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

tony duell ard at
Sat Nov 8 17:01:49 CST 2014

> > 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


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