VAX 4000-500 PSU Overload?

Robert Jarratt robert.jarratt at
Mon Dec 14 01:27:37 CST 2015

> -----Original Message-----
> From: cctalk [mailto:cctalk-bounces at] On Behalf Of Glen Slick
> Sent: 14 December 2015 01:36
> To: General Discussion: On-Topic and Off-Topic Posts
> Subject: Re: VAX 4000-500 PSU Overload?
> 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 ugly.

I just got the board out. The caps on mine look OK, but I see the ESR is at or just above the value suggested on the table printed on my meter. So I will replace them all.

> 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.

I had a look and all I can make out on one is"P654" with "143" underneath. On the second one I can see "MP" with "GI" underneath. So, presumably, the full marking is MP654/ GI 143. Presumably, one of these parts is the date code.



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