IBM AT Free to a Good Home

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Fri May 21 18:48:43 2004

> I have a "Casper" brand monochrome monitor, dumped by a neighbor when
> he moved. It has a 9-pin connector on it, though only 6 pins are
> actually fitted. It does not power up, and I have nothing to drive it

Quite normal. 2 grounds, Hsync, Vsync, video, and intensity.

> with anyhow. It is in nice physical/cosmetic condition, and is likely
> repairable. It's available for the cost of shipping to anyone who wants
> it.

I have (not up for grabs) a Zenith mono monitor. It had what I called the
'Irish PSU' (note for non UK people, over here 'Irish' jokes are much the
same as Polish jokes...). This combines the ease-of-repair of a switcher
with the efficiency of a linear PSU (for the software types, that's
roughly like combining the ease-of-understanding of machine code with the
speed of an Interpetted HLL :-)).

Seriously, this PSU started out by rectifying the mains, then fed it to a
free-running chopper with no regulation applied. This drove a little
transformer which gave out about 20V peak AC. This was rectified and
smoothed, then fed to a discrete-transistor linear regulator (which used
the green power-on LED as the reference !) to provide the 12V line used
by the rest of the monitor.

Notince I am talking i nthe past tense. One day the chopper transistor
failed. After many attempts at replacing it, and killing many expensive
transsitors, I did what I should have done in the first place. I removed
the chopper and mains rectifier circuitry and replaced it with a
mains-frequency transformer. Increased the smoothing capacitor on the
secondary side (after all, the original transformer worked at about 1000
time the frequencyt of the mains). Worked fine for several years

And then the picture collapsed horizontally and faded out. I suspected a
line output stage problem, but was amazed to find the fault was the line
driver transformer, one of the windings of which was open-circuit. That
was simple enough to rewind by hand, which is what I did. And it's still
working.

-tony
Received on Fri May 21 2004 - 18:48:43 BST