30 Apr
2011
30 Apr
'11
7:40 a.m.
-----Original Message-----
From: cctalk-bounces at classiccmp.org [mailto:cctalk-
bounces at classiccmp.org] On Behalf Of Rob Jarratt
Sent: 30 April 2011 11:11
To: 'General Discussion: On-Topic and Off-Topic Posts'
Subject: RE: H7140 PDP11 PSU
must
-----Original Message-----
From: cctalk-bounces at classiccmp.org [mailto:cctalk-
bounces at classiccmp.org] On Behalf Of ard at p850ug1.demon.co.uk
Sent: 30 April 2011 09:24
To: cctalk at classiccmp.org
Subject: RE: H7140 PDP11 PSU
http://www.bitsavers.org/pdf/dec/pdp11/1124/MP01018_1124schem_Aug8
I will add some diagnostic wires to the board as you suggest, I
don't remember a lot about TTL chips, is there a convention about
which are the power pins?
In the meantime someone else told me that the click I would hear
when connecting to the mains (before switching it on with the key)
was some
kind
of relay. I don't hear that anymore, would
that help to isolate the
problem
at all?
OK, I am looking at the schemaitcs... I will take the referenes from
this manual on bitsavers, since I guess we all have access to it.
0.pdf
This is a complex PSU, and it is not obvious at first sight what is
going
on,
but I will try... I will put page references (to
that PDF) in brackets.
The relay is K1 (p70). It shorts out a soft-start resistor in series
with
the
input to the mains bridge rectifier. This
produces 350V across C1/C2
in series.
Now much of the PSU circuitry runs off a little SMPSU. The chopper
transformer is T1 (p82). It provides about 12V across C3 (p82).
Now we need to look at the 'Bias and Interface Board'. The relay is
driven
by
E4 (p86). It's essentially a 'mains
OK' circuit, the relay is
energised
when the
votlage across the mains smoothing capacitors is
high enough to
trigger
E3a.
E3a is on the mains side of the isolation
barrier, of course. But the
relay
driver (E4) is powered from a signal called
'+5V'. This is not the +5V
that
you're expecting. It comes from the 7805
regulator E1 (p87). The input
to that comes from that little SMPSU I've been talking about. This is
isolated
from the mains.
The control circuitry for that is on page 88. Be warned that this
circuitry is
NOT isolated from the mains. It's a
relatively conventional SMPSU with
the control circuitry powered straight from the mains. Q1 (p88) is the
chopper transistor.
So, if everything's working right, this SMPSU starts up when you apply
mains
to the machine, the 'mains OK' circuit
triggers and the relay pulls in.
I would start by checking that '5V' supply, remember it's not the one
to
the
backplane. Check it at the output of E1, for
example. If it's missing,
as
I think
it will be, you need to sort out the SMPSU
I've been talking about.
But
let's find out if you need to do that.
-tony
Regards
Rob
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When I was looking at Bias and Interface board I realised that the +5V indeed be coming from elsewhere as the tracks are not
even connected
(directly) to the PSU motherboard. As you suspected, the output of E1 is
0V.
Regards
Rob
I added more wires at the input to E1 and also at the input to D9. Unless I
made a mistake or did it somehow wrong, they were both 0V as well. Looking
at the printset I suspect that this means the problem is on the PSU
motherboard. Could it be the transformer T1 on p83?
Regards
Rob