IBM 5151 (MDA display) - common faults / gotchas?

drlegendre . drlegendre at
Sat Nov 8 11:17:24 CST 2014

Hey, I had a question about this..

On Fri, Nov 7, 2014 at 11:49 PM, tony duell <ard at> wrote:

> It's a very simple circuit, the schematic is in the TechRef. One odd
> feature is that
> there is no horizontal oscillator, it just drives the horizontal section
> from
> the pulses on the Hsync (more correctly HDrive) input.

This is the same situation as the Osborne Vixen + Zenith 7" display. The
display takes in a horiz. signal (a vert. as well, but that's another
matter) which gets routed to a transistor, and then a 555 timer. From
there, one more transistor and then comes the horiz. driver & opt. section.
Anyway, the first two transistors and the 555 are collectively labeled
"horizontal oscillator".. but that's not exactly the case, is it?

I measured the horiz. input signal at roughly 16Khz; the chassis says it
uses a 15.7Khz signal, so I assume that's the true freq. So if that's the
case, it doesn't actually need an oscillator, does it? My guess was that
the 555 was more of pulse-shaping deal.. it only uses the horiz. input as a
trigger pulse, and then outputs a clean square / rectangle wave of whatever
duty they want from the 555. That pulse then gets amplified. shaped,
whatever by the next stage until it feeds the horiz. driver which in turn
feeds the HOT.

Is that what's going on in these monitors?


I'm pretty sure I have one good-working 5151 monitor here, and just maybe a
dead unit as well. If you need the worker as a reference, you're welcome to
borrow it for as long as you require it to get yours repaired. Please just
promise to be very careful with it.. ;-)

And if I have it, the other one might be a parts donor should you need
anything specific to the design, that you can't get off the shelf etc. Keep
it in mind, for when we meet-up next spring or whenever.

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