Pioneer SX-880 DC Offset High

[Brivan]

Hello, all

I have a Pioneer SX-880 on the bench that is exhibiting high negative DC offset. The receiver originally had two blown 1-amp fuses: FU2 & FU3 (I chuckle whenever I see FU2). I found D26 shorted, as well as Q29 & Q30 with shorted junctions. All were replaced with correct modern equivalents. I now have -41.5vdc coming from Q29 and +41.9vdc coming from Q30. With the P & Q jumpers in place, I'm getting about -500mV on both pins 5 & 12, with VR5 & VR6 fully clockwise. With the P & Q jumpers removed, I'm able to get about -130mV at pins 5 & 12 - still off a ways.

Protection relay has not engaged at any time so far (not sure where the threshold is and I have yet to check the PA3004 pins to make sure those voltages are all good - except for the DC offset input). I have correct voltages everywhere else I've checked, except for pins 5 & 12. Output transistors have all checked good. R255, 256, 261 & 262 test good. Am I simply at the point of increasing the value of R241 & 242 (with the P & Q jumpers removed)? I can do that, but there's obviously something else preventing me from obtaining zero balance and it would be good to find out what that is.

Any help would be appreciated! Thanks!

 

[Brivan]

Crickets. Chuckle-chuckle. I do miss Mark being around (Audiokarma's own "Soup Nazi"!). And Merlynski. Haven't heard any updates on Mark, but I'm hoping he's on the mend.

 

[Brivan]

I replaced R241 & R242 with 240K-ohm resistors, which brought DC offset in both channels down to nearly zero, with P & Q jumpers removed and VR5 & VR6 centered. I read some older posts from Mark about the +41.5v & -41.4v power supply differences (I had a 0.5v difference) contributing to the high negative DC offset I was experiencing. I didn't pull/test R411 & R412, but suspect the differences in voltage are likely being caused by resistance variations between the two. Not a big deal, so I left them as-is.

I was now getting correct voltages on the PA3004, with exception of pin 3 (around 14v) and pin 8 (fluctuating). I replaced C417 and tested Q37 and found it to have an open leg. It was replaced with a KSC2383 and the protection relay now engages.

Next I replaced C418 with a 220uf non-polarized capacitor to hopefully prevent false protection triggering at higher volumes.