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Pioneer SX-780 Goes Nuts for a Second

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Artstronauts

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I have an SX-780 that plays fine... mostly. After around 20 minutes of use, the volume of the right channel will instantly increase (by a lot) with treble-y distortion... for ONE SECOND. Then, back to normal.

1. It has been serviced and had STK-0050s replaced last year.
2. It went back to the tech for this issue, he claims he ran it for multiple days with no issues.
3. It lives in the same environment (6 feet from a computer and router) as my other vintage receiver, a Sony STR-V4, which has no issues.
4. Normal reading on power meters, but during one of these events, I haven't been fast enough to see the right-hand meter.
5. Occurs with either onboard or external phono preamp through AUX.

Thoughts?
 
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Have you tried heating the STK module for the RH channel to see if you can induce the fault? If not the STK, then try heating some of the transistors in that channel.
 
Jefft91 said:
Have you tried heating the STK module for the RH channel to see if you can induce the fault? If not the STK, then try heating some of the transistors in that channel.
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I have not—but I think that if it was heat, the problem would persist longer than one second as the receiver heats up. After this issue occurs, you might not hear it again for a while. Actually, the tech did try this, with no results.
 
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Artstronauts said:
I have not—but I think that if it was heat, the problem would persist longer than one second as the receiver heats up. After this issue occurs, you might not hear it again for a while. Actually, the tech did try this, with no results.
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Yes, your logic makes sense. Doesn't present as a thermally induced problem. Have you tried tapping components? If it is mechanical, maybe you can narrow it down that way.
 
almost sounds like something in the feedback loop is going open. Don't have a schematic to refer to here but are the tone controls by chance in the loop on this? If so it might be a problem with the treble pot or just a cracked solder joint somewhere. It goes open when it hits some magical temperature range and "fixes" itself after a moment.
 
gadget73 said:
almost sounds like something in the feedback loop is going open. Don't have a schematic to refer to here but are the tone controls by chance in the loop on this? If so it might be a problem with the treble pot or just a cracked solder joint somewhere. It goes open when it hits some magical temperature range and "fixes" itself after a moment.
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Question is, if temp related, what makes it stop so quickly and wait a good length of time before reoccuring?
 
No idea, but something losing connection kinda fits the symptoms. Likely could reproduce it with gentle taps on the board with a chopstick.
 
Do you know where your tech sourced the STK module? Are you sure it is authentic? I was under the impression that all that was available on the market currently are knockoffs and counterfeits. I didn't even want to chance it, so I ordered the discrete component kits invented by someone here at audiokarma
 
He says he's had a good source for the STKs for years. They may not be authentic, they may be good knock-offs, who knows? He's an old Pioneer warranty repair guy, so I take him at his word. If they turn out to be the culprit, I'll do the discrete components (already assembled). They weren't on my radar at the time.
 
Artstronauts said:
He says he's had a good source for the STKs for years. They may not be authentic, they may be good knock-offs, who knows? He's an old Pioneer warranty repair guy, so I take him at his word. If they turn out to be the culprit, I'll do the discrete components (already assembled). They weren't on my radar at the time.
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Sorry, I'm not trying to make it sound like he isn't trustworthy or anything. I just know I just got done doing a lot of research on STK modules, and the consensus I saw was almost none of the ones available today are authentic, and the quality of the of the ones offered is dubious at best. That is why I am doing the discrete conversion on mine when they arrive this week. Hopefully you get it solved
 
Dan13105 said:
Sorry, I'm not trying to make it sound like he isn't trustworthy or anything. I just know I just got done doing a lot of research on STK modules, and the consensus I saw was almost none of the ones available today are authentic, and the quality of the of the ones offered is dubious at best. That is why I am doing the discrete conversion on mine when they arrive this week. Hopefully you get it solved
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Oh, I know. Sadly, this is one of those deals where you take your car to the mechanic and it won't make the noise.
 
Artstronauts said:
Oh, I know. Sadly, this is one of those deals where you take your car to the mechanic and it won't make the noise.
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Unfortunately I know all to well, but from the other side. I am a Ford tech. So I also understand his side a little. Can't fix what you can't diagnose, and the first step of diagnosing is verifying the concern.
 
One thread per issue please. You already have a duplicate thread going on in the Pioneer forum, in which you've received several helpful responses. Duplicate threads mean that nobody can consistently track what other advice people have given and what you've done.

Pioneer SX-780 Goes Nuts for a Second

I have an SX-780 that plays fine... mostly. After around 20 minutes of use, the volume of the right channel will instantly increase (by a lot) with treble-y distortion... for ONE SECOND. Then, back to normal. 1. It has been serviced and had STK-0050s replaced last year. 2. It went back to the...
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