23 Apr
2023
23 Apr
'23
2:59 a.m.
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:
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).
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.)