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Soft-power troubleshooting and repair

The following information is kind contribution by our forum members Lydux and Caius and this fix was applied to X68000 SUPER and Expert HD.

The symptoms: X68000 powers on fine, but does not shut down when the soft-power button is pressed. The screen fades, Power LED does not change from Green to Red and the PSU fan keeps spinning. The same happens with a regular ATX PSU.

If this is your case then the problem is almost surely located on the IO board.

Here is the power on/off logic behind the /PSON signal (named CH2 in the schematics). Click on them to enlarge.

A condition using 4 signals is used to get the X68K turn on/off correctly :

  • Via the RTC (Real Time Clock) alarm signal (/RTC_ALARM)
  • From the expansion bus (/EXPWON)
  • Via the front power switch (/PSON)
  • Via the soft-sequence signal from the system controller (/MPWOFF)

In normal operation, all these 4 signals must go to logic High (+5v) to get a system shutdown. The problem might be due to a signal that is always stuck Low.

If you look at the Buffered IO part in the schematics, 3 of these signals are connected to the MFP GPIP port.

They can be queried via software and are connected like this :


A program for testing these signals can be found here :

X68k Power Test

Execute it more than one time. You should get :

  • RTC_ALARM = 1 or 0, depending on the current RTC configuration.
  • EXPWON = 1
  • PSON = 0

If RTC_ALARM always stays 0 :

  • Bad RTC chip (RP5C15)

If EXPWON = 0 :

  • Check the expansion riser, or remove it.
  • Check the “Remote” plug on the back, if I remember right, it's also connected to this line.

If PSON = 1 :

  • Damaged resistor R54 (should act as pullup)
  • Bad power switch

And if all is ok, it could be :

  • Bad IC28 (74LS08) (stuck output).
  • Bad Q9 transistor (shorted).
  • Bad C64 capacitor (generating noise).
  • IC29 (74LS244) might have bad output on pin 5.
  • The power signal from the main board, which I miss infos actually. A fix to this could be soldering a 10K ohm resistor between IC28 pin 10 and 14.
x68000/soft_power_switch_troubleshooting_and_repair.txt · Last modified: 2019/08/27 20:45 by