1 Nov
2015
1 Nov
'15
10:34 a.m.
its a switcher.
On 11/1/2015 10:37 AM, Robert Jarratt wrote:
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-----Original Message-----
From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Paul
Koning
Sent: 01 November 2015 16:52
To: General Discussion: On-Topic and Off-Topic Posts
Subject: Re: Testing H7864 (MicroVAX II) PSU With No Load
transformer, it's
On Nov 1, 2015, at 7:12 AM, Robert Jarratt
<robert.jarratt at ntlworld.com>
wrote:
Some of you may recall I have a faulty H7864 PSU,
which failed a while
ago with a loud pop, but no obvious physical damage. I replaced the
blown transistor (on the primary side of the large transformer), but
when I power it on, the transistor does not switch and there is no
output, so clearly there is still a problem somewhere.
I have been spending some time drawing schematics for almost the whole
thing. I am now at the point where I intend to compare it with a
working one, probing each one side by side, to see up to where it
appears to be working. It would be awkward to have two dummy loads,
just for lack of suitable equipment. Does anyone know if it is safe to
run these PSUs with no load? Would my testing be valid without a load?
If it has a
transistor at the primary side, and a high frequency a switching supply. Those want a load, usually.
Especially the older
ones.
If it has a laminated iron transformer, big
electrolytics for filtering,
and
transistors at the secondary side, it's
probably a linear regulator
supply. Those
don't need a load. You may want to do full
testing under load to observe
correct regulation, but for initial testing you can test them without.
Linear supplies are older; I think by the time of VAX you're likely to see
switchers (with the possible exception of first generation stuff like the
780, I
don't remember if those big supplies are
linear or switchers).
paul
Hmmm.... Until your email I was pretty confident this was a switcher. But
your description of a linear PSU seems to fit. I have posted three photos of
the main board here: http://1drv.ms/1KQkTBp can you tell from that what kind
of PSU it is?
The big TO-3 on the left is the one that I found had failed. The replacement
does not appear to be switching, but I don't know why.
Regards
Rob