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Old 25th February 2010, 10:21 PM   #11
durwood is offline durwood  United States
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On that diagram I believe it is the ones marked Q20/Q21. There are definitely no solder bridges.

I'll have to check this tomorrow when I have a chance. I bypassed the cumbersome and impossible to troubleshoot clamping method with a pair of hand clamps and strips of wood to make it easier to troubleshoot with the rear cover removed.
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Old 26th February 2010, 03:22 AM   #12
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If you have some heavy weather-stripping material or soft rubber, apply a strip to the wood strip. Without something to evenly distribute the pressure, you're likely to have pressure on only a few components.

If you have some channel stock (possibly available from lowes or home depot) you can make clamps like those in the following photo.
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Old 26th February 2010, 02:38 PM   #13
durwood is offline durwood  United States
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Ok this is crazy, yesterday I thought I had tested it with the components removed on the bad channel and it was no go. Today it works fine with the outputs out and the drivers out of half the bad channel. However, I suspect it is a problem with the three transistors Q28, Q79 and the one between them. I'll have some measurements in a bit comparing the good channel with the bad. I think it would be Q28 is shorted since I'm reading a short/ 23 ohms across C and B of Q20/Q21 and R59 from the diagram is ~20ohms. I mistyped before about the short between E and C.

Last edited by durwood; 26th February 2010 at 02:50 PM.
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Old 26th February 2010, 04:56 PM   #14
durwood is offline durwood  United States
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Yep, Q28 (MPSA56) on that diagram was bad. I replaced that guy put drivers (mpsa06) and outptus back in 2n6488 and it was ok but then slowly started to creep up in current. Found one mpsa06 going bad so I replaced those and then replaced all the 2n6488's because the one was bit off from the other two. Put back together and viola! It lives. Thanks for that schematic, it was exactly what I needed.

Last edited by durwood; 26th February 2010 at 04:58 PM.
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