VAX 4000-500 PSU Overload?

Glen Slick glen.slick at
Sun Dec 13 19:35:34 CST 2015

On Thu, Dec 10, 2015 at 11:42 PM, Holm Tiffe <holm at> wrote:
> Another thing is that small PCB on the DSSI storage Backplane, the Caps
> there where dead too. I thing it is the termination Power regulator and in
> my case the bad caps "simulated" a bad disk, I could'nt initialize this
> disk properly before I've changed the Caps there too..

I took a look at the DSSI & SCSI termination power supply board in my
VAX 4000 BA440 chassis. It's the small board in the storage area of
the backplane mounted in place by a single screw. There are two
identical circuits based on the LT-1086 with the adjustable output
voltage set at 5.30V  [1.25V * (1 + 392/121)]. Each circuit has a
100uF 35V 105C nichicon cap between the input and ground and the
output and ground. Both circuits on my board still output 5.3V from a
12V input, but I think the caps are starting to leak electrolyte on
the PCB and I should replace them all (4 total) before things get

Each circuit has three diodes, an SB520 and a 1N4735A for which I can
find datasheets, and a third one which looks like a General Instrument
MP654 for which I can't find a datasheet and I'm not sure if that is
the correct part number. I can't quite read the marking all the way
around the body of the diode. If someone else takes a look at their
BA440 DSSI & SCSI termination power supply board let me know if you
can make out what that part number is, or if MP654 is a GI part for
which you can find a datasheet.


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