Pioneer SX-727 Preamp DC Fluctuation

[ivandezande]

Got a SX-727 going in and out of protection. I narrowed it down to the left channel output from the AF/preamp unit on pin 14 going all over the place from -0.5 to 0.5 while the right channel output is about zero. I replaced all the transistors (KSC1845 and KSA992) first, no change, then all the caps, no change, and then even the FET's with real replacements. No change. Checked all my orientations multiple times.

Is this through hole board having issues? Should I re-replace the transistors?

 

[EastPoint]

If you unplug the rear preamp jumpers does it still do it?

 

[ivandezande]

If you unplug the rear preamp jumpers does it still do it?

Man….yup, that’s it. Time to restore the amp.

 

[ivandezande]

Replacing the input pair transistors on the amp board solved it.