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Old 8th August 2010, 08:50 AM   #11
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Are all of the output and biasing transistors clamped to the riser plate?
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Old 8th August 2010, 09:05 AM   #12
myk777 is offline myk777  United States
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Quote:
Originally Posted by Perry Babin View Post
What is the DC voltage measured between the center legs of the two output transistors for any of the channels?
The're all right around 41.4v
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Old 8th August 2010, 09:08 AM   #13
myk777 is offline myk777  United States
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Quote:
Originally Posted by Perry Babin View Post
Are all of the output and biasing transistors clamped to the riser plate?
If by "riser plate" you mean the heatsink then yes there are 8 output transistors and the 4 biasing transistors between them, they all have one screw each and are bolted to the heatsink with the thermal compound and one of those thin clear insulators each. I've never turned it on without them bolted up, the aluminum chassis/heatsink gets very hot in a hurry.
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Old 8th August 2010, 09:13 AM   #14
myk777 is offline myk777  United States
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FYI - I have a two channel dso oscilloscope if that could be of any help in the diagnosis?
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Old 8th August 2010, 09:18 AM   #15
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You can confirm that there is no oscillation on the speaker output terminals but I don't think that there's anything else you can check at this point. Have you replace any of the output or biasing transistors?
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Old 8th August 2010, 09:23 AM   #16
myk777 is offline myk777  United States
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No, I haven't replaced anything in the amp, it all looks original and unaltered. Are you thinking one of the output or biasing transistors could be bad? I'll check the speaker terminals with the scope.
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Old 8th August 2010, 10:45 AM   #17
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It's likely to be a problem common to all channels like a power supply problem but, from the information you provided, I can't see anything that would cause a the high bias.
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Old 9th August 2010, 05:48 AM   #18
myk777 is offline myk777  United States
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OK, I might be getting somewhere, was able to "fix" the amp by testing it with the o-scope, this is weird.

Scoped the 4 channels of speaker outputs from each speaker's - to +, they all seemed similar about 500mV peak to peak at ~140kHz

I then decided to check from the B- to the speaker terminals and as soon as I touched the scope probe to any of the speaker terminals + or - the idle current immediatly went to 1.0amps and everything cools off. Also does the same thing with the scope lead referenced to B+.

Next I unhooked the scope lead from the scope and I get the same behavior as above, this was with the scope lead in the 1X position, however in the 10X the idle current went back up to 7A.

I then tried a few resistors in place of the scope lead from 1meg to 4.7K with no effect on the idle current so doesn't seem to be a resistance thing. Not sure what's in a scope lead, capacitance maybe?

I guess I could sacrifice a scope lead to fix the amp...lol. Any ideas, weird one.

Mike

Last edited by myk777; 9th August 2010 at 05:58 AM.
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Old 9th August 2010, 06:04 AM   #19
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With no RCAs plugged in, what's the resistance from the RCA shield to the non-bridging speaker terminals?

Does it oscillate/overheat with no RCAs plugged in?
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Old 9th August 2010, 06:17 AM   #20
myk777 is offline myk777  United States
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A bit more info, checked the scope leads a bit closer and they have a capacitance rating of 18.5pF in the 10X position and 115pF in the 1X position, I had a ceramic 330pF laying around, tried in inbetween the B- and a speaker terminal and Bingo idle current goes to 1A. Must have a bad cap somewhere?

@Perry - Will go check right now on the resistance, and yes it overheats with no RCA's plugged in.
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