DEC H745 regulator common failure modes?

william degnan billdegnan at
Fri Nov 27 08:00:31 CST 2015

On Nov 27, 2015 8:04 AM, "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

There is a pdp 11 troubleshooting guide on Bitsavers that covers the h742
powersupply and regulators 744. 745, 754.  Assuming that is what you have.

They're all tough to work on especially the regulators but I agree with
Tony that you need to lock down the main suppl first.

That troubleshooting guide very useful just be prepared to completely
disassemble the parts.

Bill Degnan
twitter: billdeg

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