12 Oct
2025
12 Oct
'25
5:48 p.m.
I am trying to resolve a problem with a H7842 PSU from a DEC Rainbow. The AC
OK output is not being asserted. I am looking at Tony Duell's schematic
https://bitsavers.org/pdf/dec/rainbow/duell_schematics/psu.pdf and
specifically at the Power OK circuit.
I am using a test load of 1R on the 5V output and 6R on the 12V output,
which is within the spec of the PSU. I see that under load the 5V output
is only 4.4V, so I suppose it is to be expected that AC OK is not asserted.
If I (briefly!) remove the load from just the 5V output, the 5V output is
5.27V, but the AC OK output is still not asserted.
Using Tony's schematic, I looked at the input to the inductor on the
secondary side (sheet marked "H7842 PSU Sheet 3") and it looks like this:
https://rjarratt.wordpress.com/wp-content/uploads/2025/10/h7842-5v-secondar…
I can't tell if that is correct or not. If I remove the 5V load the peaks
narrow but are slightly higher.
However, the puzzling thing is the 5V Pulse signal, it doesn't pulse, it
is a steady 32V. I see that the 12V Pulse signal *does* pulse. The name
would suggest it is supposed to pulse and its inputs do pulse as shown in
the trace pictured above. I don't see how it is possible for 5V Pulse to
be at a steady 32V, even if the diode or resistor that create the 5V Pulse
signal was somehow bad. I guess this non-pulsing might be the problem, but
I don't really know what to check here, I have lifted the diode and tested
it, it seems fine.
Some more thoughts:
I can't see why the 5V Pulse signal would be steady at something like 32V
unless there was a capacitor connected from it to ground somewhere which
would get charged to a value approaching the peak value of the rectified
signal. I've hunted all around the schematic and can't see anywhere else
the 5V Pulse signal goes except to the Power OK circuit on H7842 Control
Module Sheet 2 and I can't see any capacitor there or on H7842 PSU Sheet 3
where the signal originates.
Can you check the physical unit and make sure that 5V Pulse is not
connected to anything else, especially checking for a capacitor to ground?
If there truly is no capacitor, there could be an unexpected path through
some faulty component causing this signal to get smoothed when it would
otherwise not. It's hard to see where this could be though. It is hard
to imagine where else 32 odd volts could come from in this circuit other
than by rectifying and having a capacitor smooth pulses with peak values
in that sort of range.
(The 12V Pulse signal does have a 4n7 capacitor to ground across it which
could be significant at the frequency the chopper works at. Perhaps
"12V Pulse" is not meant to be pulsing if this capacitor is open circuit?)
It would be interesting to know the values of Vstart and Vz and what the
voltage connected to the power supply pins of E1, E2 and E3 is. I suppose
Vstart should be 12V given that it comes from a 7812. I can't see anywhere
on the schematic what the power supply arrangements are for E1, E2 and E3
are or what the value of Vz should be.
Regards,
Peter Coghlan.
