Dying VaxStation 4000 VLC - help?

From: Mark Tapley <mtapley_at_swri.edu>
Date: Thu Nov 14 17:37:01 2002

Antonio said:
>I opened up my VLC and it has an H7109-C PSU in it.

Ok, good. Matches mine, so at least I know where to get another one of those.

>Can't you either borrow
>a known good one or at the very least pick up a VLC
>from ebay for less than $50?

Possibly, exploring that. But I'm reluctant to board-swap until/unless I
get reasonably strong evidence that it really is the PSU. Although in this
case, the failure is obviously not enough to fry anything, so I'm not so
likely to damage the known-good component by board-swapping.

This PSU is pretty clean and doesn't show much evidence of damage. Though
I'll bet a capacitor with bad ESR looks like a healthy capacitor.

---
Thanks for the reference, Frank! JT Computer may well end up $50 richer.
---
Tony said:
>_Sometimes_ the voltages are marked on the PSU PCB (particularly if it's
>a 3rd-party unit). But I guess you've looked there.
I did not unscrew the PCB from its mounting plate so I could see the back
side. But all the Red wires soldered down to little metal strain-relief
collars standing side by side, with resistance between them = 0. Likewise
for all the Black wires. Resistance Red/Black was about 5 Ohms, FWIW.
>What you are
>looking for is a +5V or ground signal that's _not_ directly connected to
>the appropriate PSU output (check this with an ohmmeter with the machine off)
I'm pretty sure it doesn't exist. See my earlier post.
>Buy a can of freezer spray. It's not _that_ expensive, and it'll do the
>job properly.
Plan A for the weekend involves a trip to the local elx. place. I'll try to
get freezer spray and replacements for all of the electrolytics near the
output wires on the PSU. Then I'll try to nerve myself for some soldering.
I'll keep the freezer spray away from the PSU.
----
Toth said:
>It certainly sounds like something is overheating. You might want to try
>using some cardboard to carefully direct the airflow from your hair dryer.
OK, will try it tonight.
>Do these systems have an internal fan? I haven't been inside one enough to
>remember, but if they do, is the one in your system running properly?
Two, side-by-side, mounted in the PSU housing. They suck air out of the
mainboard compartment into the PSU housing, and it then exhausts out the
far side of the case. They are not far from the connector and the
components I listed. Both of them turn when the switch goes on. (Thanks for
the suggestion, Emanuel!) Both had a *little* dirt around them, but I
cleaned that up (and both still turn). The SCSI cable partially obstructs
their apertures - I'll try to alleviate that.
>Linear Technology 1.5A adjustable voltage regulator, TO-220 package.
That's it.
... (!) "100% Thermal Shutdown Functional Test." !?!?!?
Maybe I'll give that part some stricter attention with the hair-dryer. Any
chance that thermal-shutdown function could be getting too sensitive on me?
Or that its thermal mounting has been messed up somehow? The mounting tab
looks like it could go to a heat-sink, but there's no heat-sink attached.
But there *is* a matching hole underneath in the mainboard!
How do I know if it goes into thermal shutdown?
>Low Power Low Offset Voltage Dual Comparator
>
>Datasheets can be found here:
>  http://www.national.com/pf/LM/LM393.html
Ok, what's it doing there? Anyway, I'm going to try to probe its outputs
tonight and see whether they are in different states when the machine is
locked up.
As always, I'm indebted for all your help! More reports tomorrow.
									- Mark

Received on Thu Nov 14 2002 - 17:37:01 GMT