Pioneer SX-780 Goes Nuts for a Second

[Artstronauts]

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.

 

[zebulon1]

The 780 contains those TA7136 Op-amps for the EQ and pre amp stages. I wonder if it’s causing the issue?
These chips are not aging well.
Just a guess.
Check out this current thread:

SX-750 Right Channel Phono Out

Howdy folks - I pulled an sx-750 out of storage and found that the right phono channel is dead. Sound is clean on all other inputs, so it is definitely related to phono. Continuity from input is good. I've cleaned all switches. Issue is present with headphones too and does not move with RCA...

audiokarma.org

 

[larryderouin]

ONLY the preamp stage. HA1457 is the Phono EQ IC.
Does the problem exist with the Unit in FM or AM????
Wonder if it's just the opposite, the LEFT Channel taking a dump for a second or two and coming back on. Intermittent in a transistor or STK or the TA7136P.

 

[Artstronauts]

ONLY the preamp stage. HA1457 is the Phono EQ IC.
Does the problem exist with the Unit in FM or AM????
Wonder if it's just the opposite, the LEFT Channel taking a dump for a second or two and coming back on. Intermittent in a transistor or STK or the TA7136P.

Not only the preamp stage. As I mentioned, also occurs thru AUX with an external phono preamp or other source, like an iPod or iPhone.
The overall effect is not that a channel dumps out, it's the right channel getting too loud for a second. If I turned up the volume, I could theoretically damage a speaker.

 

[larryderouin]

"Only the preamp stage" was a reference to the chips. There are 3 stages of Amplification in the 780. Equalizer (or Phono), Preamp, then amp. Phono is the only source that goes thru all 3. Everything else (Aux, Tape monitor, FM/AM )go thru Preamp and Amp. Q3 and Q4 are the TA7136 op-amps. These are suspect due to ALL sources. If it were the HA1457 then it would only have problem in Phono. Red lines denote stages. Note phono only goes to Equalizer.

 

[Artstronauts]

"Only the preamp stage" was a reference to the chips. There are 3 stages of Amplification in the 780. Equalizer (or Phono), Preamp, then amp. Phono is the only source that goes thru all 3. Everything else (Aux, Tape monitor, FM/AM )go thru Preamp and Amp. Q3 and Q4 are the TA7136 op-amps. These are suspect due to ALL sources. If it were the HA1457 then it would only have problem in Phono. Red lines denote stages. Note phono only goes to Equalizer.
View attachment 3125165

Ahhh... I see. Thanks Larry.

 

[dr*audio]

In your other thread, you said it does this even with the volume turned down. Since it does it even with the volume turned down, the problem is in the power amp stage. Most likely a noisy transistor. Buy a can of freeze spray and spray a small amount on each of the small black transistors on the power amp board, one by one, waiting at least 10 seconds for an effect. The one that causes the noise is bad.
To clarify, you are not hearing distortion, because with the volume turned down there is no audio signal to be distorted. You are hearing noise.

 

[Artstronauts]

In your other thread, you said it does this even with the volume turned down. Since it does it even with the volume turned down, the problem is in the power amp stage. Most likely a noisy transistor. Buy a can of freeze spray and spray a small amount on each of the small black transistors on the power amp board, one by one, waiting at least 10 seconds for an effect. The one that causes the noise is bad.
To clarify, you are not hearing distortion, because with the volume turned down there is no audio signal to be distorted. You are hearing noise.

To clarify, I meant "at low volume". I get what you're saying, though. I think a good test would be to run it with the volume at zero, and see if the noise occurs.