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Soft power switch troubleshooting and repair
The following information is kind contribution by our forum members Lydux and Caius. This fix was applied to X68000 ExpertHD and SUPER.
The symptom: System powers on fine, but does not go into stand-by when the soft-power button is pressed. The screen fades, Power LED does not change from Green to Red and the system fan keeps spinning. The same happens while trying to use a regular ATX PSU.
The problem is almost sure located on the IO board.
Here is the power on/off logic behind the /PSON signal (named CH2 in the shematic). Click on it to enlarge.
A condition using 4 signals is used to get the X68K turn on/off correctly :
- Via the front power switch (/PSON)
- Via the RTC (Real Time Clock) alarm signal (/RTC_ALARM)
- From the expansion bus (signal /EXPWON)
- A signal from the system controller (the soft sequence you're talking about, signal /MPWOFF).
On normal operation, all these 4 signals must go logic High (+5v) to get a system shutdown. The problem might be due to a signal that is always stuck Low.