vinylisfinyl
Member
Greetings fellow Solid-Staters, and Happy Belated Thanksgiving!
I'm working on my grandmother's Magnavox 1RP660 console stereo, chassis R204-18-20, and need some advice on how to proceed with troubleshooting a problem: When the four position Bass control is put into any of the positions below full bass, the left channel weakens and loud hissing can be heard. As the bass is further reduced, the left channel continues to weakens until no audio can be heard. Conversely, hiss continues to get louder till the lowest setting. This happens regardless of input type. The stereo sounds and behaves excellent otherwise.
I checked all transistor voltages and most seemed OK (see attached schematic). I then scoped the signal path with a 1 kHz sine wave fed into the Tape Play input and found the output of the second set of preamp transistors to be very clean (yellow is Left Channel, blue is Right). The signal is then fed into the volume pots and then into the bass control before going to the driver transistors. Coming out of the volume pot the signal looks a little noisy, but is also attenuated so I don't think much of it. However, scoping the output of the bass control, it's very clear that the signal going in is OK but coming out it exhibits more noise and weaker audio as the bass is turned down. To me this says something inside the switch or a capacitor or resistor inside the packaged circuit is probably the cause.
However, I'm unsure of the impact of tearing apart the packaged circuit you see in the picture below that looks like red shrink-wrap. It is made of unobtanium (of course!). Have any of you ever taken one of these apart? My hope is it's like a little PCB shrink-wrapped, but I want to be certain before I permanently wreck this trying to figure out what's going on here.
I'm working on my grandmother's Magnavox 1RP660 console stereo, chassis R204-18-20, and need some advice on how to proceed with troubleshooting a problem: When the four position Bass control is put into any of the positions below full bass, the left channel weakens and loud hissing can be heard. As the bass is further reduced, the left channel continues to weakens until no audio can be heard. Conversely, hiss continues to get louder till the lowest setting. This happens regardless of input type. The stereo sounds and behaves excellent otherwise.
I checked all transistor voltages and most seemed OK (see attached schematic). I then scoped the signal path with a 1 kHz sine wave fed into the Tape Play input and found the output of the second set of preamp transistors to be very clean (yellow is Left Channel, blue is Right). The signal is then fed into the volume pots and then into the bass control before going to the driver transistors. Coming out of the volume pot the signal looks a little noisy, but is also attenuated so I don't think much of it. However, scoping the output of the bass control, it's very clear that the signal going in is OK but coming out it exhibits more noise and weaker audio as the bass is turned down. To me this says something inside the switch or a capacitor or resistor inside the packaged circuit is probably the cause.
However, I'm unsure of the impact of tearing apart the packaged circuit you see in the picture below that looks like red shrink-wrap. It is made of unobtanium (of course!). Have any of you ever taken one of these apart? My hope is it's like a little PCB shrink-wrapped, but I want to be certain before I permanently wreck this trying to figure out what's going on here.
