DEC H745 regulator common failure modes?

Ian Finder ian.finder at
Fri Nov 27 15:44:30 CST 2015

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> 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

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