DEC H745 regulator common failure modes?

tony duell ard at
Fri Nov 27 12:09:57 CST 2015

> 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.

I am not sure what you mean by 'tough to work on'. If it's a physical access 
problem then what I did was to obtain a spare 8 pin mate-n-lock plug and
socket and make up a pin-pin wired extension lead about 1m long. Then I 
can take the brick out of the PSU, connected it via the extension lead and be
able to probe it. I normally sit it on top of the CPU box of my 11/45, you get
easily enough air flow from the fans to run it for testing.

If it's that the circuits are complicated, well, remember these are swtiching
regulators (not linear) but are a lot more repairer-friendly than most SMPSUs
in that nothing in the brick is directly connected to the mains.


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