Delco Model 984592 Car Radio – Part Two

Old chassis parts

I turned my attention to the remainder of the chassis once the power supply and volume control were built. I took a permanent marker and wrote the part identifier from the schematic plus its capacitance value where it was illegible or facing the wrong way.

Rebuilt chassis

I decided to replace every component. I’m glad I did, because easily half of the resistors were wildly incorrect in value and I’m sure all the old paper/wax capacitors were equally bad. I pushed the tubes in and powered up gingerly. After wiggling one tube a bit to clean the contacts I was greeted with AM static.

Rear of chassis

It was nice to see the thing fully populated with tubes again, and assembled. I cleaned it up a bit (it was rather dusty) and put the face back together.

Front of chassis

It’s far from perfect, but I think it looks okay. I might paint it in the original silver to match the back of the case.

Bluetooth board

I had done a bit of research into Bluetooth modules, and found a nice one that has an audio pass-through using a mechanical relay. When nothing is connected, the relay is connected in such a way that the audio in passes through to the output connector. With a Bluetooth device paired, it engages and disconnects the input audio and sends its’ own generated audio out. Ideal, as the radio can function as original until a Bluetooth device is connected, then it’ll feed through from that. I had started work on this as you can see; the connectors de-soldered from the board and a rather large ferrite core inductance choke connected to the power input. There was quite a lot of hash being picked up and send back into the radio and out the loudspeaker through the heater circuit. I added that choke and a couple of high value capacitors to the heater chain which silenced the noise. I replaced the audio wire that goes to and from the volume control because the original had a short circuit break somewhere inside, and had the out and back signals twisted together, inside a shield. When it was just an AM radio this was perfectly acceptable but when the Bluetooth module engages, the amount of cross-talk from the radio circuit made the end result unintelligible. I separated the radio output and volume control mix return wires in their own shielded, individual wires. That resolved 99% of the interference issues and made the audio output very clean.

Rewired

I set the radio on the side and streamed music to it for several hours. The quality of the audio is impressively good; the design of the power amplifier circuit is good and while not quite a full 12 Watts due to a lower plate voltage than usual (at a guess it’s about 8 Watts) it is a good, hi-fi design, and with a full-range loudspeaker the sound it produces far surpasses any expectations of the device. Last to-smile-at feature will be in the car, installed. With my phone connected, it represents a hands-free calling system… not a bad option for 1951!

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