hams on classiccmp

Ian King IanK at vulcan.com
Sun Jan 18 03:01:56 CST 2009

Industry folks say that the lifetime of electrolytic capacitors is about fourteen years.  Since the filter caps in our machines were significantly (i.e. TWICE) older than that (and some showed physical symptoms of degradation), we replaced them rather than deal with them one by one as they failed.  In our experience with our PDP-10 machines, this was (despite its cost) cheap insurance against periodic failures, which can occasionally be catastrophic.

The foam on the doors not only serves to cut down on dust into the machine, it also cuts down on the noise in the machine room.  That makes me happy.  :-)  Yes, the machines can be run without it.  But since our goal is restoration, we'll replace it - and yes, replace it again in ten or fifteen years.

We *are* running the machine off three separate 110V single-phase circuits - apparently you missed that part.  Despite your opinion that "it should be obvious", as good engineering practice I wanted to understand how the machine was using the input power before making assumptions.  I'd rather spend some time in due diligence than in damage control - or fire fighting!  Also, since our interest is historical restoration, we wanted to make use of the PDU as wired rather than try to bypass it with some sort of approach that would feed 110V directly to the various elements.  You may be completely correct about how buildings are wired, but our building isn't wired that way.  The EE on our team came up with an approach that's working for us.

That raises an interesting point I forgot to mention: it's important to determine that the various subsystems are properly plugged into the PDU.  When we first tried to bring up the machine, it would pop the main breaker.  After some head-scratching, we determined that this was because the fan for the memory subsystem wasn't plugged into an unswitched outlet.  If that fan isn't running, there's a signal from an airflow sensor that trips the main breaker!

Yes, if the OS doesn't boot that means you have a problem - but what problem?  DEC designed these diagnostics to tell you where that problem lies.  I chose to minimize my variables.  Chalk it up to a dozen years as a software quality assurance manager at Microsoft.  Given the expectations of the people who sign my paycheck, I chose the conservative route.

I disagree with your opinion on scoping the power supply outputs.  In my colleagues' work with the PDP-10s we have, they found that 'weird' problems were often caused by an unexpectedly unfiltered DC line - and DCOK didn't cop to it.  I have clean power - under load - to this machine.

We have a valid 6.2 license, which is a 'period' OS for this machine.  As to your assertion that we would be fine using the hobbyist license with 7.2: I'm not a lawyer, are you?  I have used the 7.2 hobbyist release and PAKs for a number of years on several of my own VAXen at home.  But this is not 'at home' and I know my 6.2 license is legal and proper.

From: cctalk-bounces at classiccmp.org [cctalk-bounces at classiccmp.org] On Behalf Of Patrick Finnegan [pat at computer-refuge.org]
Sent: Saturday, January 17, 2009 12:06 PM
To: General Discussion: On-Topic and Off-Topic Posts
Subject: Re: hams on classiccmp

> From: brad at heeltoe.com [brad at heeltoe.com]
> Ian King wrote:
> >I just got a VAX-11/780-5 up with VMS on TCP/IP....
> very nice.  Can you describe how it went?  Was the 780 previously
> in moth balls?
> -brad

On Saturday 17 January 2009, Ian King wrote:
> This machine was one of a pair bought from a collector who bought
> from a collector who bought from...?  It was interesting noting the
> differences between the two: for instance, on one the black foam was
> in fairly good shape (replaced it anyway), but on the other it was
> falling off in chunks.  Both seemed in good mechanical condition.
> The next step was to ensure electrical integrity.

FWIW, if you can be sure you're going to operate the machine in a
relatively clean environment (eg, no carpeting or other things to
generate dust), I'd forgo the filter foam, as all it'll do is crumble
and block airflow.

> So I replaced every electrolytic capacitor of consequence in the
> switching power supplies - five in one machine, six in the other -
> plus the LSI-11 boot machine and RX01 boot floppy.  This was
> expensive!

This also seems unnecessary if the capacitors were still good.  The
11/780 I got up and running has one bad psu (de-asserts DCOK every once
in a while, and causes the machine to reboot) which may be bad
capacitors, but everything else was ok, despite being stored for over
10 years.

> After all of this, we carefully brought up the machine.  We had a
> challenge because 120V three phase doesn't seem to be usual practice
> in US wiring - we had 240V three phase, but that obviously wasn't
> going to do us any good!  Carefully looking through the power
> distribution unit's engineering drawings, it became clear that DEC
> used three-phase simply to balance the current load among the legs -
> in fact, everything runs on 120V.  So we used equal care in reviewing
> the wiring of the warehouse where we keep these machines and found
> three outlets that were (a) on the same side of the 240V mains and
> (b) not sharing a breaker and circuit.  Those were connected to a
> three-phase outlet, the VAXen were plugged in and voila!  NOTE: we
> have a team member experienced with commercial power circuits.  Don't
> try this at home - or if you do, be very very careful and be certain
> that, from any of the three live blades to another, you don't have
> more than 120V.

First, I want to point out that all of the power outlets on the PDU are
120V, 20A (NEMA 5-20R) outlets, so it should be somewhat obvious from
that, that the machine doesn't need three phase power to run, no
engineering drawings required. :)

Also, your statements about three phase power aren't quite valid.  You
may have had three-phase 240V, but that is unlikely, the typical
practice in the US for non-motor loads (lighting, general power usage)
is 120/208V three phase or 120/240V single phase.  In normal use, the
three phase connections on the VAX PDU have 208V between them, and 120V
to ground, which is a standard 120/208V three phase system.

You can run the machine off of single phase power by chosing up to three
separate 120V circuits - it's ok if they're on different phases, having
240V phase-to-phase is OK - and running each one to a different phase,
and tying all the neutrals together to the neutral in the machine.  In
fact, it is a good idea to make sure that you have different phases, so
that you minimize the neutral current, otherwise you may end up with
melted wiring or fire, and selecting opposite phases will do this for
you, as the netural current from opposite phases will cancel either
other out.

Fortunately, an 11/780 doesn't draw nearly the outlet/PDU rating; I
think I measured around 24A total draw at 120V from all three phases on
mine, and load didn't raise that too much.  I've run it from three 15A
circuits (in a building with 120/208V power that I'm not allowed to put
in my own outlet for it ;), and it was ok, each phase was around 8A

> Now that the machines would power up, I scoped all the power supply
> voltages to ensure they were really DC, i.e. that I hadn't missed an
> important filter cap anywhere.  All good, so I tried booting from the
> floppies we got with the machines.

Fun.  I'd suggest putting a scope (or even better a one-shot that
triggers when they go off) "DCOK" and "ACOK" outputs from the PSUs as
more important; checking voltages with a DMM is probably more useful.

> I was able to get the basic
> console to boot - hooray! - but was unable to get the
> microdiagnostics to run.  We had agreed that successful execution of
> the low-level diagnostics was a precursor to any attempt to install
> the OS, so this was a roadblock.

Your OS is probably the best diagnostic that you have.  If it doesn't
boot, or crashes, there's something wrong, which you can pick a precise
diagnostic to examine the problem.  Diagnostics sometimes find problems
that don't really exist, and miss things that are show-stoppers.

> We had CDROM media for OpenVMS 6.2, the latest version certified for
> the VAX-11/785

7.x works fine.

> Now we were cooking with gas.  I had a valid VMS license but not a
> UCX license.

You should be able to use the hobbyist license, unless you're planning
on running the machine commercially.

Purdue University Research Computing ---  http://www.rcac.purdue.edu/
The Computer Refuge                  ---  http://computer-refuge.org

More information about the cctech mailing list