Pioneer SX-950 Recap Issue--huge pop when I flip tone switch

[EastPoint]

Hi folks,

I've been recapping a Pioneer SX-950 with no issues so far, but now I have one. I have recapped the power supply, protection board, phono board, AWG-038, and AWG-039. I replaced some transistors but not all of them. Currently the 950 plays fine, but when I flip the tone switch (middle in the group of 3) I get an incredible pop from my speakers and the receiver briefly goes into protection. The volume control doesn't affect the pop.

All the electrolytic and tantalum caps on AWG-039 are new. I triple-checked polarity against the silk-screening and it is correct. Both 1312 transistors were replaced with KSC1845s, and they appear to be in correctly. I checked for solder issues and everything looks fine. I did catch one mistake--I had put in a 10 uf cap for C19 instead of a 100 uf cap, but I corrected that and it is still popping. Any ideas?

 

[Watthour]

Did you happen to review this thread?

http://audiokarma.org/forums/index.php?threads/pop-in-pioneer-sx-950.499056/

Right around Page 5 or so I attempted to simulate the problem.

 

[Watthour]

Also note post 198 and the subsequent discussions.

C23/24 are also at play, and if the schematics are not checked carefully it might seem that those are 22 µF instead of 2.2 µF.

 

[EastPoint]

I took a look at that thread, but didn't see anything that would necessarily help me. I just replaced the caps on the board with the same uf-no 22 uf here. The pop is the same whether or not volume is up or down, and the pop happens as many times as I flip the switch. It's a hell of a pop. No other pops on startup or shutdown. Only when I flip the tone switch.

 

[UncleBingo]

Iirc, there is a silkscreen error on that board that shows cap polarity backwards, but it has been a while. I think it's the cap in pic 3 closest to the bottom edge of the board at the lower right. I had this same problem with 2 SX 850 or 950 a few years ago and I seem to remember that solving it. Check the schematic to be sure.
Also, 1 channel only or both?

 

[EastPoint]

Both channels.

 

[EastPoint]

Iirc, there is a silkscreen error on that board that shows cap polarity backwards, but it has been a while. I think it's the cap in pic 3 closest to the bottom edge of the board at the lower right. I had this same problem with 2 SX 850 or 950 a few years ago and I seem to remember that solving it. Check the schematic to be sure.
Also, 1 channel only or both?

Thank you! That cap was in backwards. It's not really a silkscreen error, just a bit confusing. The pop is now doing the same thing that the guy's 950 did in that 13-page thread, where there's a little pop if you flip the switch right after turn-on, but if you wait ten seconds then it's fine. Looks like we're good.

 

[Watthour]

There is a lot crammed in that space.

 

[EastPoint]

There sure is. I should've just put a film cap in there initially, but I ran out and was using electrolytics. I'm just glad you figured it out. It was driving me nuts.

 

[Watthour]

YOU figured it out. We just stood back and pointed.

 

[EastPoint]

Well, I really appreciate it, and I'm very glad this didn't turn into a 13-page monstrosity like that last one. I try so hard to get polarity right, but I'll be even more careful in the future.

 

[Watthour]

Wait until you run into one of these:

 

[rcs16]

The pop is now doing the same thing that the guy's 950 did in that 13-page thread, where there's a little pop if you flip the switch right after turn-on, but if you wait ten seconds then it's fine. Looks like we're good.

you managed to dig that far back and read that thread. good to go imo.
If everyone used BP ecaps & films, we would not have as many backwards cap problems
Pioneer could not afford to use more expensive film caps where we can easily use them now for refurb and repair.