27 Nov
2015
27 Nov
'15
1:44 p.m.
Yep, I've already checked out the rest of the voltages thru the machine. The AC inputs
are correct, +15 is good, +5 is a little high at 5.5v but workable and I'll tweak it
later, -5 is good, etc.
Just the darn 745 module. Fuse and connector are okay as well, no obvious signs of
catastrophic failure.
Sent from my iPhone
On Nov 27, 2015, at 05:01, tony duell <ard at p850ug1.demon.co.uk> wrote:
Hi folks,
I've begun repairing my PDP 11/40 and have tracked down the first major issue to the
H745 (-15v) regulator.
Before you dive into the brick (sounds painful!) check that the +15V DC input is present
and correct. Unlike
the +5V units (H744, etc) the H745 needs both the 20V AC from the transformer and a 15V
supply which
comes from either the PCB on top of the transformer (in an H742 supply used with the
BA11-F) or the PCB
under the transformer in the supply used with the BA11-K
If that's OK, then it's the usual suspects in the brick. The capacitors can dry
up. There is a fuse, but it
never fails for no good reason, so if it is blown you need to find out why. Power
semiconductors on
the heatsink bracket.
Whatever you do do not disable the overcurrent trip (e.g. by desoldering the collector of
the sense
transistor). If you do, and the crowbar fires then you will be cleaning bits of
transistor off the
ceiling. Do not ask how I found that out!
-tony