17 May
2005
17 May
'05
4:08 p.m.
I checked the HP 9845B and it's absolutely
dead. The fan doesn't even
run. However the fault should be easy enough to locate. It's odd though,
Be warned that thr 9845 PSU is _very_ complicated, even worse than the
PDP11/44 PSU (!).
Anyway, the input circuit isn't too bad. There's a mains-frequency
transformer (the laminated-core one on the metal bracket) that has 2
primary windings. They're in series for 230V, in paralell for 115V. The
output of this transformer provides the startup supply. There's also the
conventional bridge/doubler circuit with the 2 massive capacitors in the
middle of the PSU mainboard to supply 350V DC to the choppers
Anyway, the fans are connected in parallel with the startup transformer
primaries. So on 115V they're essentially connected across the mains.
There is a fuse at the back (10A IIRC for 115V) that I'd check first,
otherwise look at the wiring on the back panel, the barrier strip on the
side of the PSU casing (be warned that the screws that hold the cover
over that go into loose nuts inside the PSU casing, so pull the PSU
module first), etc. This is one of the not-too-complicated parts of the
machine.
-tony