For pickup: VAX-11/750 and RK07

[Chuck Guzis]

On 6 Jun 2009 at 18:17, Tony Duell wrote:

> Reminds me of the time smoke poured out of a 3rd party (Plessey IIRC)
> Unibus expanison box. It was clearly coming from the PSU area and I
> thought I could see something glowing orange inside. Turns out (after
> takign the PSU apart) that a bridge recifier had shorted and the
> orange glow was the wires linking that to the transformer -- all the
> insulation had burnt off and they were more than red hot. Replaced the
> rectifier and thw wires, no problems...

That UPS I wrote about a couple of days ago turned out to have a 
similar problem that should have resulted in magic smoke, but a 
thoughtful engineer saved me from that. 

An inspection of the curcuit showed that the UPS will not engage when 
powered on unless the batteries (4x12v wet cell lead-acid) are 
supplying current.  Once energized, the unit will supply power as 
long as AC power is present (allows you to change batteries without 
interrupting power). 

The batteries were flat.   A test with an automotive battery 
charger/tester showed that all would hold a full charge.

What caused my unit to brick was a shorted 1000uF 100V filter cap in 
the charging circuit.  But no bulging cans, magic smoke or any other 
sign that anything was wrong.  An ECO had been added at some time 
that included a 20A fuse in the secondary of the inverter 
transformer.  Said fuse was in a holder tucked away under a nest of 
wires attached to the inside front panel with an adhesive cable tie.

The unit kept operating until a thunderstorm caused the power to 
glitch momentarily.  With no battery power, the unit would not power-
cycle when the line supply recovered.  That fuse could have blown 
months ago without showing any outward sign.

Once the (blown) fuse was discovered, it was pretty obvious that 
something upstream was wrong.  A quick check with an ohmmeter turned 
up the bad cap.

The charging circuit is pretty crude by modern standards--an LM317K 
TO3 regulator on a large (4"x6") heatsink with a 50W zener and a 
thermostatic switch.  It could probably benefit with some upgrading 
to a "smarter" circuit.

This UPS has been in continuous service since 1989.

--Chuck