DEC H745 regulator common failure modes?

tony duell ard at
Fri Nov 27 07:01:02 CST 2015

> 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!


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