WTD: DEC 11/40 power supply (main unit) model H74

Tony Duell ard at p850ug1.demon.co.uk
Tue Aug 12 02:01:11 CDT 2008

> The part number on the front is "H74 a" and it's the big part of the power 
> supply in the back.  In front (looking from the side) are 5 "h754" 
> modules.  The modules and the console power are attached to the "H74 
> a"  Other H74x supplies should work.

I think you're missing some digits....

The normal PSU for the 11/40 (at least in the 21" mounting box) is an 
H742 (the suffix letter gives the mains voltage IIRC). There are slots 
for 5 regulator 'bricks', but I thought most of them were H744s (+5V 25A) 
ones. I think the H754 is -15V 10A, and I can't think why you'd need 5 of 

> Here is the wording on the unit:
> Dec Power Supply
> H74 a
> Caution
>        (( ))
> ------------
> where (( )) = the light bulb.
> System was working recently.....The supply's red indicator light no longer 
> comes on, nor do the console lights, though the fans still run.  In the

The fans _in_ the power supply -- the 3 above the 'bricks' and the one 
over the +15V regualtor board at the front) are run off the mains input, 
as is the red neon indicator at the front and, of course, the big 
transformer. If the fans are running, then mains is getting to the PSU, 
and about the only reason that the neon isn't coming on is that it's 
failed. That is not uncommon.

have you checked the AC supplies to the bricks? You should have a 20-30V 
AC feed to each one. This supply is relatively repairer-friendly in that 
the only place you find mains is on the input to the trasnformer and, of 
course ,the fans, etc. All the electronic stuff is entirely isolated from 
the mains by that transformer.

Have you checked the voltage outputs from the bricks? And the +15V output 
from the regualtor board?

I would be very suprised if you need a complete new PSU. And quite apart 
from my dislike of board-swapping (or in this case assembly-swapping), it 
would be expensive to ship a PSU due to the massive mains transformer in 
it, which is very unlikely to be the problem. 

All the components in these PSUs apart from that transformer are 
standard, and AFAIK still easily avaialble (or at least substitutes are). 
So my advice is to grab the prints, grab a voltmeter, and jump in!


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