22 Feb
2009
22 Feb
'09
7:54 a.m.
-----Original Message-----
From: cctalk-bounces at classiccmp.org
[mailto:cctalk-
bounces at classiccmp.org] On Behalf Of Robert Jarratt
Sent: 20 February 2009 22:45
To: cctalk at classiccmp.org
Subject: Powering up a 20-year old MicroVAX II
I have just collected a MicroVAX II which has been in storage and has
not
been powered on for 20 years. This is my first machine of such an age,
unfortunately I am not particularly knowledgeable at the electronics
level
(I studied circuits academically 25+ years ago and can solder a bit,
but
that is as far as it goes). I know I will need to treat it carefully in
order to get it working again. I plan to open it up and make sure I
clear
out any debris etc, but beyond that I need advice from those with the
experience and knowledge that I lack on how to go about powering it up
carefully.
Thanks Rob
After removing all the boards and leaving just the disk and tape drive for
load I impetuously decided to try powering it up. I knew the PSU (model
H7864) was set for 110V and made the switch to 240V (I am in the UK). When I
connected the power cord, after a few moments there was a loud pop, followed
by another before I could pull out the power cord, smoke rose from the PSU.
This sounded just like when I had once accidentally made a 110/240 mix-up.
The question is, could it be that I had not made the switch to 240
correctly, or could this just be down to the age of the PSU?
----
*Rob, possibly you made the 110/240 switch wrong, but also possible is
that the PSU just wasn't ready to be turned on like that. Electrolytic
Capacitors (of which there are many in that PSU) tend towards
non-functionality the longer they sit unused. Without those caps
working right, the PSU will do pops, smokes, and other alarming things.
Then good luck fixing it.
What I do with an old PSU like that is test each cap prior to it ever
being powered up.. both for capacitance and ESR. Usually, some or all
of the caps need reforming or even replacement. Only after the caps
are back to health, do I then give the PSU power... at first with the
smallest load I can get away with.
If it's a switcher PSU (as that one is), I'll bring it up quickly to
about 90V using a variac... then in 5V increments every 1/2 hr after
that to 130V, then back down to 120V. This in the USA.
jS
*