Pioneer SX 939 Speaker pops

[JD Hoffman]

Hi.. had my original Pioneer SX 939 gone over and repaired amp… seems the high/low cut … tone off switch and -20 db buttons are causing a pop in speakers when pushed in/out… so my question is can I go the deficit/faderlube or is this more of transistor replacement type of fix… my tech is 120 miles away and would like to avoid that! Lol… thanks

 

[larryderouin]

Try the De-Oxit/Faderlube 1st. The 838-939-1010 push switches are somewhat problematic. I remember there was a capacitor or 2 on the tone control board near the front of the board that could/would cause this problem, but it's been almost 10 years since I did my 939. @markthefixer would know exactly what cap(s), provided that they ARE the problem.

 

[markthefixer]

one or both channels?

go to the pre-out / amp in jumpers, phono ground a black dmm lead and probe the red dmm lead into the jumper (one at a time)
with volume turned to zero. see if there is a dc voltage on one or the other.

turn on and off the high/low cut, tone off and -20db buttons while taking readings. tone off, cuts off will read the flat amp output, tone on will read the tone amp output.

a power supply fault could alter the biasing on those boards and actually reverse the needed polarity of the coupling caps.

but you will have to open the unit bottom to read those voltages at the power supply board.

of course if the caps are original, those (c1,c2, c13, c14, c47, c48) are the dreaded CSSA Sanyo sky blue solid aluminum caps - 100% guaranteed failure to short circuits...

Open the bonnet, and the bottom and inspect for them...