Hi there,
I am trying to fix an old Marantz PM250. There is no amplified sound, although there is a very quiet, distorted sound from both channels. As both cannels behave in the same way, I suspected common circuity to be at fault, e.g. power supply. I used oscilloscope to check the signal at the input of the Volume module PG00 - looked great and the output - the waveform is distorted. The output is connected to the input of pre-amp stage, so I suspected that the power supply for pre-amp was faulty. Sure enough, the emitter of Q812 that supposed to be -11.4V shows approx 1V positive. I immediately suspected burned Zener Q805 or regulator transistor Q812, however both of them, as well as the capacitor C804, were fine. More to it, when I disconnected emitter of Q812 from the PCB it shows -11.4V as it should. It points to the issue somewhere else.
The question I was going to ask - are there any obvious candidates to be checked? I could start cutting PCB tracks trying to find a culprit, but before that wanted to ask if there is a better approach...
Thanks in advance,
I am trying to fix an old Marantz PM250. There is no amplified sound, although there is a very quiet, distorted sound from both channels. As both cannels behave in the same way, I suspected common circuity to be at fault, e.g. power supply. I used oscilloscope to check the signal at the input of the Volume module PG00 - looked great and the output - the waveform is distorted. The output is connected to the input of pre-amp stage, so I suspected that the power supply for pre-amp was faulty. Sure enough, the emitter of Q812 that supposed to be -11.4V shows approx 1V positive. I immediately suspected burned Zener Q805 or regulator transistor Q812, however both of them, as well as the capacitor C804, were fine. More to it, when I disconnected emitter of Q812 from the PCB it shows -11.4V as it should. It points to the issue somewhere else.
The question I was going to ask - are there any obvious candidates to be checked? I could start cutting PCB tracks trying to find a culprit, but before that wanted to ask if there is a better approach...
Thanks in advance,
Hi Qbfbk,
I think you've proven the Zener is probably OK. Seems to me that the only suspects are very low Beta in the transistor, or a resistor that's gone way high in actual value.
Referring to the schematic, I suggest reconfirming that the right end of R806 is about -36V, then measure left side of R806. The schematic suggests it should be -28.8V, but given you're seeing +1V at Q812 emitter, I can imagine only a few scenarios:
1. Q812 might be very hot because it's trying deliver large current, but can't because of failed preamp circuit. Observed voltage at R806 will be much lower than nominal -28.8VDC. Or it might be relatively high because no collector current is flowing.
2. Q812 regulator output may be delivering very little current; culprits could be very low current gain in the transistor (base and Zener voltage near 0 due to low current gain.) The same result could could occur because R808 has drifted high in value--- you could tack another 3.3k in parallel as a test.
3. R806 may have drifted high. Consider the observed voltage across R806 and how hot it seems to be. These may be clues.
Let us know what you observe. Good luck!
I think you've proven the Zener is probably OK. Seems to me that the only suspects are very low Beta in the transistor, or a resistor that's gone way high in actual value.
Referring to the schematic, I suggest reconfirming that the right end of R806 is about -36V, then measure left side of R806. The schematic suggests it should be -28.8V, but given you're seeing +1V at Q812 emitter, I can imagine only a few scenarios:
1. Q812 might be very hot because it's trying deliver large current, but can't because of failed preamp circuit. Observed voltage at R806 will be much lower than nominal -28.8VDC. Or it might be relatively high because no collector current is flowing.
2. Q812 regulator output may be delivering very little current; culprits could be very low current gain in the transistor (base and Zener voltage near 0 due to low current gain.) The same result could could occur because R808 has drifted high in value--- you could tack another 3.3k in parallel as a test.
3. R806 may have drifted high. Consider the observed voltage across R806 and how hot it seems to be. These may be clues.
Let us know what you observe. Good luck!
Hi BSST,
first and foremost, thank you for the reply.
I found a wire jumper that connects (e of Q812)-11.4 V to some of the pre-amp circuitry which I was able to disconnect wit minimum fuzz. I have measured resistance of the pre-amp circuit to the ground - it was in hundreds of K Ohms in both directions, suggesting no catastrophic failures of the pre-amp. I have turned the power on and measured the current supplied by Q812, it was about 16 mA. It made no sense at all. 30 odd V across Q812 and it is still cold, no sparks, no fumes
. Then I went back to your reply and started thinking . R806 drifted high. Not just high, but to some magic value of 3.5 K. It explains everything. The regulator is starved from current and just can't do its job.
While I am educated and have some equipment beyond a multimeter at home, I don't have much experience repairing old analogue stuff, so it never occurred to me that a passive element could fail. I would rather suspect every single transistor, then every zener, then all caps...
So the old bugger is working now. Repair cost - between one and two cents. Listening to the sound of it - priceless.
So thanks again, you helped a lot!
first and foremost, thank you for the reply.
I found a wire jumper that connects (e of Q812)-11.4 V to some of the pre-amp circuitry which I was able to disconnect wit minimum fuzz. I have measured resistance of the pre-amp circuit to the ground - it was in hundreds of K Ohms in both directions, suggesting no catastrophic failures of the pre-amp. I have turned the power on and measured the current supplied by Q812, it was about 16 mA. It made no sense at all. 30 odd V across Q812 and it is still cold, no sparks, no fumes
. Then I went back to your reply and started thinking . R806 drifted high. Not just high, but to some magic value of 3.5 K. It explains everything. The regulator is starved from current and just can't do its job. While I am educated and have some equipment beyond a multimeter at home, I don't have much experience repairing old analogue stuff, so it never occurred to me that a passive element could fail. I would rather suspect every single transistor, then every zener, then all caps...
So the old bugger is working now. Repair cost - between one and two cents. Listening to the sound of it - priceless.
So thanks again, you helped a lot!