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Aporte original por: Michael Karcher ,


I troublehot (and fixed!) that very problem just the other day. You can get schematics for that unit at electrotanya. The symptom you describe happens when the main voltage does not turn on (quickly enough) when the unit leaves standby mode. The start-up process is actually quite complex (possibly it is meant as short-circuit protection): Power is turned on by a high level on the base of Q951. The high level there turns on the transistor. The first thing that happens is that the main power rails (+9V, +6.2V and +3.3V) are pre-charged through the diodes D951 and D952 (a dual diode). The total current available to pre-charge the power rails is limit by R950-R953 to around 20mA. Only when all of the power rails are charged to around 1.0V, the emitter voltage at Q951 is high enough to turn on Q953 which drivers the main power switch transistor Q952. The +6.2V and +3.3V regulators are powered from the output of Q952 and thus also turn on after Q952 turns on.

In the case of my broken unit, the pre-charge didn’t manage to bring up the +9V rail above 100mV (which can be measured for example at pin 1 of IC302). This was caused by excessive leakage current in C329. I temporarily “fixed” it by accidentally shorting the switched +9V line to the unswitched +9V line, which slightly reformed the capacitor. The permanent fix is to replace the cap.