'Bolters helping 'Bolters is a beautiful thing!
This Tech Tip was originally put together after we tried having a Radio Clinic in our forums. It was such a hit, we decided to keep the Radio Bench as a regular forum. The original tip was written in February 2006.
Here are some more Tech Tips regarding old radios:
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!
No worries ... come on in
Here are some repair hints to help you repair your radio in your old truck. Hopefully, this series of "Frequently Asked Questions" (with the answers of course!) 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 in The Radio Bench forum. Come on by ... even if you want to just lurk!
Q. What kind of setup is needed to check out [and maybe repair] an old tube type truck radio?
Depending on how far you intend to pursue problems yourself:
There are voltage dropping resistors that will do the job. For a six volt radio, use a 1 ohm, 50 watt resistor. Here is a typical listing of one from a mail order vendor: Another solution is a linear three terminal voltage reducer with a proper heat sink attached, which is available from some suppliers specifically for higher amperage applications like radios and heaters. The radio can also be converted to 12V by changing the tubes, vibrator and transformer, which can be done by some repair services if you're not up on electronics enough to attempt it yourself.
Yes, if you can find a DC to DC converter to step the 6V up to 12, but it might be preferable to convert the truck electrical system to 12V.
If your radio just doesn't work, there are three primary things that may cause it to not work: the vibrator, the capacitors, and the tubes - most frequently in that order.
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:
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, requiring 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.
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.
Q. If a tube tester registers a good test for a tube, does that mean the tube will work correctly?
The short answer is no. If a tube tests poorly on a tube tester, then it should be discarded. If a tube tests as good, it means that the heater is working and that it passes a test under specific conditions. However, these conditions may not be the same as the ones in your radio. The very best test for a tube is how well it performs in an otherwise healthy circuit. Sometimes it is handy to maintain a "test radio" just to check tubes, vibrators and speakers.
For the radios of the late 1940s and the 1950s:
The push buttons 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 it is slow, and delivers a fair amount of force. However, the push button 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.
At the Stovebolt, we encourage our members to get involved in their old truck restoration to the extent that they are comfortable. In the radio forum, we try our best to assist folks repairing / restoring their radios. A number of individuals have successfully used our help to do this.
An old Stovebolt - The original American Idol