NEC P1250+ dynamic focus

This can sometimes happen with substituted flybacks. In some sets, it is the components in the HV multiplier that handle the dynamic focus using RCL combinations.

If your set does use discrete dynamic focus circuits, I would strongly suggest getting the service manual to troubleshoot this. I have only seen discrete design for dynamic focus in the very high end TV sets, or in studio broadcast monitors.

If the HV is too far out of specs, the dynamic focus may be effected. This can also happen if the CRT is defective.

Have you tried swapping the two focus leads to the CRT?

On Wed, 23 Mar 2005 04:29:34 -0500, "Jerry G." put finger to keyboard and composed:

Slightly OT, but can you explain how these dual focus controls work?

Feature

- 14" ~ 20" Color Monitor

- 31.5KHz ~ 100KHz Horizontal Frequency

- Double Focus Pack

- Dynamic Focus Capacitor Intergrated

AFAICS, accurate dynamic focus control could only be achieved by modulating a DC (static) voltage on the focus grid with an AC correction voltage that is a function of the beam's H&V position. How does the above FBT get its vertical positional information? Is the dynamic focus limited to the horizontal direction? Is the second focus control an input or an output from the FBT?

- Franc Zabkar

can

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board?

No not yet, could that have this effect?

TV and monitor equipment is very limited due to the cost. In the case of using an RCL constant in the HV multiplier, it is true that the correction is mainly on one plain. In looking at the aspect ratio, the H distance is greater than the V distance, so this works to the advantage of allowing some V error.

In the high end monitors they employ sophisticated circuitry that modulates correction to the focus voltage according to both the H and V beam position. If you look in to the high end of monitors that the broadcasters and studio production houses use, they have a true dynamic focus drive circuit. In some of the very high end models, the dynamic focus is uPC controled.

Jerry G. ======

Yes, it can be the cause of your problem. I'd give it a try. At least you would eliminate it as a possible cause.

On 24 Mar 2005 03:51:13 -0800, "Jerry G." put finger to keyboard and composed:

Thanks for the explanation. My one remaining concern is how this fixed RCL combination can work over the full horizontal frequency range of the monitor, say 31.5KHz ~ 100KHz, hence my question as to whether there was an external input to the focus block.

Yes, I've seen those, but only from the outside.

- Franc Zabkar