(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