On 2025Oct 11,, at 2:45 AM, Robert Jarratt via cctalk <[email protected]>
wrote:
>
> 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-secondary-side-rectifier-output.png
> 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.
>
> Does anyone have any suggestions please?
- The design configuration:
I don’t know of a single name for this configuration of PS, but it’s comprised
of:
- a switching driver
- which drives an isolating pulse transformer
- which drive inductive step-down buckers (for the +5 & +12 outputs)
- and some final filtering around the outputs
The isolation transformer performs some degree of V step-down but most of the
voltage step-down-&-control is being performed by the bucker formed from a
winding of the shared-core inductor "16-20387-01” and the 2•2 diodes. Those
2•2 diodes are a 1•2 rectifier and a 1•2 buck-discharge diode.
From my observation this configuration seems to have been introduced in the 80s
or came into wider use in that period.
This is a very common/standard configuration for switching supplies these days.
The point of all that is it’s not a full-wave (per 2 diodes) centre-tap
rectifier feeding a filter, and it’s not a ‘simple’ single-stage switching
supply.
With the bucker(s) in there, the input pulses to the bucking inductor may be
significantly higher than the target output voltage, which is evidenced in your
scope trace where the input pulses are 42V peak >> 5V output.
The "5V-pulse" signal is picked-off from the input side of the bucking
operation, before the bucking+filter energy storage, so the pwr-OK circuit can
get an early sense of pwr fail without having to wait for the energy storage to
all discharge.
- The schematic:
. . .
Was preparing this when Peter’s second message came in. So:
<snip>
And yes, everything Peter says about IC powers pins, Vz, and cap.
I’ll add though, to check all 6 of those pins, traced or ohmed-out, both -pin
and +pin.
The circuit design implies that the E1 “GND” pin probably isn’t GND, but rather
“-12V”. (Might be true for E2 & 3 too, so to check them).
But there are two "-12V” supplies in the schematic. Are they actually connected
together or are they separate?
You might also check/confirm what is on those IC power pins when powered-up.
- AC-OK:
I take it you don’t mean “power-OK”, but rather the (no-number) pin on the
“Logic Power” connector fed by the 2N5638 JFET.
How are you observing “AC-OK”, is there anything connected to it?
What are you expecting as ‘good’ indication?
The JFET output is open-drain, so there won’t be any voltage there without
something external to provide it.
Based on presumptions about the IC power connections and the circuit, it looks
like AC-OK-good is high-impedance, while not-good is low-impedance to GND.
Or stick a 10K R on there to a +V.
Or you could look at the output of the pwr-OK comparator instead.
