Thanks to Finalcaleb, the AS/400 PSU repairing could be started last week

According to the initial review the PSU, he found the root cause:

• 5V AUX circuit was faulty – this issue was the driver of the front panel power outage.
• Small corrosion can be found on the panels, while few through-hole elements fallen out from the soldering tin.
• The Standby/AUX power produced by the TOP223 FET+Regulator combo IC. This looked partially faulty, this caused most probably the PSU hiccups.. The input B+ high voltage current limiter R1 resistor burned and the signal lengthened. The secunder side of AUX the the feedback circuit’s near to outgoing rectifier diode at C7 10uf-os condenser parameters were faulty. The diode was heated – 16 Ohm ESR could be measured, while 1-2 Ohm should be the factory value. The TOP223 input power C3 47uF capacitor is old as well. The other capacitors benchmarks were acceptable, but will be refurbished as well.
• R1 resistorás value has to be defined, expected between 2- 5- 10 Ohm necessary. The TOP223 replacement is available on the market. It looks the AUX psu side was the faulty. The primer side of the PSU looks healthy based on the outgoing voltages. Repairing in progress by Finalcaleb.
• Photos below: