27 Jul
2011
27 Jul
'11
2:43 p.m.
Agreed on the scope connection: hook up the ground. :-) And check the filter caps -
unless they've been replaced in the last fifteen or twenty years (check the date
codes) they are Prime Suspects in the sort of noise you're seeing (unless it's
from your scope setup). It's worthwhile to pull the big ones and perform a leakdown
test on them.
Another question: are you certain that all the NPG lines are feeding through? I don't
know whether a break in the NPG chain will cause BUS ERR on the display - I've never
tempted that fate - but if the machine had a DMA device like a disk controller in it at
some point and it was removed, the small G727 is not sufficient in that slot. You need a
G7272, that also jumpers CA1 and CB1 - or if you find a SPC slot that doesn't have
those two connected, you can just replace the jumper on the backplane. (I'd check it
for you on my 11/34, but I have too much stuff stacked in front of it right now - sorry.)
That's a guess, as I'm not sure if an NPG failure gives you a front panel error or
just hangs the machine. -- Ian
________________________________________
From: cctalk-bounces at classiccmp.org [cctalk-bounces at classiccmp.org] On Behalf Of
Pete Turnbull [pete at dunnington.plus.com]
Sent: Wednesday, July 27, 2011 1:55 PM
To: On-Topic and Off-Topic Posts
Subject: Re: Power Ripple in DEC BA11-K
On 27/07/2011 12:14, Jochen Kunz wrote:
Not necessarily, I do it all the time. And the lack of a local ground
is almost certainly where all that hum and noise is from.
On Wed, 27 Jul 2011 11:31:34 +0200 (CEST)
Christian Corti <cc at informatik.uni-stuttgart.de> wrote:
How did you
connect your scope? Looks more like a ground problem with your
measurement setup.
This could be the problem. I don't have an isolating
transformer. GND
of the scope and GND of the PDP-11 are connected to earth of the mains
plug. Because of this I connected both to the same power strip. I did
_not_ connect the GND clip of the probe to GND of the PDP-11. If I had
done this, I would have created a ground loop. This, of course, _will_
create problems. But if the M930 is enough and if the voltages are OK,
why can't I halt
the CPU with the control panel? There are NPR jumpers in the backplane
and grand cards in emty slots.
Classic sign of something wrong with the bus grants. If the grant cards
are all the small type, are they the right way round? Is there one in
every slot, including the last (slot 9)?
--
Pete Peter Turnbull
Network Manager
University of York