D'oh - Re: PDP-11/23 H786 PSU Fault Finding

David Comley david_comley at yahoo.com
Mon Jun 29 17:04:53 CDT 2009

> On 6/29/09, Tony Duell <ard at p850ug1.demon.co.uk>

> > My first question is 'do you have enough load on the
> 5V line?'. If you
> > don't, the chopper circuit is running very 'lightky',
> and probably the
> > 12V side isn't getting enough power to work properly.
> I've certainly had
> > DEC supplies where the 12V output is low untill I
> loaded the 5V output.

--- On Mon, 6/29/09, Ethan Dicks <ethan.dicks at gmail.com> wrote:> 
> There's a load-board for the uVAX2000/VS2000 box for use if
> you don't
> have an internal hard drive.  AFAIK, that's why it's
> needed - to
> ensure enough load on the +5V line for the +12V regulator
> to work.

Well, as both Tony and Ethan noted, you do indeed have to load the +5V line up before the +12V line will regulate properly.

I put a low wattage automotive turn signal bulb across the +5V line and the +12V line jumped right back to where it should be.

Many thanks. I can now reassemble the 11/23 and get it back in the rack.

Don't you love it when stuff works ! Now I just have to fix that RLV12...


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