Pioneer SA 9500 Left Channel Cutting Out

I recently replaced some transistors on it with some advice from this community, but now I have a new problem, or, more accurately, an old problem that I forgot it had because I hadn't used it much for a good while before the transistor job.

It will play perfectly for 30 seconds or so before the left channel fades to almost nothing. After several minutes, it might come back up, but fade back out again after around 30 seconds. It does this on all inputs and whether or not the tone controls are on.

I was checking through all the mechanicals to see if I could either cause or reverse the behavior (mostly just moving things around hoping to find a bad solder joint or something), but nothing I tried seemed to change it except for one thing - on the pre/power switch, if I bridged the "normal" (meaning not separated) and common contacts, for even a split second, the left channel would immediately kick back on to full volume and go for a minute or so before cutting back out. So thinking there was something wrong with the switch, I connected the pre in/power out jacks on the outside, but the amp exhibited the same behavior. Even more odd, it didn't seem to matter which position the switch was in. Bridging those contacts would still cause the left channel to kick on and eventually go back out.

I considered just soldering jumpers on the switch to basically set it permanently in the "normal" position, but this seems like a bad idea if I'm not 100% sure that the switch is the issue. I'd think that if the switch was at fault, the channel would only work while I had those contacts bridged and not continue to work for a period of time afterward. And it would work with pre in/power out jacks connected.

I should also mention that I cleaned up the protection relay contacts with 800 grit sandpaper and contact cleaner, so I don't believe the problem lies there.

Anyway, I'm now unsure of how to continue troubleshooting the issue.
 
If you don't use those rear panel jacks at all, try soldering the 3 switch terminals together in each channel.
If that does not get both channels working, then it isn't the switch.

The switch is pretty likely though, since it's never used, and the contacts will have oxidized badly.
If you have some Deoxit, you can try it in the switch instead of soldering the terminals together.
 

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Thanks. I was thinking that if I connected the pre out to the power in externally, that I was taking the switch out of the equation, but reading the schematic again (and I've pored over it quite a bit, so I don't know why this eluded me), I see it's still going through the switch. And thinking about it further, I have no idea what gave me the idea that it wouldn't be.

I did try my best to get some contact cleaner in there and work the switch back and forth, but I know that can only do so much. I think I'll try to take the switch apart and clean it. Because if that doesn't work, I can just solder the wires.
 
I was saying that soldering the terminals is a good next test.
If that does not get it to work, then the problem is elsewhere.
Just jumper and tack solder all 3 terminal per channel with a short bare wire.
 
Yes, I understood that. I was saying that I thought I was accomplishing the same by connecting the jacks, but realized that I definitely wasn't.

I took the switch apart last night. It was pretty corroded, so I'm thinking that's the problem. I cleaned up the contacts with sandpaper and contact cleaner, nice and shiny now. I just need to wire it back up. If that doesn't fix it, I'll try soldering the contacts.
 
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Then it could be a wire connection at the near end or far end. Or is this a pcb mounted switch?
When you say 30 seconds, is that 30 seconds after power is applied, or what?

Or it could be one of the several electrolytic capacitors that are connected to these points.
If the capacitor connections look ok, you might try replacing the capacitors that are directly
connected to the preamp output or the power amp input.

However if the behavior is the same with the contacts shorted, it may not have to do with the switch at all,
and some more testing is then in order. If you have a scope, monitor the signal (with AC coupling),
and find the point where there is no dropout at all. Then work forward from that point to later in the signal path.

Just leave the shots on the switch until you find the problem.
 
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I should also mention that I cleaned up the protection relay contacts with 800 grit sandpaper and contact cleaner, so I don't believe the problem lies there.
Did you cross connect the left and right channel preout/main in to see the problem is the premp or power amp section?
If the problem is the power amp section, my suggestion is use a headphone to make sure the speaker selector is not the culprit, and Bridging the output relay contacts to confirm if the relay is ok.
 
If the capacitor connections look ok, you might try replacing the capacitors that are directly
connected to the preamp output or the power amp input.
I would join in on this advice - these capacitors are old and can fail in more than one way.
The 30 seconds delay would hint towards some DC buildup or drift somewhere in poweramp input stage (that DC can be shorted by "bridging the switch") but instead of spending time on attempting to track down the failure I would consider faster and cheaper to replace the 3 capacitors before Q1 (C1, C3 and C5) and resolder all resistors in that area as a starting point.
Checking 35V power on pin 5 would also do no harm.
sa-9500_poweramp_input.jpg
 
Did you cross connect the left and right channel preout/main in to see the problem is the premp or power amp section?
If the problem is the power amp section, my suggestion is use a headphone to make sure the speaker selector is not the culprit, and Bridging the output relay contacts to confirm if the relay is ok.

I ran a source straight through the power in jack without any bridging of the switch contacts, and that worked perfectly. So it seems the problem lies before the power amp. Also checked with headphones and get the same behavior as through the speakers, so I think we can discount the speaker switch. I'll try bridging the relay contacts, but I'd be very surprised if the issue is there. I cleaned those contacts pretty thoroughly.

Madis64, I did a recap when I first got the amp, a little less than 10 years ago, so those caps are all fairly new. Also, it seems to work perfectly through the power amp only. Voltages are good too, all within 1V.
 
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Did you cross connect the left and right channel preout/main in to see the problem is the premp or power amp section?
If the MODE selector is on REV position and the problem go to right channel, the culprit is on the left channel before the switch.
If the problem stay on left channel, then the culprit should be on left channel after the switch.
 
If the MODE selector is on REV position and the problem go to right channel, the culprit is on the left channel before the switch.
I would doubt that - all the active circuitry of the preamplifier is after the mode switch (where you swap or mono the inputs) so in REV position you are just swapping the RCA inputs (channels, to be correct). Easy way of testing would be to set the mode to mono.
 
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So as I was checking some things out last night I bumped the volume pot (not bumped up the volume, but the pot itself) and the left channel came on briefly. After it went back out, I tried something else that was suggested to me, which was to jack the volume up to see if that brings the left channel back, which it did. Again, briefly. But another try or two and the channel stayed on for quite a while, until I turned it off. I took the pot apart to clean up, but I didn't see any visible corrosion. I also had to remake one of the cables to it because of a brittle wire (I was hoping maybe that was the problem), but I'm seeing the same thing. Channel goes out and I give a few quick jacks with the volume knob and it comes back.