phase linear 400

gene french

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guayabo de bagaces, guanacaste, costa rica....
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music...the healer of souls...
got two front panels left to refurbish...
waiting on led kit from joe for one...
the other came with bad meters...
i have found a reasonable replacement...
used edison filaments for light
reprinted the dials...
whittled on front cover...
getting there....
 

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May 25, 2023
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Mount Vernon, WA
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Bikes, baseball and big amps.
Good going Gene. Did you go with white? PS I think I have this pl400 tamed after going through and replacing all the 10W resistors, and the .001 capacitor. I went ahead and installed the WA output relay, and the DBT has quit lighting up. My final challenge is the right channel has .350 bias, and the left has zero. I have a new set of MJ21196G coming, and I hope that solves that issue. Also waiting on a couple WO control boards for another PL400 and PL400II from Joe.
 
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Mount Vernon, WA
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Bikes, baseball and big amps.
Well, I'm back, did you miss me? hahaha. So here is where I am at. The PL400 with PL14 board. I have now replaced the transistors with new 96's, (bottom row are 410's). My DC offset on both channels is almost undetectable. The Bias on the left channel is set at .360, however the right channel bias is at .550 and is not adjustable with the pot. The amp fires up, the output relay does it's job, and the tight channel (farthest from the transformer) R25 resistor slowly burns up. I takes a couple minutes, but eventually it gets super hot and smokes. I've replaced and the symptom remains the same. Anyone know what is causing too much voltage to that resistor? I traced the schematic, but don't see a clue.
 

Gepetto

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Well, I'm back, did you miss me? hahaha. So here is where I am at. The PL400 with PL14 board. I have now replaced the transistors with new 96's, (bottom row are 410's). My DC offset on both channels is almost undetectable. The Bias on the left channel is set at .360, however the right channel bias is at .550 and is not adjustable with the pot. The amp fires up, the output relay does it's job, and the tight channel (farthest from the transformer) R25 resistor slowly burns up. I takes a couple minutes, but eventually it gets super hot and smokes. I've replaced and the symptom remains the same. Anyone know what is causing too much voltage to that resistor? I traced the schematic, but don't see a clue.
Please include a snippet of the schematic you are referencing for the R25 callout. PL has many schematics, none very consistent. We want to get to the heart of the matter without a wild goose chase. 0.550 bias is not a good situation. Likely caused by a defective Q7 or Q10 on the control board schematic.

1712667059943.png
 
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May 25, 2023
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Mount Vernon, WA
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Bikes, baseball and big amps.
I swapped all the transistors from side to side, and there is no change. Also took bias readings from both 10ohm resistors on the right channel. The one closest to the end of the cabinet reads .550, the one on the other side reads .07. Neither respond to adjustment of the pot.
 

mlucitt

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Jacksonville, FL
Did you replace the the 10 Ohm resistors (R38 and R39) on the backplane?
What is the voltage on R25? Is it positive or negative referenced to Ground?
Is you lift on leg of D12, does R2 still get hot?

The amplifier might be in a permanent state of "current limiting". If you remove the yellow wire on the control board for the Right channel, does the resistor still get hot?
 
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