Magnavox all transistor schematic12/5/2023 ![]() I believe R101 and R107 have to do with the push pull system of Q22 and Q21? I read somewhere that you can run into oscillation problems and thermal runaway when this section acts up? I can't find any other bad parts. I am just now learning about amps, so forgive me for using terms incorrectly. I believe the problem in the right channel was pulling the whole power supply down, too. The left channel also had low voltages, but no heat. So a ton of load is being put on the output transistors and R101, R107. The voltages at Q21 and Q22 were both pulled down low. Q21 and Q22 both test fine (even after the heat). Also, transistors Q21 and Q22 became very warm so I removed power. I replaced R101 and R107, but they both become very hot. In addition to a recap, I found that R101 and R107 were both open (see arrows). I attached a schematic of the Right channel: AF and Output. Now, I do have audio from the right channel via the "Tape Record" output, so this confirms the problem is in the right channel AF and Output amp? I also tested the speaker itself. This happens in all modes (Tape, Phono, FM Stereo). The Left channel works perfect, but the right channel is dead at the speaker. It was a wedding present and means a lot to her.I am working on repairing a 1970s Magnavox stereo console. The son asked me to look at it because it belongs to his mom who is widowed. ![]() Looks like the oscillator isn't working so digging into that right now with no luck. Now onto the FM band where I get one station, the same station all up and down the dial. After replacing the RF amp and fiddling with the bias, got it working pretty good. I had trouble with sensitivity of the AM band and tested and retested everything and found nothing. ![]() I got a Sams print for it to help me along and identify where things should go. I carefully unsoldered all those components, tested them, and reinstalled them on the top side of the board where they belong. Plus I got a bag of components with it too. Why he messed with the radio and did that with all the parts baffles me. The guy took most of the components in the radio section off the pc board, then remounted them on the bottom, the foil side. The son went to get it back from the guy and that's when he asked me to look at it. A simple head adjustment right! She had a guy look at it, he took all the guts out of it and had it over 6 months. I was told the woman that owned it told her son the only problem with it was the 8 track player was having crosstalk between tracks. I never should have taken on this project. I thank you so much as it would have been useful. Now onto the FM where I believe the oscillator is not running! I ended up experimenting with changing the bias voltages on the transistor by changing resistor values hoping to get more gain and viola, success! I don't know how good of a performer this unit was in the day but it now plays decently. I don't know what the secondary should be but I find it strange that the antenna would be defective being that the windings are only one layer, what could go wrong! So I retested all the components in the RF stage and nothing. This exonerates the primary of the antenna for me. When I measured the antenna, it turned out to be 475 micro-henries, so I am thinking we are in the ball park close enough considering the accuracy of the settings on the dial. What I found was the inductance calculated out to be approx. Then using some math, calculated what the inductance the antenna should be to resonate at these frequencies. I took measurements at 550kz, 1 MHz, and 1.6 MHz on the dial assuming the dial is accurate. I took capacitance measurements of the tuning gang in parallel with the antenna after disconnecting everything around it. Ok, got back to work on the Maggy and did some testing on the ferrite loopstick antenna. I will be posting in the want adds for this antenna. ![]() So I guess what I'm asking is anybody have any experience with this unit and how common would it be to have a defective ferrite loopstick antenna? I tried looking on line for a replacement but it is futile. If I touch the base of the RF amp transistor it improves greatly. The tuning condenser shows no shorts between the plates. I know 2 ohms isn't much but in this case it's half of what it should be. According to the schematic the antenna primary should measure 4 ohms but only measures 2. Everything leads me back to the antenna circuit which doesn't have much in it. I have tested all the components in the RF amp section and even replaced the RF amp transistor. Did an alignment on it with hardly any improvement. With the volume all the way up, I can barely here 2-3 stations. I don't know how well these units worked but it should be better than it is. Working on a beautiful Magnavox 70s AM/FM 8 track record changer chassis #R321-12CA floor model for a friend. I know it's a dinosaur but it means a lot to him. Hi all, I don't know if this is the place for this but here goes.
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