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M500T Carver help?
M500T Carver help?
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Old 3rd January 2019, 09:20 PM   #1
T3GTerry is offline T3GTerry
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Default M500T Carver help?

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?
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Old 3rd January 2019, 10:12 PM   #2
llwhtt is offline llwhtt  United States
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Since the output of the opamp is inside the feedback loop everywhere you check the signal it will look distorted. Though it could be the opamp that wouldn't be my first choice. Start comparing DC voltages with the good channel.

Craig
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Old 3rd January 2019, 10:21 PM   #3
dennismiller55 is offline dennismiller55  United States
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Check for cold solder joints in and around this opamp.
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Old 4th January 2019, 01:27 AM   #4
T3GTerry is offline T3GTerry
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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.
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Old 4th January 2019, 02:21 AM   #5
T3GTerry is offline T3GTerry
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Also the left rail is cold while the working right side is SLIGHTLY warm. Normal.
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Old 4th January 2019, 03:11 AM   #6
sgrossklass is offline sgrossklass  Germany
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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.
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Old 4th January 2019, 03:18 AM   #7
T3GTerry is offline T3GTerry
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Ok I will admit I have no clue which one is the bias transistor. As for the BE voltage check ....... I will get back with that info in a few. I am sure I checked but I don't remember.
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Old 4th January 2019, 03:28 AM   #8
T3GTerry is offline T3GTerry
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Ok. I checked the voltage between the B & E legs of the drivers and .5 vdc on the good rail and barely .2vdc on the left rail.
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Old 4th January 2019, 11:24 AM   #9
dennismiller55 is offline dennismiller55  United States
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Q109 is the Bias Transistor. It located on the heatsink.
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Old 4th January 2019, 03:30 PM   #10
T3GTerry is offline T3GTerry
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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.
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