Phase Linear 400 bias adjustment, odd values

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menno1000

Member
Joined 2020
#1
Hi All,
I am an intermediate diy-er.

I have here a PL 400. I got it in, checked dc output and was on channel A close to zero on channel B 14 mv.
The amp sounded pretty nice already in this state.

I started to swap the filter caps. One cap waz doen to 3000 uF. And after the swapped the caps ( to Rubycon 10000 uF) i measured the bias.
Channel A was easy to get right to 0.380 accros the 10 ohm resistor.
Channel B was really low ( 0,0 50) i could not get it any higher than .220 with bias pot turned all the way up.

Any suggestions where to start checking?
Driverboard recap? Or powertransistors?

Thanks for your help!


Menno
 
#3
The voltages are are allright . No changes there. I dont think the cap swap influenced the bias. I did not test it before the fresh caps. Dc on ouput did not change too.
The amp still sounds pretty good too. Better than before actually.
 
#4
There's no reason for the bias to suddenly have a problem just from replacing the power supply capacitors.
The rail DC voltage might just go up a bit. The bias pot could be dirty or worn, and intermittent.
 
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