PDP-11/34A / BA11-K PSU

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Tue May 27 18:09:00 2003

> > (the section in 'The Art of=20
> > Electronics' 2nd edition is a good staring point),=20
> Unfortunately I don't own this book. When I tried to order it from .de,
> the cost was prohibitive.=20

It's not cheap in the UK (about \pounds 40, I think), but it is _good_. I
think it's worth the money. Of course I don't know what price you were
quoted...

>
> > and these DEC PSU=20
> > bricks are a pretty standard step-down regualtor circuit.
> Well, it looks like the H7441 has some funky design tricks.=20

Oh, the H7441. For some reason I assumed you had standard H744 5V 25A
regulators. Those also use the 723.

>
> > The only unconvention feature is that the controller is=20
> > a 723 chip, not a special SMPSU chip.=20
> The H745 has a 723, the H7441 has two OPAs and two NE555. The H745 looks
> quite simple and I think I understand what the different parts do, but
> the H7441??? One OPA seams to do some over current protection, one the
> voltage regulation. So the NE555 are doing the ON / OFF timing??? Have
> to dig deeper into that...=20

Probably one 555 is an oscillator, the other is the pulse-width modulator
or something. I don't have the H7441 printset to hand. But to be honest,
problems with the control circuitry are very uncommon in these bricks
(and indeed in SMPSUs in general) -- most of the problems are in the
power-handling part.

>
> Unfortunately it doesn't seam as simple as the SGI Indigo2 PSU repair I
> did today. I had a close look at the PCBs of the PSU and saw
> electrolyte... The machine seams to run stable again, now that I

As I said, most SMPSU faults are capacitors or power transistors :-)

> replaced the electrolyte capacitor in question. :-)=20

-tony
Received on Tue May 27 2003 - 18:09:00 BST