1 May
2011
1 May
'11
2:32 p.m.
-----Original Message-----
From: cctalk-bounces at classiccmp.org [mailto:cctalk-
bounces at classiccmp.org] On Behalf Of Tony Duell
Sent: 01 May 2011 20:56
To: cctalk at classiccmp.org
Subject: Re: H7140 PDP11 PSU
part
I measured 8.6V across pin 1 and 8 of E2.
So the 555 timer is getting power, and 8.6V is enough to run a 555. So why
isn't the PSU working?
OK, I know what I would do next :
1) Remvoe and test (properly) the chopper transistor. It's the most likely
to fial after all.
OK, I will do that. My multimeter has a diode check range, but I don't know
how to interpret the results, I will just have to tell you what the readout
says.
2) Remove the Bias and Interface Board and find some way of powering that
control circuitry from a bench suppy (or even a 9V battery). Of course you
can't then link it to the mains parts of the H7140 (like the chopper
transfromer), but now you can connect a 'scope to it and see if that 555
is
oscillating, if the chopper transistor is being
driven, if the regulation
feedback circuit works, etc.
I have to remove the board to attach diagnostic wires anyway, so removing it
is not a problem. Attaching it to an oscilloscope is a big problem, because
I don't have one.... Have you got any recommendations for a suitable basic
one that I could find for not a lot of money on eBay? I keep thinking I
should get one at some point anyway.
Regards
Rob