10 Oct
2015
10 Oct
'15
10:14 a.m.
If I remember correctly I had a problem in this area that turned out to be a loose/dry
joint where the two thermistors that are across the relay joined together. In my case it
was an intermittent fault though.
Regards
Rob
-----Original Message-----
From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Josh Dersch
Sent: 10 October 2015 16:46
To: General Discussion: On-Topic and Off-Topic Posts
Subject: Re: DEC H7140 (11/40) power supply revisions and general advice?
something working.
turns on the relay.
Interesting idea, I?ll look into that. Thanks!
- Josh
>
>
>>
>> Capacitor C4 looks to be involved with the START-UP DRIVE signal
>> circuitry (which drives the relay) so the behavior I'm seeing makes
>> sense if C4 died and took a couple of things with it (or if something
>> else died and took C4 with it).
>>
>> Here's where it gets kind of odd -- I spent some time testing diodes
>> and transistors in the related area near C4 and while doing so I
>> noticed that there are four diodes (D1-D4) listed on the schematic
>> that are missing from my board. "Missing" as in someone clipped them
>> out at some point -- there are just nubs of the leads left. I'm not
>> sure why this would have been done, but there were a number of ECOs
>> applied to this board (a few wires and resistors added) and I don't
>> want to assume that if I just put four new diodes in that it won't
>> cause other problems. Anyone know if there were other revision
>> levels of the bias/interface board that would have done away with
>> these diodes? Anyone have an H7140 they can easily crack open to
>> compare? (It's actually relatively easy to get to, if you can get to
>> the
>> supply...)
>>
>> Thanks as always,
>> Josh
On Oct 10, 2015, at 4:15 AM, Mattis Lind
<mattislind at gmail.com> wrote:
2015-10-10 5:42 GMT+02:00 Josh Dersch <derschjo at gmail.com
<mailto:derschjo at gmail.com>>:
Hey all --
First of all I haven't poked around in the PSU of my 11/44 since it
worked fine when powered up. But the design of the H7140 is a lot
similar to the design of the H7104, the PSU of the VAX-11/750 which I
worked quite a lot with.
http://www.datormuseum.se/computers/digital-equipment-corporation/vax-
11-750
<http://www.datormuseum.se/computers/digital-equipment-corporation/vax
-11-750>
> Once again, I find myself in over my head debugging a power supply,
> this time an H7140 from a PDP-11/44. Here's the skinny:
>
> I examined the supply physically before experimenting and found a
> capacitor on the Bias/Interface board that was leaking, bursting and
> rather burned-out looking (not a great sign) -- this is capacitor C4
> in the printsets on Bitsavers (
> http://bitsavers.informatik.uni-stuttgart.de/pdf/dec/pdp11/1144/MP008
> 97_11X44sys_Dec79.pdf) Everything else looked OK physically; I
> replaced the obviously bad capacitor at C4.
>
> When power is applied (plugged in, breaker switch flipped to "On")
> the relay does not click - based on my readings of the manuals this
> should happen after the bias voltages are up to spec. I measure
> 308VDC on the lugs on the top of the memory board, so that's at least Getting
to other points to test voltages is a bit more difficult,
especially with those high voltages in the way, what a nice design :).
The previous owner said that it happens that the relays get stuck. The
relay is there to limit the inrush current so when it clicks it
bypasses a couple of power resistors. Regardless relay state you will
have the full voltage over the capacitors. (at least this is the case
in the H7104). If the relay get stuck then the power resistors get
very hot. The relay drive circuit detects that the voltage is high enough then Maybe something in this circuitry is bad. In this
case you could force
it to on but then I think you should power it from a variac
Thanks. I should have mentioned, but I tested out the relay and it?s not stuck
and seems to be working fine. It definitely looks like the relay control circuitry
is not running properly.
Switching the front panel switch to
"Local" (or any other position)
has no effect -- no fans, no LEDs, nothing. I've double-checked all
the wiring and everything looks OK.
Have you verified that you have the 12V BIAS voltage? If not you could
probably generate the 12V BIAS voltage using a lab power supply just
to see if the main switcher is working properly.
I used an insulation transformer and a variac and then forced the
relay drive to on. In the H7104 the same circuitry also enabled the
main switcher so unless I did this there were no fun at all. I
supplied 12V using a bench supply. It made it possible to check
everything was working ok without any high voltages.