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Hafler DH500 No gain in one channel
Hafler DH500 No gain in one channel
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Old 2nd May 2018, 12:28 PM   #1
sepolansky is offline sepolansky  United States
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Default Hafler DH500 No gain in one channel

Strange problem here. After relocating the amp from one house to another, one channel seems to have lost gain. Both 'scope and voltmeter indicate output is equal to the input in one channel, the other functioning normally.

Fuses are fine, output relay is fine. +/- 90 volts is on the PCB (19c), the bias is set at 350mA, offset I adjusted to below 0.01V. So, to my mind, there is at least DC functioning from input to output. But why no gain?

Output stage and drivers are followers, so no gain there, but I'm terrible with SS schematics. Thought about some kind of resistive failure causing 100% feedback.......

Suggestions/ideas on where to look?

Thanks ahead of time,

Stuart
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Old 2nd May 2018, 04:18 PM   #2
llwhtt is offline llwhtt  United States
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C8 in the feedback circuit might cause that problem. Doesn't explain why moving the amp would cause the problem though.

Craig
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Old 2nd May 2018, 04:27 PM   #3
sepolansky is offline sepolansky  United States
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Quote:
Originally Posted by llwhtt View Post
C8 in the feedback circuit might cause that problem. Doesn't explain why moving the amp would cause the problem though.

Craig
Thanks Craig, I'll check that out tonight.

I replaced lots of caps and did some simple mods several years ago. It's been working fine for a long time. Maybe something like a stray piece of wire fell onto the board, but I've had that thing at every angle imaginable and nothing has changed, so maybe a shorted cap.

Stuart
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Old 2nd May 2018, 04:35 PM   #4
llwhtt is offline llwhtt  United States
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Usually when that feedback capacitor goes bad/open is when you loose gain/bass.

Craig
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