Here's part one of the repair of a 1965 Magnavox solid state Astro-Sonic console stereo, using the R204 series chassis. You may recall that I made a video of this same stereo when I first obtained it and I thought it was a '66 model; but, after opening it, I found that it was made in mid-1965. In this video, I cleaned all the controls and replaced all of the electrolytic capacitors. One capacitor was open and the rest measured 2-5 times higher than their stated value when measured on a modern capacitance meter. When a capacitor test much higher than expected on a modern cap meter, that usually means that the cap is leaky. Sure enough, when I tested the old capacitors on an old-style Eico capacitor tester from the '60's, they showed leakage. Most of the original capacitors are Nichicon-branded Japanese capacitors. Nichicon is a good brand that I still use today; but, after close to 50 years, they are showing their age and need to go.
After the controls were cleaned and the capacitors replaced, the audio quality was much better; but, the FM performance is not as good as I think it should be. I tested all three transistors in the FM front end (RF amp, mixer, oscillator) and they all seem to have some leakage. These are all PNP germanium transistors that cross over to an NTE126. When tested on the diode test function of a DMM, I measure the expected .3 volts in the forward direction from B to E and from B to C; however, when I reverse the leads, I'm getting about 1.3 to 1.8 volts when I should not be getting anything. In the past, I've ran into germanium transistors that tested like that; but, would still work. So, I decided to take some voltage measurements at the transistor terminals to help determine if the transistor is functioning properly or not. At $10+shipping/ea, I'd rather not replace any transistors that I don't have to. If I have to replace a transistor, I will likely first try a more modern (and cheaper) silicon device. Sometimes, a silicon device will work in place of a germanium device; and, other times, the bias voltage is off enough that a silicon device won't work.
After making voltage measurements, I found 0V on the collector of the FM RF amp transistor when there should be -15V at the collector. In the next video, I'll attempt to find out why the -15V is missing.
As far as FM performance, I can semi-decently receive the strongest local FM stations; but, I have to turn the volume up higher on FM to get the same degree of loudness that I get on AM. As far as weaker FM stations, they are either very faint or non-existent.
The AM band, with it's tuned RF front end, seems to be fairly hot.
Once we get the radio working correctly, we'll move to the record changer. This model uses a Collaro Micromatic changer, like what's in most Magnavox products from that era.
6 окт 2024
