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21st September 2007, 12:46 AM
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diyAudio Member
Join Date: Dec 2006
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marshall g-80 oscillates
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?????
judging by the size of the VAS compensation caps, they had stability problems with this design..... i'm posting this in musical instruments as well
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