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

drlegendre . drlegendre at
Sat Nov 8 21:24:37 CST 2014


Ok, then I guess I +really+ don't get it.

Why does the Zenith +need+ a horiz. osc. when it also requires the horiz.
input signal - which just happens to be at the exact same frequency
(15.7KHz) as the horiz. osc? What is the horiz. input used for, then, if
not to set the frequency of the 555 output?

The 555 acting as a monostable (aka one-shot) triggered by the horiz.
signal makes more sense.. I'm not saying that's the case, just that I can't
see why it would need be otherwise when a properly-clocked signal is
already provided for it. Then again, if it works the way I am presuming,
the 555 could maybe be replaced with a Schmitt trigger - eh?

On Sat, Nov 8, 2014 at 5:01 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?
> The IBM 5151 has no horizontal oscillator at all. The input signal goes
> straight to the horizontal
> driver stage.
> This Zenith monitor does have an oscillator. The 555 is an astable
> (compare it with the standard 555
> circuits).
> -tony

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