Sorry, got dragged into something else, back on this now. I know about the cap error on the schematics, haven't replaced those 2. Again, it works if the +5VD feedback input to the regulator board is broken, doesn't if connected.
I have checked U1371C. The ref input is correct. The opamp is working. If I disconnect J233 and put the dummy load on the inverter side, PSU starts fine, voltages are fine but a bit high, since there is no feedback. I can then drive the regulator side +5VD with an external supply and see that U1371C pin 8 does the right thing, going positive if the external supply is low, going down towards zero if the external supply is too high. So, that's working fine. But, removing the external supply and reconnecting +5VD from inverter board to regulator board, ticking. Monitoring various bits with a scope, PSU starts, +5VD starts up at about 5.4V. U1371C pin 8 starts decreasing in voltage, which is correct. But, it overshoots too low and the inverter shuts down. This is puzzling because the feedback loop clearly is at least semi-functional. This is a strange beast, the opamp at balance is basically running full gain, the loop being closed by the feedback to the inverter then thru the +5VD line back to the opamp. There an AC feedback loop on the opamp, clearly to provide some damping for the long round trip. Back to being confused. Oh, I also looked at the feedback at pin 8 on the opamp. Goes quickly high when the PSU is turned on, then starts heading down towards 0, exactly as expected. (High means output is too low). But, it never stabilizes at ~1V where it should end up, just keeps going down until the PSU shuts down. Repeat every second or so.
Bill
