Thursday, February 10, 2011
Easy Fix on the 1966 Ford Truck Radio
The '66 Ford Truck is keeping me busy! There is plenty of work to do but I have been having fun playing around with the radio I picked up on Ebay. As soon as I received it, I promptly yanked the cassette radio that the PO had installed and chucked it.
The first thing I did was disassemble the radio to expose the circuit board and check for any obvious signs of failure: burn marks, corrosion, etc. I was relieved to find that none of those were present or at least not obviously so.
Despite the internet's propensity for disseminating vast amounts of knowledge, I was unable to source schematics, but I did find some minor information on my radio.
Turns out that decoding the A4TBT stamp on my radio's box lets me know that it is a transistorized AM truck radio built by Bendix (or using Bendix guts) in 1964. Nice to know but hardly of much help to me.
I did some minor repairs so that I could get a speaker attached to it since it appeared that the originals had been yanked out and proceed to power the radio on. There was a very audible hum pumped to the speaker as soon as the switch turned on despite the volume level.
Here is where I noticed I had made a mistake: these radios are powered from a 12V battery, a DC source. While the power supply I had hooked up for testing was at the correct voltage, it was a switched power supply! These sort of power supplies are prevalent in today's electronics because they are more efficient but they are completely unsuitable for testing an old car radio. Instead of pulling the battery out of the truck to use for testing, I bought this power supply which is regulated, adjustable, and lends itself quite well for a future of electronics tinkering at my bench.
Reading up on old radio repair, it seems that the culprits in most hum situations are paper and electrolytic capacitors. Over time they dry up causing their values to stray. Inspecting the radio turned up only two capacitors that fit the bill so I made a really crappy picture in google documents and then removed them.
my local electronics store, picked up replacement electrolytic capacitors, and soldered them into place. The next power test (now with new capacitors and an appropriate supply) played fuzz but no hum! I was able to improvise an antenna out of alligator clips and a long strand of hookup wire to make a loop antenna and it picked up local stations!
Now that it is functional, it is nigh time to hack this thing!