Yamaha RX-V1400 Protection circuit experience?

My Yamaha AV-Receiver RX-V1400 goes back in stand-by mode immediately after turning power on via the On/Std-by button. I managed to go into Diagnostic mode and get a "PS PRT:037" message on the display. According to the service manual, that indicates a Power Supply issue (not a main amp DC issue, different message). I needed to replace two Voltage stabilizer (5V, 12V) and now all voltages check out as per spec. Still the same error. I can go into Diagnostic mode and bypass the protection circuit: The amplifier works fine. The relay control circuit works fine. As this circuit is controlled by a CPU I have difficulties in identifying the root cause. Any suggestion?
 
Thanks! I am not familiar with these Yamaha AVs, so I have a few questions: Do I need to attach a Monitor to get the reset code? Can I also do it with the unit's display? How do I reset the unit (the manual is not very clear about that)
 
Yes I did. Being in Diagnostic mode it seems (according to the display) that (which?) voltage is fluctuating showing "PS😛RT XXX" where XXX is somewhat between 005 and 039. However, I can not see that while checking the individual voltages. I would expect a larger ripple than acceptable in order to generate this reading. I am trying to disable the protection circuit "sensor" line to the CPU to narrow in the faulty circuit, but somehow it seems I am running in circles. In diagnostic mode I checked all power amplifier channels attaching speakers and applying a music signal through the RCA connectors. All fine with one exception: I can not activate "Speaker B". (same circuit as Speaker A"). The respective relay does not work. Perhaps that is a clue?