Roland KC-550 mute circuit

[soundinvst]

Thanks for the forum! Googling my music electronic questions seems to always lead me back here.

I need help with a KC-550 that seems to have a mute circuit problem. I have replaced 4 jfets on the preamp board (Q18-21) and one on PS-1 (Q29) to get from utter silence to low output to the main speaker. The headphone output is loud and clear.

I made an audio probe to plug into a mini amp and traced the signal through the jack board into the preamp at CN1 out at CN17 to the PS-1 board all the way to the Drain pin on Q29. No signal to the Source or there after with one notable exception: a brief second at power-up and power-down.

I have ~-7v on the Gate which I figure is turning the fet "off" (with the exception as noted above). What is supposed to turn it "on" again? What part, if any, does the "Mute TR" circuit figure into this? Does TR stand for trigger or transistor (as in 2SC2878A)? Why is the Headphone circuit unaffected?

I read ~-8v on pin 8, and ~+15v on pin 9 of CN12 of the PS board (the mute fet and TR circuits, respectively). All other voltages appear to be normal.

Thanks for any help you can provide.

RM

[Dave H]

The FET is turned on when C13 on the PS Board charges up. -8V is an odd voltage for CN12 pin 8. It should be either -15V or +15V. Check if C13 is leaky. MUTE TR connects to a load of transistors so I guess it stands for mute transistors.

Edit:
Does it unmute if you short Q4 base to emitter on the PS Board? MUTE FET should measure +15V and MUTE TR -15V with Q4 B E shorted.

[Jazz P Bass]

Why ignore the fact that the -15 rail is low?

Forget the mute circuit until you get the power supply fixed.

[Dave H]

Why is the Headphone circuit unaffected?

I read ~-8v on pin 8, and ~+15v on pin 9 of CN12 of the PS board (the mute fet and TR circuits, respectively). All other voltages appear to be normal.

The headphone mute transistor (Q31,Q32) are marked 'NIU' (Not In Use?) on the schematic and are not on the parts list.

Jazz,
pin 8 and 9 on CN12 of the PS board are the mute signals.

[soundinvst]

Thanks for the input...

Dave was on the right track when he mentioned Q4, which happened to be crossed up with Q41. Fixing that got the polarities and voltage right on the mute fet and mute tr circuits and she sprang to life. The LV +/-15v rails (pins 2 and 7) were never out of bounds. I double-checked; still good.

I've got some midrange distortion going on though. Almost sounds like too much gain. There is little or no DC on the speaker, up to 5mV. Cross-over distortion maybe? I sprayed all the I/O jacks but it didn't help the problem