30 Apr
2011
30 Apr
'11
2:18 p.m.
-----Original Message-----
From: cctalk-bounces at classiccmp.org [mailto:cctalk-
bounces at classiccmp.org] On Behalf Of Tony Duell
Sent: 30 April 2011 20:25
To: cctalk at classiccmp.org
Subject: Re: H7140 PDP11 PSU
(and
it
I added more wires at the input to E1 and also at
the input to D9.
Unless I
You beat me to it ;-). That was what my last message (sent a few minutes
ago suggested).
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?
It could be the transformer, but it's unlikely. The trap for the unwary caught me the first time) is to assume that T1 is a
normal mains
transofmrer. It isn't. It's the transformer of a switch-mode power supply.
The clue is that one end of the primary widnign fo this transfotmer goes
to
the rail that DEC call '300V', that is the
positive side of the mains
smoothing
capacitors.
Most of the circuitry for this SMPSU is on the bias and interface PCB. I
think I
mentioned the chopper transistor in one of my earlier
messages. I would
start by checking that. If it's shorted, then soem other components, or
mnaybe even PCB tracks, will be open.
I will try to think of some sensible tests to do on this circuit. The main
problem is that it's not isolated from the mains, so you cna;t just clip a
'scope onto it. If you try that at the very least you'll blow fuses, more
likely
you'll do more damage to the unit (and to the
'scope probe).
But as I said, start by finding nad checking the chopper transistor.
-tony
If I am not mistaken, the chopper transistor you are referring to is Q1
(p88). I checked the resistance with the component still soldered onto the
board. Looking at it from the front (with a plate behind connected to what
looks like a diode), the resistance between the middle and the left pin was
170Kohm (or 1Mohm, the multimeter scaling confuses me somewhat, I get
different numbers depending on whether I put the dial on 200K or 2M, either
I don't know how to use it properly or it is not a very good multimeter),
the resistance between the middle and the right pin is about 2Mohm. The
resistance between the left and right pins is about 90ohms. Not sure how
valid it is to check these with the component in circuit though.
Regards
Rob