING CIRCUIT FROM ONE HAND TO THE OTHER LEADS STRAIGHT THROUGH YOUR HEART!

Q.) Where can I find some information on original Advance Design series radios?

Q.) I took off the cover! What to do? What to do? It looks like the far side of the moon in there!

The tubular wax coated paper capacitors are "out-dated" technology, and they deteriorate with age. It's best if you substitute modern film capacitors for all of them right at the start. Modern replacements can be found from a number of sources, including just radios.com which is a good reference regarding capacitor types. There will also be one to three electrolytic capacitors, sometimes together in a "can" about the size of a vacuum tube, which should also be replaced with physically smaller modern equivalents.

Vibrators are a step in converting the truck's DC voltage to the AC that the radio power transformer uses. If the vibrator is quietly humming, it may be working correctly. If it makes no sound it will more likely be bad, meaning no AC voltage to the power supply and therefore no DC high voltage to run the radio, so the radio can't function! The old mechanical vibrators do fail with age, and replacements now are solid state, pretty much a 'lifetime' component.

In the initial testing, if the radio is powered up with the vibrator installed, there will be one of three outcomes:

1. The radio will work with no problems.
2. The radio will show some life but not work properly.
3. The radio will exhibit the symptoms of a short and will: trip the power supply, or blow a fuse (if used), or one or more components will start to burn. In this case you learn nothing except the radio's condition just got worse - you've let the smoke out!

The best way to start testing is to remove the vibrator and power the radio up. There is almost no chance of a component burning under this setup because there is no high voltage DC supply to run the radio. This will allow you to check if all the tubes that are supposed to warm up, do so. Tubes that are working properly will be warm or hot to the touch. In most cases, tubes with transparent glass will allow you to see the red glow of the heater.

An explanation: With the exception of a very few tubes like the 0Z4, they generally have a "heater" inside which heats the central electrode (the cathode) so it will emit electrons from its surface. Without this heat, the tube is dead. When the heater goes in a tube, it is no longer functional. Also note that the tubes in a 6 volt radio, which require 6 volts to run the heater, will have tubes numbered 6xxx, and the 12 volt radio, requireing 12 volts, will be numbered 12xxx.

Also you can check that the off/on switch works.

After checking that all the tubes light up, you can now install the vibrator, power it up and watch very carefully for any signs of distress. A typical tube radio may take several minutes to warm up before operating correctly.

Q.) What is the best approach to follow in restoring a radio?
A.) The best sequence is as follows:

First, open up the radio and take photos of the inside and outside. These become a very handy reference. If a brittle wire breaks off, this step can save you a lot of time in searching the circuit diagram and the chassis to figure out where it was attached! Clean out any obvious dirt that you can reach. I usually like to use methyl hydrate ("wood alcohol") and Q-Tips, to get rid of the dirt.

Do the initial testing without the vibrator. Proceed to replace all the capacitors, one by one. As you are doing this, you will have better access to the underlying chassis as each capacitor is removed. This will allow you to clean out more dirt. As this proceeds, you should try and check every resistor that you come across by measuring it's value and checking against the schematic value. If the reading is 20% or more higher than the listed value, it is defective. If it is lower, then there is something else in parallel that is preventing a good measurement. You should recheck it after all the capacitors are replaced. After the capacitors are done, replace any defective resistors. Also note that under the small metal plate covering the area under the power transformer, there is an important capacitor called the "buffer" capacitor. Don't neglect to replace this.

After these repairs, it's time to test the radio with the vibrator installed.

The pushbuttons on your radio are used to "instantly" move the tuning apparatus to the specific area on the dial that each is set to. The tuning control or knob, allows you to tune up or down the dial continuously. This knob works through a worm gear drive, so is slow, and delivers a fair amount of force. However, the pushbutton could never move the dial against the opposition of a worm gear drive. Therefore there is an automatic clutch, which disengages the worm drive when a button is pushed. If the operation of this clutch is compromised, it will slip and the tuning control will not move the dial. The most common cause of failure with the clutch is a dried out and warped rubber friction element.

v. February 2006