Dying VaxStation 4000 VLC - help?

From: Mark Tapley <mtapley_at_swri.edu>
Date: Thu Nov 14 08:42:00 2002

> The PSU is an H7109-00.

Mine says Model Number: H7109-C on it.

>>Most DEC machines have at least one power-OK line from the PSU.
>
>I want to figure out which that one is.

I think there is no such line. On the mainboard connector to the PSU, there
are Red (3), Black (4), Orange (1), and Blue (1) wires.

Red are all connected together, and all go to appx. 5 V when the system is on.
Black are all connected together and to ground and are always at 0 V.
Orange is one wire, and goes to appx. 12 V when the system is on.
Blue is one wire, and goes to appx. 12 V. when the system is on.

The red and black cross-connections are true on the mainboard and on the
PSU. They are not all adjacent on the connector, which makes not much sense
to me, but that's how it is. Red and Black on the mainboard connect to the
corresponding colors on the Disk power connector.

As for voltage: I really need a good DVM. Calibrated the 12V setting at
work against a good 4-digit fluke DVM, so I think my reading of 12.1V for
orange and -12.0 for blue is close. Did not have time to check at 5V, but
afterward measured 3 Ray-o-Vac Renewal D cells in series at 4.7 V, so I may
not be way far off. If so, my reading of 5.3 to 5.4 for the Red group is
worrisome. This reading is consistent even at the Disk power connector.
Anyone else have a good VOM, a working VLC, and 3 Renewal cells? Or just
the VOM and either of the other two?

>I have not figured out correlation between temperature and working status.

I know more about it, now. Turned it on cold, it ran for a while, then
failed, then started to try to boot again. Blew the hair dryer on it, it
went into hard fail almost immediately and did not try to boot. Blew a fan
on it, it went OK and actually ran far enough to give me a login prompt. In
both cases I was trying to aim at the part of the mainboard near the power
connector, but the system is so small I was probably affecting most of the
mainboard and possibly a lot of the PSU as well.

Near the power connector are two power-looking components (as in they have
big leads, and in one case a heat-sink attach point, whereas nearly
everything else is microscopic surface-mount stuff). One is a
power-transistor looking thing with a heat-sink attach point (but no
heat-sink) that has 3 pins and says

LT1086CT
9151

The right leg is connected to 5.3 V (Red-wires).
The center leg stays around 3.4 V, system running or failed.
The left leg stays around 2.15 V, running or hung (as far as I can tell).


The other is an 8-pin DIP that says

M9124
LM393N
QST

Are these two part of a crowbar circuit, or reset circuit, or some such?
Should I be able to see some clue about why the system hangs by looking at
their pins?

                                                                - Mark
Received on Thu Nov 14 2002 - 08:42:00 GMT