Help w/DEC H742 +15V...

Paul Anderson useddec at
Tue Apr 7 22:28:10 CDT 2009

I think the +15 feeds all the regulators. It is regulated by a 54-09730 or
something like that. It's located in the high front of the H742 by the small
muffin fan.
I hope this helps. Also, the 11/40 has a lot of jumpers that have to be set
for the options you have(KW11, KE11, stack limit, FIS, etc). I know everyone
says to check power on the backplane, and not the molex, but now you know
why this is not always the way to go. Sorry, I don't have prints handy

On Tue, Apr 7, 2009 at 7:24 PM, Josh Dersch <derschjo at> wrote:

> So I'm now the proud father of a DEC PDP 11/40, which should be a fun
> project/adventure to get running again.
> I have been testing out the power supply and everything looked good this
> morning, at which point I figured I'd populate the CPU backplane and see
> what would happen.  The answer is: not much -- occasionally a few LEDs would
> light up, but the machine was completely unresponsive.
> I went a bit further and tested the voltages at the backplane, and while
> checking the +15V, my probe slipped momentarily, bumping a nearby wirewrap
> pin, causing a small spark.  Now, where I once had +15V, I have 0V.  And
> since the -15V regulator is powered (in part) from the +15V line, that's
> dead too.
> I was hoping, after reading through the documents again, that it would be
> something as simple as fuse F1 on the power control board, but no such luck.
>  The fuse is still good.
> Any suggestions for what's most likely to have blown when I screwed up
> here?  Any recommendations for working on this supply? I'm really looking
> forward to pulling this thing out of the rack to work on it -- this thing
> looks heavy :)
> Thanks,
> Josh (still slapping himself for screwing this up...)

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