On 2023-Apr-22, at 3:53 PM, Peter Coghlan via cctalk wrote: On 2023-Apr-22, at 1:07 AM, Rob Jarratt via cctalk wrote: >> This seems to be because I measure a steady 0.6V on pin 6 of the transformer >> (p4, PSU Sheet 3). I just can't imagine where it might be coming from as the >> chopper won't be running. I had previously removed the transformer and there >> are no shorts between the pin 5-pin 6 winding and any of the other pins on >> the transformer. I checked all the DC outputs of the PSU when powering the >> 7812 from the bench, both on a working PSU and the non-working one. They are >> all at zero except the -12V output on the non-working PSU, which is +0.6V. >> But the voltage can't come out of nowhere. > > I've looked at it some more. On page 5 (control module sheet 1), at the > non-inverting input of E1b, there is a 75k resistor and 16k9 I think? > resistor between -12V and V2 which is derived from Vstart. Perhaps this > would account for the +0.6V? > > (Ignore what I said earlier about the possibility of one of the two diodes > connected to pin 6 of the transformer being shorted. The 51 Ohm (or is it a > 5 Ohm?) resistor across one of them would look like a short compared to 75k.)
Following on Peter's observation above, note that under normal (as opposed to test-bench) operation, a limited -12V is supplied at startup by the same little mains transformer that supplies VStart (schematic: PSU sheet 1). This startup -12V would appear to be or may be required to bias things correctly for startup to proceed. It follows that the -12V current-sense shutdown being observed is not (necessarily) part of the fault being looked for, but just a consequence of the absence of the -12V startup. Also, minor note: not indicated on the schematic is the power supply pin connections for the comparator ICs (E1,2,3) (or I haven't spotted them). Presumably the +supply is VStart, but you might check and record whether the -supply is GND or is it -12V (and check all 3 ICs).
