Phase Linear 400 bias, advice

[Menno1000]

Hi all,

Wel i did sone work. Intalled the WA speaker relay, WOPL caps, fresh MJ21196G on the outputs, fresh WO bias transistors, emmiter resistors and 10ohm bias resostors. Works again and sounds nice but.. the bias of the right channel remains low ánd different on pos neg side. Seen from the inside: Right row :179mV left row 205mV. This is with pot at max setting.

What i also did, tested and tried:
Check and replace D6 D7 D8. Funny, when i replaced with new 2n4148 the bias dropped actually. I put the old ones back in.
I replaced all elco's , one by one. Sounds better but no difference in bias.
What i measured that after R18 the DC is significant lower than on the good channel.

Anyone has suggetions how to debug this any further?

Thanks for your attention!

 

[gene french]

tough one...just like christine...
i worked on her several months on and off...with 2 different boards...both boards recapped....
i wish you the best!!!!

 

[Menno1000]

And did you fix it with christine?

 

[gene french]

And did you fix it with christine?

i am not the sharpest crayon in the box...
but no, i did not!!!...i enjoyed immensely trying...i appreciate a challenge...after all...this is a learning hobby for me....finally, i gave up...when i started walking in circles...i ordered the wo control board and had the mj...96s for such an occasion...it is pulling voight pipes duty as we speak...i still have 3 stock 400s and 3 wopls....i derive great pleasure from this!!!

 

[Menno1000]

Ok. Yes i understand. Did you have under bias too?

 

[gene french]

one side...just like you...
even the 220 version transformer...
kinda like de ja vous

 

[Menno1000]

What did you do or try?

 

[gene french]

i changed stuff, until i kept doing more damage than good...
then i switched boards and repeated the process....
this took place over several weeks...i would lay it down and start again...
my cooling off period!!!
if you have the manual...use a light and trace the trace map...maybe a run is broken or something...
i tried all i knew...and what others suggested...
finally, i relented...
lol
plus you have no idea what others did ahead of you, when they tried to repair...
many of ebay amps are ones others gave up on too...
lololol
i was warned...but, i had to find out for myself!!!
and it was fun trying...
there are a bunch of really smart guys here...they helped me a lot!!!!

 

[Menno1000]

So with the second board the same bias thing was going on?

 

[gene french]

similar ...it all started with onesided bias problem sililar to you.....that i could not solve. .got in a rabbit hole....i still have one unadulterated 14a board if i ever need it...and several scrap boards that i cant trust...i use my system daily...often 8 to 5...often at high volume levels...reliability is an issue...
i will not chase down many more problems on a stock board...if not easily repairable for me...i will upgrade...i have wa dcp boards...new power caps...led meter boards in all...at a minimun...

 

[gene french]

and of course...all are recapped...

 

[Menno1000]

Ok. Well i am not alone in this)
Was the altered bias fault looking inside at the amp the right channel? power supply left , bindings to the right.

 

[gene french]

left channel...and as you, it played...just could not adjust to specs...some before/after pics...i also had a straight pl14 board that i messed with...problems just kept multiplying. .hole kept getting deeper...
lol

 

[mlucitt]

Right row :179mV left row 205mV. This is with pot at max setting.

It could be the Bias Adjustment Potentiometer, or several other components on that 50 year old board.
It is not advisable to run the amplifier with incorrect bias voltage. The sound will not be affected until it is; the bias is controlling the DC voltage supplied to the Output Transistors, it has nothing to do with the audio sound.

Bite the bullet and order yourself a new White Oak Audio Rev G1 "Dual Mono" Control Board; then you can say, "Why didn't I do this in the first place?"

 

[Menno1000]

Thanks. Yes, i got the Rev G1 board already here.
The next step.

 

[wattsabundant]

Hi all,

Wel i did sone work. Intalled the WA speaker relay, WOPL caps, fresh MJ21196G on the outputs, fresh WO bias transistors, emmiter resistors and 10ohm bias resostors. Works again and sounds nice but.. the bias of the right channel remains low ánd different on pos neg side. Seen from the inside: Right row :179mV left row 205mV. This is with pot at max setting.

What i also did, tested and tried:
Check and replace D6 D7 D8. Funny, when i replaced with new 2n4148 the bias dropped actually. I put the old ones back in.
I replaced all elco's , one by one. Sounds better but no difference in bias.
What i measured that after R18 the DC is significant lower than on the good channel.

Anyone has suggetions how to debug this any further?

Thanks for your attention!

If you have an accurate meter check the 10 ohm resistor values.

 

[Menno1000]

Thanks, I did check the 10 ohm resistors with an accurate meter. They are brand new mf and spot on 10 ohm.

 

[gene french]

good luck!!!!

 

[Izzi]

Check the bias variable resistor, their carbon to pin contact surface might fail with time, moisture and thermal cycles.

 

[gene french]

how did this work out???
enjoying the music??