22 Feb
2009
22 Feb
'09
10:54 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
*
22 Feb
22 Feb
11:11 a.m.
--- On Sun, 2/22/09, js at cimmeri.com <js at cimmeri.com> wrote:
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?
Could be. What cabinet is this in? A BA123? Those have two power supplies. One for the
disks and half the cards in the backplane, the other for the other half of the backplane,
and perhaps fans. I forget the distribution. But when you remove the power supply,
you'll see that there are two seperate boards there.
You may very well have blown up one because of improper voltage - or perhaps you
didn't have it loaded enough. Old switchmode supplies need a load in order to work
properly. Normally, they just don't output voltage, but I've seen them burn up
before.
-Ian
12:58 p.m.
*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.
Do you mean that it will be difficult to fix, even if I can find someone who
really knows what they are doing?
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.
I only have a basic multimeter so I don't know if I would have had the
necessary equipment to do this, do you have any advice on the minimum
equipment needed? Now that there has been some damage is it sensible to
replace the blown capacitors and any other ones that don't measure well?
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.
I looked up variacs but there seem to be an awful lot of different ones,
again any recommendation as to the minimum I would need?
By the way, I am aware that PSUs can be very dangerous to meddle with when
you have limited knowledge. How long should I leave the PSU between any
tests to allow the capacitors to discharge? The label on the PSU says to
leave it 5 minutes, I suspect it should be longer.
jS
*
3:33 p.m.
js at cimmeri.com wrote:
*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.
I have to say, for all the talk of failing caps in power supplies I've
only ever seen one electrolytic cap fail *ever*, and that was last week
in a one-year-old graphics card that has hardly ever been powered off...
Gordon
23 Feb
23 Feb
2:37 a.m.
On Feb 22, 2009, at 3:33 PM, Gordon JC Pearce wrote:
Heh. I've replaced four tantalum capacitors in the past week alone.
-Dave
--
Dave McGuire
Port Charlotte, FL
*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.
I have to say, for all the talk of failing caps in power supplies
I've only ever seen one electrolytic cap fail *ever*, and that was
last week in a one-year-old graphics card that has hardly ever been
powered off... 
24 Feb
24 Feb
12:07 a.m.
On Sun, 22 Feb 2009, Gordon JC Pearce wrote:
js at cimmeri.com wrote:
So, you didn't have the unique pleasure of having a batch of bad
motherboards that had counterfeit caps? Poppity poppity poppity!
--
David Griffith
dgriffi at cs.csubak.edu
A: Because it fouls the order in which people normally read text.
Q: Why is top-posting such a bad thing?
A: Top-posting.
Q: What is the most annoying thing in e-mail?
*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.
I have to say, for all the talk of failing caps in power supplies I've only
ever seen one electrolytic cap fail *ever*, and that was last week in a
one-year-old graphics card that has hardly ever been powered off...
5601
days inactive
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days old
5 comments
6 participants
participants (6)
-
dgriffi＠cs.csubak.edu -
gordonjcp＠gjcp.net -
ian_primus＠yahoo.com -
js＠cimmeri.com -
mcguire＠neurotica.com -
robert.jarratt＠ntlworld.com