Pioneer SX-680 Audio muted in Amp circuit - debug help needed!

rocknroller

Active Member
Problem: All audio sources are muted in the power amp section - no output from speakers and no blown fuses.
Using Aux as the input source path, I get audio past the preamp at the test point jumpers 72/44 and 43/72. Audio through caps C511/C512, but not through Q503/504 (Listed as 2SK117-GR or Y, 2SK34-C or D - I guess these are audio gates?). See schematic below

Pioneer sx-680 amp section.jpg
Following back through D503/504 (which are good) to the left hand side of the circuit, this seems like the controller of those gates but Q502 collector has a bad voltage of -8v. Why?? This may not be the ultimate source of the amp issue, but seems to be why the audio isn't progressing any further in the circuit.
 
Update - Mute source problem found (still have major amp issues though see below) - Mute source was caused by a open 33 ohm resistor in power supply section, causing a 12v output instead of 45v, just in case anyone else has this issue in the future.
Pionner sx-680 Power Supply.jpg

**Now back to more amp issues.
Now have good audio through Q503/504 and into Q501/Q502 but not out. Good audio at emitter junction on both, but no audio out left channel base (0.8v) and audio out right channel base (14.6V) is heavy distorted and lower gain than the input. I had a used replacement that I put in 502 and that cleared up much of the distortion on the base output, changed that voltage down to 11.5 after swap, but final output from STK chip still low and heavily distorted. Input voltages to STK chip (both) look wacked. Swapped the original Q502 to left channel Q503 but make zero improvement, so something on the left side is pulling that base output voltage down to 0.8 and it's not Q503 itself.
Pionner sx-680 amp.jpg
 
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