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Old 18th Oct 2021, 8:58 pm   #21
AdrianH
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Default Re: 5FP7 project starting with the focusing coil.

It takes me a few days to do the metalwork and build up the parts for these projects, but the next stage has been completed to a point. The EHT unit is working and provides approx 3.8kV for the final anode, 250 to 260 Volts for g2 and down to -42 volts for the control grid.

I have used a EL91 7 pin valve to drive the LOPT, a few component changes to get it up near 10KHz frequency, but it seems to be be working OK. seeing how long the EL91 will last as it is not meant to handle the voltage spikes but the one in the home made TV is still OK.

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Short video

https://youtu.be/el3mzvLhkHc

Be safe.

Adrian
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Old 22nd Oct 2021, 7:52 am   #22
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Default Re: 5FP7 project starting with the focusing coil.

Just wondering about adding an iron wire wrap over the focusing coil, does anyone know if it makes an improvement in that it may need less current through the coil?
Was thinking of two layers of 1.2mm, would it have any adverse effect?

Cleaned up an old Radar frame and started to mount parts in it to save all the mess.
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Old 22nd Oct 2021, 8:49 am   #23
dalekmoore2007
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Default Re: 5FP7 project starting with the focusing coil.

Quote:
Originally Posted by AdrianH View Post
Just wondering about adding an iron wire wrap over the focusing coil, does anyone know if it makes an improvement in that it may need less current through the coil?
Was thinking of two layers of 1.2mm, would it have any adverse effect?

Cleaned up an old Radar frame and started to mount parts in it to save all the mess.
Attachment 243929

Adrian
Looking good Adrian

Yes the Focus coil was always a problem to me in 1990 mainly didn't want to wind a new coil or add to what i had .
You would think a round permanent magnet would be a easier fix .

Magnetic Focus
In early television tubes and in some high quality tubes such as tubes for film scanning, the focus lens takes the form of an electro-magnet or permanent magnet producing, over a short region, a magnetic field parallel to the axis of the tube. This magnetic field acts to bend the electron paths towards the axis and by correctly adjusting the strength of the field, the point of focus may be made to occur at the fluorescent screen

Iron wire sounds like it would help i would try it .
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Old 22nd Oct 2021, 9:07 am   #24
AdrianH
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Default Re: 5FP7 project starting with the focusing coil.

I do have a permanent magnetic focus unit as well, but would like to be able to do it electrically. I may still have to get hold of some magnetic rings to centre the spot, unless I find a way of using the deflection coils for this as well as scanning. I believe doing the 400 scan lines will be OK with valves, just not sure about the 6Hz frame rate, that seems very slow, even when compared to 50 Hz frame rates.

The mains power supply will be next together with some drivers to get a raster, but, it could be some time as it is a holiday half term here and the wife is now at home.

Forgot to say I have a MW6/2 scan unit and that uses iron wires over the deflection coils, I guess it may do two things: -

1) Possibly concentrate the magnetic field lines into a more compact area as not as much tray magnetic field outside the scan assembly and
2) Lessen the effect of other magnetic fields from power transformers etc.

Adrian
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Old 22nd Oct 2021, 9:23 am   #25
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Default Re: 5FP7 project starting with the focusing coil.

Quote:
Originally Posted by AdrianH View Post
I also picked up some gear with another 5FP7 in it yesterday, it has an old LOPT in it which I am trying to figure out what it is from and it's connections.

Everything seems to be connected, as in everything has a resistance from one pin to another, so no separate transformer winding to go to the scan coils It looks as though it would have used pins to plug into the LOPT originally.

I was thinking that I may be able to add windings to it on the other leg of the ferrite to get 250 and -50 Volts?

So if anyone recognises the transformer i would be grateful.

Adrian
Looks very similar to the LOPT in this Rediffusion Mk7 TV thread. https://www.vintage-radio.net/forum/...54#post1416354

John.
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Old 22nd Oct 2021, 9:30 am   #26
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Default Re: 5FP7 project starting with the focusing coil.

Indeed it does, i will have to keep an eye on that thread to see if a circuit comes available.
Many thanks

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Old 22nd Oct 2021, 9:46 am   #27
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Default Re: 5FP7 project starting with the focusing coil.

https://www.radios-tv.co.uk/communit...2/#post-108145 Circuit of the cable version in this thread over on Vrat.

John.
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Old 22nd Oct 2021, 10:01 am   #28
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Default Re: 5FP7 project starting with the focusing coil.

Thank you John appreciated.

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Old 22nd Oct 2021, 10:08 am   #29
dalekmoore2007
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Default Re: 5FP7 project starting with the focusing coil.

Quote:
Originally Posted by AdrianH View Post
Just wondering about adding an iron wire wrap over the focusing coil, does anyone know if it makes an improvement in that it may need less current through the coil?
Was thinking of two layers of 1.2mm, would it have any adverse effect?

Cleaned up an old Radar frame and started to mount parts in it to save all the mess.
Attachment 243929

Adrian
Looking good Adrian

Yes the Focus coil was always a problem to me in 1990 mainly didn't want to wind a new coil or add to what i had .
You would think a round permanent magnet would be a easier fix .

Magnetic Focus
In early television tubes and in some high quality tubes such as tubes for film scanning, the focus lens takes the form of an electro-magnet or permanent magnet producing, over a short region, a magnetic field parallel to the axis of the tube. This magnetic field acts to bend the electron paths towards the axis and by correctly adjusting the strength of the field, the point of focus may be made to occur at the fluorescent screen

Iron wire sounds like it would help i would try it .
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