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Reduce Scan Size on a Video Monitor?

Back in Ye Olde Tymes the tube sets used to have Width slug-adjusting coils, as well as pots for Height.

New stuff seems to have no adjustments whatever for Width, and only for Height if you are lucky.

I would like to be able to eliminate overscan on some monitors, to see the whole video signal. I have one supposedly Underscan-capable monitor that still does not show the corners of the video. Underscan monitors are harder to come by and expensive.

What part does the factory select to adjust the width? Can you stick a coil or resistor in series with the yoke winding to reduce the width? Will any of this mess up the geometry, disturb the high voltage, or result in a frantic call to the fire department? Thanks.

Reply to
Clive Tobin
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It's usually digital these days. The configuration is often stored in EEPROM. Many TV sets have a secret hidden setup menu that you can use to adjust these parameters. Most such TV sets can be smoked by setting bad parameters :(

Reply to
larwe

Thanks.

What parameters exactly are they changing to set the width and height? The drive current to the output transistors, or something else? Couldn't this be overridden maybe by changing a resistor value in a feedback loop or somewhere?

I don't imagine they have some mysterious expensive invention in there like a digitally programmable choke.

Reply to
Clive Tobin

The most recent TV schematic I've looked at was probably ten years old, so I don't know fersure. I'm making that statement based on what I read in sci.electronics.repair and also from the service manuals I occasionally get into. (Every TV set I've ever owned has been poorly adjusted from the factory, particularly with respect to convergence).

In fact, now I come to think of it - your question would probably get a better answer in sci.electronics.repair - why not try reposting there?

Reply to
larwe

I'd guess that some EEPROM values (digital) are passed through an A/D and applied to the appropriate sweep generation circuits as reference values (min and max ramp voltages). Other digital values are used directly as counters for time delays.

In fact, I believe some monitors generate sweep signals using direct digital synthesis of the waveforms. Its much easier to handle various types of geometry correction (pincushion, etc.) in software and then generate the sweep voltages by reading them from a look up table.

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Reply to
Paul Hovnanian P.E.

I saw an illustration to do exactly this in a Don Lancaster book. (The original Secret Money Machine - it was used as an example of a sketch vs. a printed illustration). Cut one wire to the yoke and add a coil in series, that can be shorted out by an SPST toggle switch when not required. I _think_ it was just on the horizontal but I don't remember. I don't have the book anymore and I don't remember the specs on the coil. This probably applied to a garden-variety TV set of ~1980; it may not work as well on a 1600x1200 VGA monitor.

I know that when line voltage is low, the image on some sets shrinks. If the set B+ is adjustable (with a pot), maybe turn that down and see what happens?

Matt Roberds

Reply to
mroberds

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