DEC H7260 PSU fault

Jules Richardson jules.richardson99 at
Thu Jan 25 21:29:08 CST 2018

On 01/25/2018 07:16 PM, Glen Slick via cctalk wrote:
> On Thu, Jan 25, 2018 at 3:06 PM, Jules Richardson via cctalk
>> OK, the top PSU of the pair in this machine appears to be healthy. The
>> bottom one, however, sits at around 2.5V on both the +5V and +12V outputs.
>> That's without any cards, and the card-cage fan / temperature sensor
>> unplugged, i.e. just with my dummy loads in place, so it looks like that PSU
>> has issues (oh, and I tried decoupling the PSU from the backplane and
>> loading it via the drive connector, so it doesn't appear to be a backplane
>> issue).
> Looking here at page 107 of the maintenance print set, BA123-A Basic
> Enclosure Power Harness Wiring Diagram
> Did you disconnect the wiring harness from J10 completely? If you
> disconnected the temperature sensor module or the card-cage door
> interlock switch I would expect that the power supply might not
> operate normally.

I believe that I tried it both connected and disconnected, with no change 
in behavior, but I'll double-check (not until tomorrow evening now, though).

I need to look at how the signals on J10 interact with the entire PSU 
internals - it certainly seems to get a -15V feed from the lower PSU of the 
pair (the one that's causing me issues), but many of the wires run to the 
fourth, small PCB that's in the entire PSU assembly; I assume that one 
handles fan control as it also connects the the PSU fan and to J7 (which 
supplies the fan in the mass storage area)

It seems strange to me that one PSU of the pair would sit in an 
under-voltage condition due to a sensor issue while the other one would 
continue to operate normally, though; at this stage I think it's more 
likely that I've got a PSU fault (or a fault with the rectifier/relay board 
which supplies it - I'm most of the way through tracing a schematic for 
that board, so I can test some voltages sensibly tomorrow)

Will let you know how it goes...


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