Edge Focus & Scan Coil Design

[peter scott]

In trying to display the Indian Head test pattern on the old Emiscope 6/6 it becomes very obvious that focus near the periphery of the circular screen deteriorates quite badly. One obvious thought is whether the electron path length is different towards the edges but manipulation of the focus control makes absolutely no improvement so there has to be some other mechanism.

   

Looking in Fink "Principles of Television Engineering" he describes the effects of non-uniform electric or magnetic field. I wonder how design of scan coils changed to achieve the more uniform field that gives the good edge focus that we see in later sets or whether some other cause is involved?

   

Peter

[peter scott]

Another thought has just occurred to me. Perhaps the scan coils are just too low on the neck? I did did remove the push-about coil because it distorted the focus but it also served to hold the scan coils nearer to the cone.

Must refit the push-about coil.

Peter

[Terry]

The Stock Exchange Cable TV system I've mentioned before used computer generated text and modified domestic monochrome receivers.

The text was 30 rows of 48 characters which virtually filled the active display area (the sets were under-scanned so that all the text was visible).

The initial sets were 19" & 23" and tended to suffer badly from defocusing in the corners - particularly the 23" tubes. Later we standardised on 20" sets only.

I sometimes wondered if it might be possible to generate parabolic waveforms, similar to those used for convergence in colour sets, and use them to modulate the focus voltage in the corners although, as many tubes also suffered from severe astigmatism in the corners it probably wouldn't have been an ideal solution.

I wonder if it would have worked?

[peter scott]

Thanks Terry, I guess these were 110 degree sets. I can well imagine different electron beam path lengths in these sets and varying the focus to take account of that might give some improvement.

With the Emiscope 6/6 I think the radiused screen probably doesn't cause much path length variation over the scan and whilst my focus control does give a reasonable range of focus adjustment in the screen central area the point of best central focus is also the point of best edge focus and any move away from that point only worsens the edge focus.



My scan coils were down from the start of the cone by about 4 or 5mm due to me removing the push-about coil but with this coil in place and the scan coils fully up to the cone I get no improvement in edge focus. If I had to pass a judgement I would say the edge focus is very marginally worse with the scan coils hard against the cone.

Peter

[ppppenguin]

I think dynamic focus was used in some 110 degree colour sets and also in some professional monitors.

[Mark Hennessy]

...and in plenty of widescreen sets too.