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Old 21st September 2007, 12:44 AM   #1
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Default marshall g-80 oscillates

i'm going to post this under solid state also.
i've got a marshall g-80 (g8080) amp that came in blown up with parts missing. i ordered the stuffed board from Korg, and finally got it after a 3mo backorder. put in the new board (which came with the output jack board attached) and it oscillates at 200khz. looking at the schematic, the compensation caps are 470pf, and the integrator cap on the op amp is 100pf..... in other words TONS of compensation. i replaced the 470pf caps with polycarbs instead of ceramics, and same thing. 200khz oscillation is about 2vrms. anybody got any ideas?????
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Old 9th October 2007, 04:24 PM   #2
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found the source of the oscillation........
the shop has a terrible ground system and long signal cables. every time i unplugged the test oscillator, the 200khz went away.

says a lot for keeping an eye on the test setup.......
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