Plimpington2
Super Member
I have a Marantz 240 that I am trying to rebuild. I am doing one channel at a time and no matter what I do, I cannot get it to function properly. I am on wall voltage and get good +/- 58 volts to the amplifier board. I do not have the OPT's in, and just running a small speaker and my scope from the drivers. When there is no load, it passes a signal just fine. However, as soon as I apply an 8 ohm load (a speaker or a dummy) the top of the waveform distorts. The bottom looks fine.
The schematic for this thing is below:
I took these measurements with the input shorted and an 8ohm load on the output (but the waveforms are with a signal generator 1000 cycles, very low voltage). The OPT's are out. Green denotes a good voltage or a good part. Red is a bad voltage. As you can see, the collectors for the pre-drivers (Q507 and Q508) are wildly wrong. The bias transistor (Q801) and it's friend (Q521) have been checked and replaced and checked again. And again. The bias pot works wonderfully. And yet, I am getting +10 or +11 volts where I should have +0.7 and -0.7. As a result of this state of affairs, Q511 (the PNP Driver) is producing what I think is an appropriate half wave:
Whereas the Q510 (the NPN driver) is doing nothing at all. Flatline. I have also checked the waveform at the collectors of the two pre-drivers (Q507 and Q508) and they are identical and look like this:
Even the top of this waveform is distorted (and it gets much worse if I were to inject a signal at the inputI have tested and retested every transistor on this board, as well as just about all the resistors). The voltages I do have (in green) would seem to suggest things are MOSTLY going ok, but clearly something is very wrong. I will say that I do not have the bias transistor mounted to the heatsink. It's just floating in air. But that really should not make that much of a difference, should it? I can't move bias one bit with the pot. The DC offset works just fine. I can get it within a few mv of zero no problem. Finally, I have removed the "current sense" transistors (Q516 and Q517) just to remove that variable. However, in or out, I get the exact same result.
Any help is very appreciated.
The schematic for this thing is below:
I took these measurements with the input shorted and an 8ohm load on the output (but the waveforms are with a signal generator 1000 cycles, very low voltage). The OPT's are out. Green denotes a good voltage or a good part. Red is a bad voltage. As you can see, the collectors for the pre-drivers (Q507 and Q508) are wildly wrong. The bias transistor (Q801) and it's friend (Q521) have been checked and replaced and checked again. And again. The bias pot works wonderfully. And yet, I am getting +10 or +11 volts where I should have +0.7 and -0.7. As a result of this state of affairs, Q511 (the PNP Driver) is producing what I think is an appropriate half wave:
Whereas the Q510 (the NPN driver) is doing nothing at all. Flatline. I have also checked the waveform at the collectors of the two pre-drivers (Q507 and Q508) and they are identical and look like this:
Even the top of this waveform is distorted (and it gets much worse if I were to inject a signal at the inputI have tested and retested every transistor on this board, as well as just about all the resistors). The voltages I do have (in green) would seem to suggest things are MOSTLY going ok, but clearly something is very wrong. I will say that I do not have the bias transistor mounted to the heatsink. It's just floating in air. But that really should not make that much of a difference, should it? I can't move bias one bit with the pot. The DC offset works just fine. I can get it within a few mv of zero no problem. Finally, I have removed the "current sense" transistors (Q516 and Q517) just to remove that variable. However, in or out, I get the exact same result.
Any help is very appreciated.
