H7821 power supply in MicroVAX 3100, SCSI disk enclosures and others

Peter Coghlan cctalk at beyondthepale.ie
Fri Jan 31 05:58:56 CST 2020

The recent discussion on BSC protocol prompted me to dig out my Microvax 3100
with DSH32 synchronous serial interface.  It had been idle in storage for
several years and it wouldn't power up, only giving a brief flash on the
diagnostic LEDs and a quick twitch of the fans.  There was a slight smell, like
the stale air that comes out of a deflating tyre.

I took out the H7821 power supply and found that five identical brown 1800uF 25V
electrolytic capacitors on the output side had leaked.

The SCSI disk enclosure where the machine's system disk lives required several
power cycles to get it to run at all and it died as soon as the disk tried to
spin up.  It turned out to also contain a H7821 power supply which had a
similar issue with the same five brown capacitors, although not as extensive
as in the main unit.

I found a second disk enclosure which had seen little use and grabbed the power
supply out of that to put in the MicroVAX.  It worked well enough to test with
but there was a ring of goo around the bottom of one of the brown capacitors
which was worst affected in the other units.  Time to order a batch of
replacement capacitors and figure out what else has been damaged.  While it is
not the worst I have seen, access to these power supplies for repairs is quite
difficult and it is really difficult to debug them safely while they are
running with the cover off :-(

If anyone has anything with H7821 power supplies in them, I suggest checking
on these capacitors.  If anything with these power supplies is in storage, I
suggest ensuring it is stored the normal way up as this should limit the
ability of the goo to escape and spread around the power supply.

And there I was thought I was being safe enough by removing the nicad battery
packs some years ago...

Peter Coghlan.

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