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

Brent Hilpert hilpert at
Sat Nov 8 22:26:08 CST 2014

On 2014-Nov-08, at 7:24 PM, drlegendre . wrote:
> Tony,
> 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?

No, in a strict sense, a TV or monitor doesn't 'need' a horizontal or vertical oscillator, it just needs the V & H sync pulses to start the ramp generation at the proper time.

Historically however, a TV had V & H oscillators so that:
	1. A raster would be present when there was no signal, in part so it didn't burn a hole in the center of the screen.
	2. The raster would ride through sync pulse dropouts on weak or marginal signals, rather than collapsing sporadically.
	3. The horizontal oscillator needed to keep going, consistently, so the CRT HiV would be generated and stable.

(One might argue 3 mitigates 1, no HiV, no burn).

Once you have the oscillators, you still need the sync pulses to keep the raster scan ramps in phase sync with the video picture information.

Early computer monitors were just slightly hacked/improved versions of TV monitors and old design habits die hard.

As mentioned,  some designs did do away with the oscillators, when the sync could be relied upon to be present and stable. IIRC, the Commodore PET with it's built-in monitor was another of those.

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