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Old 4th June 2006, 08:49 PM   #1
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Default GainClone troubleshooting, major problem

I recently built myself a GainClone using the kit from audiosource.com, the LM3875 kit. I built it in a dual mono configuration. Tested one channel only first, everything was perfect. Hooked up the other channel to the toroid, switched the amp on, and got a VERY loud tone out of the new channel, with what looked like a ton of DC offset. I can only estimate that it was a 60Hz tone.

Switched it on and off a couple times. What is weird is that the other channel was still playing just fine. After about 10 seconds of testing, the bad channel let out the magic blue smoke out of the LM3875 chip . I haven't turned it on since to see if it is totally fried.

I haven't tested the voltages on the output of the rectifier board on the bad channel, yet, but that is my next step. I'm going to order another 3875. I was wondering if something else can be the culprit in this situation? I have no scope, just a multimeter, where should I test?
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Old 4th June 2006, 08:56 PM   #2
Netlist is offline Netlist  Belgium
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phorensic,
It is totally fried.

You should be able to find the problem without a scope. Most common are wrongly connected wires or a faulty soldering connection. Check everything twice before applying power again.

Good luck.

/Hugo
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Old 5th June 2006, 04:51 AM   #3
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I meant www.audiosector.com instead of audiosource, but I digress.

I quadruple checked the bad channel and physically it is identical to the good channel. My only guess is that I put too much heat into one of the components with my 30W iron. All the solder joints looked good and not cold. I was stumped. I'll post up with results of the replacement.
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Old 5th June 2006, 05:03 AM   #4
DIAR is offline DIAR  Finland
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Are you sure that signal and ground wires are not connected somehow?

Have you disconnected all other amplifiers from the preamplifier output?
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Old 13th June 2006, 02:19 AM   #5
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All wires were connected, I'm sure of it. All the solder points looked perfect. Every component was oriented correctly. My only guess, like I said, is that I put too much heat into something with the iron.

I'm not sure what you mean by disconnecting all other preamps, the source was a computer at extremely low volume.

One of the problems I have is that I don't have a scope, so the only way for me to test the rectifier is to test voltage. I get ~+/-38Vdc--identical to the other channel. I figure if it's putting out +/-38V that there probably isn't anything wrong with *it*. That leads me to the amp board, which I am basically going to rebuild with new resistors, a new LM chip, and leave the caps in place.

Maybe you guys might know which resistors I need to order. I ordered the audiosector.com classic LM3875 kit. The resistors are definately not Panasonic. Are they Phoenix? Which model(s) exactly? Because they have like 3-4 different 1/4W and 1/2W models.

**Update: R3 was smoked, not the LM3875 chip like I previously thought, however it is probably damaged nonetheless**
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Old 13th June 2006, 07:34 AM   #6
DIAR is offline DIAR  Finland
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I had a huge DC offset with my LM3875 when I had another poweramp connected to my passive preamp. I guess there was dc offset at the input of the other poweramp and it was amplified by LM3875 amplifier.
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Old 13th June 2006, 02:00 PM   #7
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With the exact same source, the left channel was working beautiful. And the optional Zobel network was in place.
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