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Old 24th August 2008, 08:06 PM   #21
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A 10k would work.

Tie the wiper of the pot to the right-most leg (legs down, looking at shaft). The outer legs would go into the board.

The low level oscillation won't generally cause any problems and probably isn't audible but it shouldn't be there.

The instability is likely from too little damping in the circuit. If you get it biased properly, the oscillation should go away.
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Old 3rd February 2010, 02:11 AM   #22
spooney is offline spooney  United States
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well I know this is forever old but I finally picked this amp up again. I soldered in the 10 k pot and adjusted it until I got .000 volts across the emitter resistor.I ran the amp for about 25 minutes as loud as my power supply would let me go and the amp heated up a bit and I re checked the bias and it was the same. The amp appears to play fine but the newly repaired channel gets hotter than the original channel. it isn't way hotter but it is a good ten to fifteen degrees hotter. Does this indicate the bias is incorrect? I'd also like to note that the original channel bias measures at .001 volts but after about 20 minutes it was sitting at .008 volts. I have never had a problem with that channel. Is it worthwhile to re adjust the bias for the unrepaired channel? Should I try to match the newly repaired channels bias to the original? I have yet to put the o scope on it yet to see if that low level oscillation was still present.
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Old 3rd February 2010, 02:08 PM   #23
spooney is offline spooney  United States
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I am beginning to think this amp has some kind of curse. I was putting it on the scope last night and then went to check the bias and i slipped off one of the emitters of one of the outputs and shorted it to the heatsink. The output looks really funny now when i run a sine wave on either channel. Just another thing to fix.
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Old 3rd February 2010, 02:52 PM   #24
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In the owner's manual, the emitter voltage is 0v. Since they are providing voltages accurate to the mv in the same chart, I assume that the bias voltage may be OK at 0.000v. If you can find the reason for the oscillation and can get a clean signal with the bias at no more than 0.001v across the emitter resistors when the amp is hot, that's probably how you should set it.

If both channels are distorted, you may have damaged a ground trace somewhere. Does the RCA ground still read ~0 ohms to the non-bridging speaker terminals?

How does it sound?

Try connecting your scope ground to one of the non-bridging speaker terminals.
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Old 4th February 2010, 04:19 AM   #25
spooney is offline spooney  United States
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Found it! When I shorted the emitter to ground it blew the shield ground in my test head unit. I repaired that and the output looks normal again and I am not seeing any oscillation after setting the bias. I haven't been able to fully heat it up again yet so I'm not sure if the issue with one channel being warmer than the other is still there but like I said in an earlier post its about a 15 degree difference. It worries me a bit but for all I know it could be normal.
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Old 4th February 2010, 09:56 AM   #26
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Do the transistors on one side get hot quicker than on the other side? Touch the face of the transistors while it's playing. This will be easier if you check it when the amp is cool.
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Old 4th February 2010, 11:24 PM   #27
spooney is offline spooney  United States
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to the touch it doesnt appear that one side gets hotter any faster than the other. when i use my temp probe i am only seeing maybe a 5 to 10 degree difference between the channels and it stays consistant regardless of how long or loud its playing. The bias voltage is holding better in the repaired channel than the original. I ran the amp at approximately 50 percent power for 25 to 30 minutes and heated it up until it was too hot to touch and it appears to be holding up well so far.
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Old 4th February 2010, 11:35 PM   #28
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Drive the amp with a test tone and confirm that both channel are producing the same output voltage. If they are, bridge the amp into the test speaker or dummy load. Does one channel still heat up more quickly than the other?
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Old 4th February 2010, 11:47 PM   #29
spooney is offline spooney  United States
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the repaired channel has slightly higher output voltage at all volume and gain levels.
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Old 4th February 2010, 11:49 PM   #30
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What's the difference?
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