Pioneer SX-1250 distortion when balance knob is full right

[Kingbleu]

When using a scope I can see distortion / oscillation on the right channel when the balance knob is full right.
If I reverse the jumpers I can see distortion on the right channel when the balance knob is full left.
If I switch the power amp boards the distortion stays on the right channel.
Pushing all the buttons switches and other functions does not affect the distortion / oscillation that I can see on the scope.
Main rails, Bias, and DC Offset are all set to specs.

I recently did a full restoration using all the parts from mattsd's recap list. I also replaced the output transistors. The big filter caps were done a few years back. Keep in mind, that this problem was apparent before the restoration and I swore that doing a full restoration would fix the issue. I also cleaned all the pots and switches.

Any ideas.

 

[Kingbleu]

Here is a link to watch a live video of it

 

[rcs16]

First determine if the oscillations are in the pre-amp or power amp section by removing the jumpers and scoping the pre-amp o/p's

 

[Kingbleu]

After removing the jumpers and scoping the pre-amp outputs the problem is exactly the same. So it has been determined that the problem is in the pre-amp section. I also redid the video above with my signal generator set at the right frequency this shows the oscillation more clearly.

 

[I LIKE MUSIC]

Take this for what it is worth, but it looks like you have your oscilloscope set to trigger (sync) to the upper trace. When the upper trace goes away there is no triggering (sync) for the lower trace and your are just seeing the result of the horizontal sweep running free.


As a test, set the triggering to the lower trace and see what happens.

 

[Kingbleu]

.

 

[llwhtt]

I LIKE MUSIC is correct, the scope is being triggered by the left channel and when the left channel signal goes away the scope doesn't know what to do. I can't tell from your picture what trigger choices you have. What is the switch in the upper right corner under the BNC connector?

Craig

 

[I LIKE MUSIC]

From your picture, it does appear that your scope is set to trigger (make the trace stable) using channel one (the upper trace on your scope.

If you set the switch to channel 2, it is likely that what you see will change channels (balance control position)



In simple terms, the scope must know when to start the trace so the trace is stable. It can get this information from either channel 1 or channel 2, depending on the switch position. When the waveform on the selected channel goes away (as it does when you move the balance control) there is no information for the scope to use to produce a stable display.

 

[Kingbleu]

You nailed it. When I switch the trigger to channel two the oscillation also switches channels. Thanks for teaching a beginner. Now I will button her up and connect the HPM100's.

 

[I LIKE MUSIC]

BTW, it is not oscillation that your are seeing, it is just the visual result of a none triggered (synced) waveform.

 

[Kingbleu]

Not oscillation got it thanks I LIKE MUSIC. And just to confirm after the restorarion the SX-1250 sounds more amazing than ever. Taking the time to perform a full restoration paid dividends.

 

[rcs16]

Good call, the old pilot error