Switching power supply problem

[keith13d]

I'm looking for some knowledge here. I have a Sony Sa-Wx700 Sub woofer that is giving me some grief. I have isolated the problem to the +/-12 generation circuit. (see attached image). According to the schematic, I am suppose to get +14.7 volts on the collector of Q5. I am measuring 28 volts. This is causing the 4.7 ohm resistors to burn out. The supply ac voltage is ~18 volts @ 100Khz square wave. Does that sound right? (double checking tomorrow). I basically don't understand the purpose of C860(0.082uF) and D836. Any insight would be greatly appreciated.

[luka]

I think that upper rail is charged via C820 in positive, while negative period comes, that diode block the positive voltage. Cap is discharged via D836

As long as everything else is ok, and squere wave is not higher then it should be, and all those diode are ok, it should still work. My guess that C820 does not discharge. coz it sound you have voltage doubler somewhere, since 28v is 2x 14v

Check if D836 is ok