21 Oct
2007
21 Oct
'07
3 p.m.
Hi Guys,
I have a "VAXserver 3100" which afaik is the same hardware
as a Microvax 3100. This unit has a bad power supply.
The power supply is labled: MODEL H7822-00
Anyone have schematics or other technical information?
I've never worked on this supply (and don't have schematics), but it
can't be that odd, can it???
In case anyone has run into this before - the symptoms:
This supply appears to be two "mostly separate" supplies
in one box. Both boards have connection to AC power.
One board generates +5, -5, +12 and -12v, which is the
sent via jumper wires to the second board - There it
appears to be monitored by a "power good" circuit, and
passes directly to the connector for the VAX mainboard,
and one of the two disk drive power connectors. All of
these rails look good.
The second supply generates +5 and +12v which is applied
only to the second disk drive power connector. This supply
constantly starts and stops - with or without a load. I
can see the output voltages coming on, then going off
over and over.
Wouldn't it be best to find out why it's doing this? My _guess_ is that
either one output is rising too high, tripping the crowbar, and thus
shutting the supply down, or, less likely, the supply to the control
circutiry has failed (the reason I mention the latter is that that fualt,
caused by a dried-up capacitor on the _primary_ side of the supply, once
led me a merry dance in a Tektronix X-terminal). I'd be inclined to start
by checking _all_ the electrolytics on that second PSU board with an ESR
meter. An high ESR capactior on the output side can put some
'interesting' high-ish voltage spikes on the output.
Since this supply drives only the second disk drive
power connector, the system shoud come up, however it
does not - I believe the reason for this is that the
power monitor is either holding the system in reset,
or providing a "bad power" indication which otherwise
halts the system because the second drive supply
voltages are not stable.
The connector to the VAX mainboard looks like this
(viewing end of power supply cable)
-12 +5
Gnd +12
Gnd +5 [Key ]
Gnd Gnd [Tab ]
+5 Gnd [Here]
+5 -5
?1 ?2
The ?1 and ?2 signals appeare to be control signals.
All power rails look good. With a supply borrowed
from a Vaxstation 3100 (smaller supply without the
extra drive connector/supply, but same main pinout),
I see 4+ volts on ?2 and some low but non-zero voltage
on ?1 - with the bad supply, both ?1 and ?2 are at
0v.
Can anyone tell me the names and description of
function for the ?1 and ?2 signals - I'd like to
know exactly what these are supposed to do as the
supply comes up.
Being a DEC supply, these might well be open-collector signals, pulled up
on the mainboard. At least one is likely to be power-OK. The other might
be a second power-OK (some DEC machines have separate ACLO and DCLO
signals), it might be a line-frequency [1] clock interrupt signal. Have
you tried a 'scope on the signals from the good supply?
[1] After Will's flame last night, I won't call it 'mains frequency', for
all that's what wee normally call it over here. 'Line' to us is the
horizontal deflection on a raster CRT display, so 'line frequency' over
here is what you'd call 'horizontal frequency'....
-tony