Pioneer SX-980 with Erratic DC Offset

[Brivan]

I've have a Pioneer SX-980 with erratic DC offset on both channels. I'm able to adjust it so it "centers" around 0v, but it will vary maybe 300mV in either direction. It seems to calm down a little after warm-up, but is still very unstable and might vary as much as 50mV in either direction. Idle current can be set and stays put. Power supply voltages have been checked and are more or less within tolerance.

Additionally, the protection relay engages almost immediately after power-up. Thought it was worth a mention in case it was somehow related to the erratic offset.

The receiver powers up normally when connected to a DBT - bright bulbs for a second, then down to dim. Voltages checked with DBT bypassed.

I've run across one or two of these receivers where the offset voltages wiggles a little, but nothing like this. Would someone have a suggestion as to where I should start looking for the cause?

Thanks!

 

[Fred Ramser]

Hi, I would start by cleaning the trim pots with dioxit. I use a perm marker to mark the trim pot original setting. Apply fader lub twice. 1st application work pots, blow dry with can air, 2nd work pots, let dry for 24 to 48 hrs before power up.

 

[Brivan]

Hi, I would start by cleaning the trim pots with dioxit. I use a perm marker to mark the trim pot original setting. Apply fader lub twice. 1st application work pots, blow dry with can air, 2nd work pots, let dry for 24 to 48 hrs before power up.

That sounds like the best plan, right now. I did work the pots back and forth several times to see if that made a difference, but it didn't seem to. I also should note that the pre/out main/in jumpers have been removed in case there was any influence on the problem coming from the preamp section. I'll report my findings back here. Thanks for the reply!

 

[Brivan]

I did the cleaning procedure and powered the receiver up to see if it made any difference, but didn't notice any change - I was hoping to see even a slight difference, but the offset voltage still varies around 100mV either direction. I'll respray and rework the pots and let them sit while I work on other parts of the receiver, but I'm wondering if the QSA979 (Q1) input differential pairs are possibly the source of my problem. I seem to recall these being a potential problem area from posts I've read in the past.

 

[Brivan]

It's amazing the kind of havoc a crappy solder connection can cause. Yesterday, when I started the problem down, I took voltage readings after the receiver had been powered up for awhile, so the readings looked somewhat stable. When I started in, again, today, the readings on the -50v and -23v supplies were low and varying. I was getting correct voltage readings at the emitter of Q1 (measured on the top side of the board), but about 15 volts lower at pin 22, even though the schematic showed them as being connected. I flipped the power supply board up and was still getting a 15 volt difference between the solder around the emitter leg of Q1 and pin 22, but no voltage difference when I tested between the actual emitter leg of Q1 and pin 22. It appears the smoking gun has been found. I resoldered all of the pins on Q1, Q4 and Q5. I now have nice, steady voltages everywhere and was able to set the DC offset normally. I reset the idle current, as well - it was about 15mV too high on both channels.

Next, I'm off to the protection board to see why the relay engages almost instantly after power-up. I guess about the only thing it could be is C1 has gone bad and is no longer charging, allowing the relay coil full voltage from the get-go. Will post my results.

 

[Brivan]

I pulled and checked C1 and it tested fine. I replaced it, anyway. Next, I checked R4, R13 and Q2. All tested good and all showed correct voltages. I checked voltages to the PA3004 IC, as well as to the pins on the protection board - all were good. I monitored pin 3 of the PA3004 during power-up and it instantly went low - wasn't sure if this was normal (and most of the delay was caused by the charging of C1?). Next, I tried to force the relay board into protection mode by turning the offset fully up and fully down on one channel, but the protection board relay stayed engaged. I verified the out-of-range offset values present on pin 4 of the PA3004, so it looked like the PA3004 wasn't responding. I pulled the original PA3004, installed a socket for the replacement and installed the replacement PA3004. Problem gone. Never had one of these give me problems, before - let alone having a protection relay that wouldn't go into protection mode (it's always the other way around).