Compare all the diodes and resistors from both boards, as in check D501 on one board and the other, see if any are different, sometimes not all, you get fluctuating voltages with bad voltage regulating diodes or bad solder joints on transformers or elsewhere.
It is either the PS-ON is fluctuating or the VCC that is generated by the standby power supply power transformer AUX winding is fluctuating that is why the PFC Voltage on the main caps and the 12/20V are fluctuating.
Verify that the PS-ON is constant, if it is then check the VCC Voltage of the PFC IC and the 12/20V SMPS IC. May be the filter caps for the VCC from AUX winding are bad.
ps_on does cycle every 3 secs after you push on/off button.
+5V is steady
+12V cycles every 3 seconds 0 - 12.2V
+20V cycles every 3 secs 0 - 21.33V
I've checked both boards - ohming out resistors. checked caps. checked diodes. I'll check again tomorrow
The LM431 to-92 did fix the as431 that were bad. This shunted regulator should have been a higher rated part. The SMD is only 0.28 watt where the to-92 pkg is 0.8 watt
Right now you have PS-ON issue. You need to run the power supply board by itself and connect 1K resistor between PS-ON pin and the standby power supply output to force on the power supply manually and see if the PRC and the switched power supplies are steady or not.
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