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Old 23rd May 2010, 05:54 PM   #1
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Default DH 220 lost a channel

My DH 220 lost a channel. I wasn't rocking it and it doesn't appear to be a blown output. This amp has been in regular use for many years. I gave it a cap job last year when it started sounding crappy and it came back to life. I suspect I might have hit it with a big DC transient whereas I drive it with a tube preamp. So... both the DC rails are fine. I replace Q1 - Q6 as well as D1 - D8 thinking I mighta smacked it's input and all of that was to no avail. Here is a copy of the schematic and voltage chart. Most of the voltages are close enough so I only listed (in red) the ones that are way out of line. Any suggestions are greatly appreciated. I cannot afford a new amp right now (daughter in college so I won't be able to afford a new amp for quite some time). I'm strictly a tube-head so these new fangled transistor thangs confuse me.

Thanks!
-Richard
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Old 23rd May 2010, 06:11 PM   #2
Mooly is offline Mooly  United Kingdom
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It's much easier when it's front of you

Gotta ask ... fuse F404 Is it OK... had it blown ?
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Old 23rd May 2010, 06:23 PM   #3
djk is offline djk
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Q9 voltages don't match the measurements on Q12, Q13.

Check Q9 or C6 for shorts.
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Old 23rd May 2010, 06:24 PM   #4
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Looking at the input stage, i would say check Q2, though I find strange that Q3 and Q4 having that wrong voltage on their emmitors still keep the correct voltage on their other pins. They sould be closed and their collectors at +DCV.
Could you measure Q3 & Q4 voltages again?

Be carefull when measuring as with solid state, a short circuit, even in the input stage, can blow the outputs, all the stages being DC connected.
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Old 24th May 2010, 12:59 AM   #5
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It's fixed thanks to a few clues from a very bright friend. The clue was the positive voltage at the collector of Q2. His suggestion was to look for current flow through R9. There was none therefore it must be a bias issue. Bias is sourced and balanced via P1. Q1 was sitting happy so it had to be somewhere between P1 and Q2. That's pretty much R6 & 7. R6 was wide open. I replaced it and the amp started working. I popped the rail fuses and inserted current meters and the offset was still dead on, even after all these years and all the unnecessarily replaced transistors.

Thanks for the suggestions.

Case closed.

-Richard
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