Can anyone help? I have an M500T that someone gave me. I tested it and the left channel is distorted. If I feed an equal signal from a sig generator into both channels the left is showing slightly less output at the meters and the ears detect the same thing. I was tracing the signal path with an o-scope and the signal takes a totally different appearance as it comes out of IC101. I am not sure if the IC is causing this or if a nearby cap is the problem. Any ideas?
i didn't find any bad joints. BUT when I attempted to set the bias, the right channel test points went great. Not so good for the left side. NO VOLTAGE SHOWING when trying to set the bias. Checked the trim pot resistance and it is working fine but adjusting it changed nothing. ALSO there is voltage to the to the collectors of the large transistors BUT there is no incoming signal to the bases and nothing at the emitters. They are on but nobody is home.
Sounds like the bias transistor might be shorted or, more likely, has no current going through it at all. Is there any voltage between the bases of the output or driver transistors? If not, it's probably that. Otherwise one of them may also have gone open - some checking of B-E voltage drops may be in order.
Thanks Dennis. I don't feel so bad now. I was studying the schematic last night and had concluded that Q109 was indeed the bias transistor. I don't feel totally clueless now. The schematic shows that there should be 1.7 volts at the collector of the transistor. Base voltage measured -1.1vdc, C=+1.2vdc, E+ -1.7vdc. A long way from the 1.7 volts the schematic shows. And nothing like the voltages I found at the other bias transistor. I tried adjusting the trim pot and while it does change the numbers a little it does not even remotely resemble the other side and does nothing to change the garbled sound on that channel.
If you notice the circuit is symmetric, what's on the collector of Q109 should also be on the emitter but opposite polarity. Q109 is controlling six PN junctions, three on the positive half, Q111, Q115, and Q119/121 and three on the negative half, Q113, Q117, and Q123/125. I counted the outputs as one PN junction as they are in parallel. Each PN junction voltage drop is approx. .6VDC, the turn on voltage, and since there are three of them in series the total is approx. 1.8VDC, so that's where the 1.7VDC comes from. The conduction of Q109 is controlled by SVR101 and surrounding components. The harder Q109 conducts, the less voltage drop across collector and emitter, the colder the bias which is what you are experiencing. You only have 2.8VDC should be more like 3.4VDC. If your problem is actually in the bias circuit your voltages should still be symmetric in regards to the collector and emitter but they are not. Try measuring the collectors of Q101 and Q103. Notice there is a mistake on the schematic, the collector of Q101 should be same as the collector of Q103, 73VDC NOT 7.4VDC. Remember to look for symmetry between the + and - halves of the circuit.
Craig
Craig
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