Restoring the Philco 76 Radio
Well, I have finally got around to finishing my 1929 Philco 76 AM radio. This radio is an AC set by Philco, it uses TRF circuitry and tunes the AM band.
When I first got the radio, it more or less worked, but suffered from problems with the power supply, as many radios of this vintage (or newer) do. Specifically, it fell to me to replace the power supply filter capacitors, which I did shortly after I got the set (some years ago, now). After finishing that, I found that the field coil on the electro-dynamic speaker was open and therefore unusable. I cobbled together a way to replace the function of the field coil for the radio’s circuits, but I kept my eyes open for a correct speaker replacement. I found a new speaker (new to me, anyways), and installed it. Having finished that, I found the radio would play well for several minutes, but gradually the audio would fall away in volume.
So, I took it upon myself to begin re-capping the entire set at that point. With all new audio-stage capacitors, and the RF coupling caps replaced, I felt I’d have some success. When I got it all back together after the recapping, and plugged it in, I found that I had very low volume even though the volume control was maxed. In fact, in spite of the low volume, the audio that was being provided was distorted, as though the amplifier was working its hardest to give me even that small amount of sound.
Thus my suspicion fell to the interstage audio transformer…which had already been replaced at some point in the long distant past. Since the available schematics do not provide information on the impedance of the original transformer, and the unit I had was bad, it was difficult to find a suitable replacement. Some research online, however, helped me. Another person with, I think, a Philco 87 had used a Hammond 124a transformer on his set. Having looked up the specs for that particular transformer, I was delighted to find that one of my spare replacement transformers for the Atwater would probably do the trick -- at least they had very similar DC resistances. So, I quickly installed it to test it -- and it worked! This is probably a testimony to the lack of electrical diversity in 1929 radios, haha!
I was pleased, and quickly started buttoning everything up. For the first time in years, the radio chassis is actually bolted into the cabinet (never saw the point while it was mid restoration). Reception is good, audio quality is suitable (it sounds better than the Atwater, which has an odd flutter I can never seem to trace), so I have no complaints. The only other task I may undertake is to replace the power cord -- it’s in generally good shape, but brittle. For now I have the set connected to a power strip that I shut off when not in use…but will be ordering some cloth-covered cord stock when I get a chance.
Enjoy this video of the Philco in operation!
