Line/frame scan coils

[AdrianPH]

This is just a general question out of interest, I think I have an idea of the question but I am often wrong.

Could a scan coil from a transistor TV be used in a valve set?  I think the answer would be no as the transistor sets could supply more current to the coils and hence less turns required to deflect the beam, where as for a valve set at the higher voltages, your have less current therefore need more turns to get back to the ampere turns needed.

But there must be a point where you have to much inductance to get a good scan at say 405 or 625 line speed and I also wonder what this value could be.  More turns also means higher resistance.  Is there any general guidelines?

Adrian

[ppppenguin]

Design of coils is always a compromise. But you've always got a line output transformer so, within reason, you can match any coil to any line output stage.

You can never have too much inductance as such, but the stray C may be too high to give you fast enough flyback. And the resistance may be too high. So for higher scan rates you would tend to design with fewer turns and need more current.

In an ideal world you apply a constant voltage to the scan coils which causes the current to rise linearly. At the end of the forward stroke you switch off the current, resonate the coil with a capacitor which generates a high voltage pulse and causes flyback. Then you turn the switch back on and let the energy stored in the magnetic field flow back into the supply. That's called energy recovery, usually done with an efficicency diode. Of course nothing is ideal so it's all a bit more complicated.

[Murphyv310]

Remember my Frankenstein Bush TV22?
It for a long time ran a Sony TV 9-90 CRT and scan coils perfectly, the LOPT is a Thorn 950 Jellypot.
https://youtu.be/-t8e48pEXcc

[AdrianPH]

I was not considering the LOPT stage at present all this tends to be magic to me especially when I build my projects.

I looked at a Youtube video yesterday about mechanical NBTV and it got me thinking about the amateur SSTV and stuff and the thought of building a simple RX with one of the long persistence Radar tubes I have. The issue would be making my own scan coils for it and how I would go on about it.

I also picked up some junk the other day and in it was a Toshiba chassis with a 280TB4 CRT, no idea what the TV was as the only label on the chassis I can find says TET-5005 which could have been just for the EHT unit.
When I looked at the scan coil on Tosh to the scan coil on the Ferguson 991T there seems to be a lot less turns on the Tosh, it may be the size of the tube neck that is confusing me, hence me thinking it's transistorised and needs less turns, therefore the question.

On the question of EHT I was thinking of just using an rf osc style EHT unit again, but this is many projects off yet.
Is there a place to still get these jellypot's anywhere or in reality would any LOPT be able to go from 405 to 625?

Adrian

[ppppenguin]

If you're playing around with ideas, there's a lot to be said for separate EHT and line scan. One of the easiest ways to make EHT is with a diode split LOPT salvaged from an old CRT computer monitor. Though these are getting scarce now and some are valued by retro gamers.

Google "diode split transformer eht generator" for ideas.

The scan coils for a wide angle modern CRT would be ill suited to a old narrow angle CRT. A lot of radar CRTs are electrostatic deflection anyway. This means either the X and Y plates are up at full +ve EHT so the scans have to coupled via HV capacitors. Or the cathode is at -ve EHT so the video coupling is a problem.

[AdrianPH]

Not getting notifications again.

The radar tube I was considering is a 5FP7 which has magnetic focus and deflection. I have two of them and doubt they would get any other use.

I will do a google search as you suggest. I do have the LOPT out of the Tosh, just wish I knew what model it was.

Adrian

[Kalee20]

Certainly for frame scan, resistance of the scan coils tends to dominate over inductive reactance, and that is independent of the number of turns (double the turns and the inductance quadruples, but so does the resistance).

For line scan coils, the same applies though luckily here, the numbers are such that inductive reactance dominates. But as Jeffrey says, more turns starts to add extra stray capacitance, which hurts. So, fewer turns are often chosen.

Whether valve or transistor is immaterial as if you drive the coils via a matching transformer, that can match to anything. Certainly circuit impedances are such that it might be possible to drive the coils directly without a transformer from a solid-state circuit. Though, DC blocking still needs to be considered, after all, you don't want the spot to be only scannable from centre to right, and from centre down, only...

[Mark Hennessy]

Posts regarding notification emails moved to here: https://golbornevintageradio.co.uk/forum...p?tid=8552

[Murphyv310]

Hi
In addition to my earlier post. When it comes to line drive for a swap to different coils most issues can be resolved by working out or selecting the coupling cap to the scan coils (S correction cap) non linearity will be reduced with careful selection.
I've never had an issue with substituting any "wrong"coils in the frame circuit even by fitting modern ones in a valve set.
I've also tried a 14 inch CRT and coils from a Thorn 1590 portable to the TV22 and it works perfectly with absolutely no component changes, the LOPT like I say is from a 60s Thorn 950 valve set.

[AdrianPH]

Looks like the junk chassis I picked up was the Toshiba 11TBE, so presently looking for a circuit for that, it could provide me with a portable HT source?


Adrian