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PPI pro mos 450 pulses when powering on
PPI pro mos 450 pulses when powering on
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Old 25th November 2020, 12:45 AM   #11
GEM 592 is offline GEM 592  United States
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Thank you Perry, you are the best. I will update after the replacements.
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Old 1st December 2020, 04:28 AM   #12
GEM 592 is offline GEM 592  United States
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Hello,

I carefully checked emitter resistors, and replaced all 10 outputs in the channel in which the bad outputs were found.

I reassembled the amp for a bench test.

The same symptom is occurring - when remoting the amp on, the power led just pulses (and my lamp dims like current is being drawn.)
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Old 1st December 2020, 04:49 AM   #13
GEM 592 is offline GEM 592  United States
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Now I've found at least one bad output in a group on the other side of the amp.

I don't remember seeing these last time, I guess I just missed them. Is it possible to blow outputs without even connecting speakers?

I suppose I should just lift those out a see if it powers up.
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Old 1st December 2020, 05:05 AM   #14
GEM 592 is offline GEM 592  United States
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OK I lifted the lowest base-emitter resistance device, rest of group read OK.

Did power-up test and it appears to power up normally now.
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Old 1st December 2020, 05:09 AM   #15
Perry Babin is offline Perry Babin  United States
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Was it another 6487/6490?

It's possible to have virtually any component fail without a load. If it's another output transistor, they could be failing due to age.
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Old 1st December 2020, 05:11 AM   #16
GEM 592 is offline GEM 592  United States
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Yes, and just one in the corresponding channel on the other side of the amp.

This one is a 90, before it was and 87.

The channel I replace still reads OK after letting the amp stay on idle for a few minutes.
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Old 1st December 2020, 05:23 AM   #17
Perry Babin is offline Perry Babin  United States
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The outputs can overheat at idle if not clamped down. There should be no bias current (and no heating) but the biasing can drift.
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Old 1st December 2020, 05:37 AM   #18
GEM 592 is offline GEM 592  United States
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Thank you. I have been clamping it. Almost ready to retry.
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Old 1st December 2020, 05:47 AM   #19
Perry Babin is offline Perry Babin  United States
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You should make absolutely sure that the transistors lay perfectly flat on the heatsink. If not perfectly flat, the cover may not push them flat.The edge of the transistor can dig into the insulator and leave the transistor off of the sink (attached).
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Old 1st December 2020, 06:59 AM   #20
GEM 592 is offline GEM 592  United States
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OK all four functioning. Testing. Sounds right so far.
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