Accuphase P-300 DC offset way to high

[pakko]

Hello everybody.

I got this Accuphase P-300 in my hands and had to rebuild the Speaker select switch and recap the boards.
Situation now is a DC offset of 2,3V on each channel. Relais are working - amp is playing on both channels.
Bias is around 170mV and not really adjustable. As I tested all transistors in both channels are OK.
But something is clearly not OK.
I hope somebody can help me search..... thanx

 

[llwhtt]

The front end runs from regulated supplies, are the supplies OK?

Craig

 

[pakko]

The front end runs from regulated supplies, are the supplies OK?

Craig

As source I used an iPod if this is the question...

 

[slimecity]

Do a search on this site for replacement transistors - I had the same issue on my P300 years ago - so sorry I cant recall the details however Echowars helpfully helped me thru replacing larger power regulator transistors on both amplifier boards, which can cause this issue.

I am surprised that the relay is closing with this voltage appearing at the outputs, given how sensitive the protect circuit is in this amp.

 

[llwhtt]

No, not the question. Check ALL of your power supply voltages.

 

[pakko]

No, not the question. Check ALL of your power supply voltages.

I gonna do this and see if everything is OK on the power supply board! Let you know...

 

[pakko]

Do a search on this site for replacement transistors - I had the same issue on my P300 years ago - so sorry I cant recall the details however Echowars helpfully helped me thru replacing larger power regulator transistors on both amplifier boards, which can cause this issue.

I am surprised that the relay is closing with this voltage appearing at the outputs, given how sensitive the protect circuit is in this amp.

I thought the same at this point. The relays is pretty tuff on this amp. I gonna check on advise!

 

[pakko]

OK - I found a faulty transistor on the power supply board. Q3 was down and I replaced it. Now the output on the +-70V rail is OK (connect 2 and 9) but the +-25V is still to high. It is by 33V at the moment (connect 4 and 6) - should be +-25V
What do you think...?

 

[pakko]

OK - DC offset and Bias are adjustable now... good!

 

[llwhtt]

The +/- 25VDC regulators are called tracking regulators. D7 in the - regulator is the reference for both the + and the - regulators. The - regulator tells the + regulator what to do, if the - regulator is off so too the + regulator. Since both are off by the same voltage it tells you the + regulator is working fine and is doing what it is told. So you need to concentrate on the - regulator first. Find out what D7 is and then measure the voltage across it.

Craig