170DC inverting input phase

[ocdecio]

I rebuilt a blown amp board on a 170DC, and I am now testing with a scope before connecting any load to it. One thing that is puzzling me is that I get a perfectly formed wave, on all frequences, or a perfect square wave.
However the wave is shifted 180 degrees. The other channel doesn't do that. The new channel has most small signal transistors new, and brand new pre-drivers/drivers (the ones on heatsinks).

I checked everything I could, compared with the other channel and coudn't really see a difference. I am wondering if anyone saw this happening and could give me an idea as for the cause.

 

[dankik]

What are you using as a scope trigger, one of the two channels? I've seen some waveform inversions that were a scope triggering issue rather than a difference in phase on one channel. Do you have the phase shift coming out of the pre-amplifier stages, or only the main amp?

 

[avionic]

Look at page 9 diagram in the service manual. You have and inverting buffer and an non-inverting buffer feeding the complimentary class A

 

[ocdecio]

What are you using as a scope trigger, one of the two channels? I've seen some waveform inversions that were a scope triggering issue rather than a difference in phase on one channel. Do you have the phase shift coming out of the pre-amplifier stages, or only the main amp?

This is an amp, no preamp. I will look into possible trigger differences.

 

[ocdecio]

Look at page 9 diagram in the service manual. You have and inverting buffer and an non-inverting buffer feeding the complimentary class A

I'll look hope I can locate them. Could this be an issue with the dual FET?

 

[avionic]

Could this be an issue with the dual FET?

That would be the cascoded FET differential amp.. I don't think so.