IBM AT Free to a Good Home

From: Don Maslin <donm_at_cts.com>
Date: Fri May 21 22:56:25 2004

On Sat, 22 May 2004, Tony Duell wrote:

> > 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 :-)).

Well, we in the colonies used to enjoy that type of joke, but
over the years have managed to hyper-sensitize just about all
ethnic groups, such that you put your health - physical or
financial - in jeopardy if you tell one. It tends to bring out
dirty looks, unfriendly comments or the "everyone is picking on
me" society - aka: American Civil Liberties Union - which can get
expensive in keeping some lawyer well off.

                                                - don

> 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 - 22:56:25 BST