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Old 3rd June 2005, 05:52 PM   #1
taj is offline taj
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Default Troubleshooting amp - help needed

I'm just learning electronics and i'm stuck at this problem. I'm not sure what to test next.

I finally fixed some major power supply issues with my 'learning project' Dynaco Stereo 80 so it runs now. One channel works fine, but the other is very ill. Very low volume with lots of hum and noise.

I measured the voltage at all the test points (see link to service info/schematic below) and the bad channel is reading about double the voltage of the good channel on most rail points, but the problem is not with the power supply since it's offering up the same supply connections through resistors 16 and 17 (which are both fine) to each channel.

Test points #3 & 4 (on PCB-19) should both read the same (36v), but #3 (for the bad channel) is about 70 volts. Points #5 & 6 should both be 37.5 volts, but #5 (for the bad channel) is about 70 volts. I can't figure out why though. I'm not sure where to look next.

Schematics: http://home.indy.net/~gregdunn/dyna...ST80/index.html

Any help is appreciated.
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Old 3rd June 2005, 07:55 PM   #2
moamps is offline moamps  Croatia
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Check Q3, Q5, R16 and R17.
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Old 4th June 2005, 08:16 AM   #3
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Thanks. Will do.

TAJ
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Old 6th June 2005, 04:24 AM   #4
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Default thats it.

Looks like Q5 was the culprit. Thanks.

Anyone got a 2N3055 kicking around their junk pile?

..TAJ
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Old 6th June 2005, 04:35 AM   #5
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That's gotta be the most common transistor in history. I think even RatShack has them.
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Old 6th June 2005, 04:42 AM   #6
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Default q5

That a yes?
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Old 6th June 2005, 04:56 AM   #7
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I don't...it's too much of a plane-Jane device for me. Got a RatShack up there?
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Old 6th June 2005, 05:05 AM   #8
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Might be one a few igloos over... I guess I'll have to strap on the old snow shoes and hitch up the dogs.
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Old 9th June 2005, 04:44 AM   #9
lubbie is offline lubbie  Canada
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Check out main electronics in Van..
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Old 9th June 2005, 05:07 AM   #10
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Thanks Ralph.

..TAJ
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