DEC H7260 PSU fault

Jules Richardson jules.richardson99 at
Fri Jan 26 19:15:02 CST 2018

On 01/25/2018 09:29 PM, Jules Richardson via cctalk wrote:
> 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).

OK, I just verified that - it doesn't matter whether the J10 harness is 
connected or not, same behavior.

The outputs of the main relay/regulator board which feeds both supplies 
appear to be the same, so I don't think that's at fault. I think my next 
step is to trace out the wiring for the temp sensor, along with that fourth 
"fan control" PCB within the PSU module so that I can better understand 
what it's trying to do.

After that... well, I need to work out how to get the entire supply 
assembly to start up while it's open, so that I can work on it outside the 
machine - then I can at least start looking for differences between the two 
supplies (boy, are those things complex, but maybe I'll get lucky and it's 
a bad solder joint or cap somewhere)



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