phase linear 400

gene french

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If it's a stock board, one of the front end differential transistors is failing. On every PL14 board I've seen in the last couple years, Q3 or Q4 has failed. I'm pretty sure it's Q4. Replace both with MPSA93. Make sure to read the service bulletins for pin outs in the service manual.
will do...
thanks so much...
it is stock but recapped board...
 

gene french

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well, i changed out q3 and q4 on both channels...
and i also did the latchup mod...
it ran a full day...then quit working again....at first coming on intermittenly, then stopped altogether ...
the led on dcp is very dim...relay will not cycle...
i ordered a whiteoak replacement control board...however i would still like to solve the puzzle while i wait ...
it is a spare...so i have time...
 

ABebar

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Anyone got time to help me troubleshoot a stock(ish) PL400 issue?

I picked up an 8-fin 400 on ebay for $250. Guy said it was working fine when he pulled it. I took his word for it and immediately busted her open (without testing anything first) and put in new filter caps from Joe and a DC protect from Don. Powered it up on the DBT and Variac all the way up to 120vac and didn't hear the relay click. Checked the DCP terminal voltages and found over 35vdc output on both channels.

Any suggestions on where to start? I obviously should have tested the unit first to know if it was something I did, or if I got a unit that was already passing DC, but that ship has sailed.
 

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Sunnbobb

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I had this issue once. Leaving the dbt connected, I took the variac out of the system and powered up. Everything was groovy after that.
 

ABebar

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Yep. I actually had to turn off the overhead light over my workspace just to insure that it was coming on at all as I was increasing the Variac voltage. It was a super soft orange glow that faded after each increment of the dial. Normal startup. I was shocked that the relay didn't click.
 

gene french

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i had a similar problem when one of my stock units quit....had dc on rite channel...mr don recommended q3 and q4 changed...i had a shorted speaker that inflicted more damage...i ordered a wo control board...those boards were fun to fix...and i learned a lot..but they are 50 years old...and not reliable imho...i still have 2 stock..all others converted...i found that when it starts failing...its like a runaway train...
 

ABebar

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Another item that I didn't think was worth mentioning is that when I flipped the control board over to install the DCP ground and DC wires to the backplane network I broke the white/green wire off of the right channel right at the control board terminal. I stripped and resoldered it and didn't think much of it. This unit looked fairly stock internally. Board looked clean and unmolested.
 

ABebar

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I had this issue once. Leaving the dbt connected, I took the variac out of the system and powered up. Everything was groovy after that.
No luck. Removed the Variac, plugged the DBT straight into a surge protector power strip and got the exact same results - 38vdc at both of the output terminals.
 

ABebar

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Honestly, I didn't get that far. I only checked the terminals on the DC protect board, confirmed the proper AC and DC supply voltages, found the +38v offset and stopped.
I did just realize that I have a blown AGX-5 line fuse. Not sure if that is a cause or symptom.
 

ABebar

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Now we're getting somewhere. Replaced the fuse, started to bring up slowly on the Variac. Didn't get very far and saw the VU meters pop up to -1 and flutter and got a bright dbt bulb. Shut her back down before the fuse blew.
My hypothesis? Something is definitely wrong. :)
 
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