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Old 7th Oct 2021, 6:27 pm   #1
AdrianH
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Default 5FP7 project starting with the focusing coil.

The eventual plan is to build a valved old fashioned SSTV/NBTV monitor using this ex RADAR tube for the display. It is also a learning exercise for me, in that I hope to understand more of the principles of electro-magnetic deflection.


Although I believe, a LOPT will not be driving the scans for such a slow unit I also want to understand these better and how they work and are calculated etc. So as time goes on expect lots of questions that may seem trivial, but please understand before taking up the hobby I was not a TV engineer.


The coil former was turned from black nylon, 42.35mm diameter and 52mm long where the wires will go. I then parallel wound 7000 turns of two 0.16mm dia, enamelled wire, giving a max rating of around 12mA.


It takes 66mA to focus the spot at 2KV EHT and 82.5mA at 3.5KV EHT. Scan coils and static deflection to follow at some point.

https://youtu.be/BU0NfyYazbI


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Old 8th Oct 2021, 5:29 pm   #2
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Default Re: 5FP7 project starting with the focusing coil.

Looking at what you're doing - If you haven't already done so I'd suggest you visit the late-1970s RSGB "Radio Communication Handbook" and its section on 'image communication' which includes a design for a SSTV monitor using a 5FP7, the circuit to produce the EHT, and schematics for the line- and frame-scan drivers [both using vintage Germanium AD161/AD162 transistors].
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Old 8th Oct 2021, 6:12 pm   #3
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Default Re: 5FP7 project starting with the focusing coil.

OK thanks for the information, I will do so and have a look at it, the plan is to be all valved if I can long term. The initial idea is to learn as I go, so different scan coils for 405 TV and NBTV if I can.

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Old 9th Oct 2021, 10:46 am   #4
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Default Re: 5FP7 project starting with the focusing coil.

Hi Adrian !
I see we are very like minded on the CRT interest ...Nice tube .
Below is my you tube channel top line my NBTV SSTV system 240 line with a very low frame rate keep it all in audio bandwidth .

https://www.youtube.com/channel/UC9e..._E3FpVgyObW8LA

Yes as you know i too with valves are trying the same sort of idea my tube is faster than yours ..

I infact had your CRT tube about 1990 before i moved to Melbourne and made a SSTV with it ,it stated out as a failed project by another constructor who gave it to me , so i didn't have to wind the focus coil thank goodness ...but it sort of put me off focus coils as i found after a bit of use the coil heated up a bit and would defocus the raster spot a bit .
i think it would of needed more turns to fix this .

I would work on deflection easy to burn your CRt with out this .or run it very defocused.
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Old 9th Oct 2021, 10:53 am   #5
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Default Re: 5FP7 project starting with the focusing coil.

Quote:
Originally Posted by AdrianH View Post
OK thanks for the information, I will do so and have a look at it, the plan is to be all valved if I can long term. The initial idea is to learn as I go, so different scan coils for 405 TV and NBTV if I can.

Adrian
Here's second CRT SSTV build
http://www.taswegian.com/NBTV/forum/viewforum.php?f=19

I will check your video out on the first post bit later tonight when i have more time .

But looks good from a quick look .

You are doing your Project very similar to me one problem at a time and see how it gos i never know how the project will end up like just what i want it to do .....i will post more on mine soon i was working onitit today .
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Old 9th Oct 2021, 7:09 pm   #6
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Default Re: 5FP7 project starting with the focusing coil.

A bit of lathe turning making two formers from Nylon bar and winding two deflection coils for DC testing. the next step would be to get all the gear out to obtain the voltages again. I need to come up with a simple means to get variable EHT, 2 to 6 KV. 250 Volts and -50 volts. But until I figure things out that is a way off.
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I will give it a try and see how the deflection goes.

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Old 9th Oct 2021, 9:27 pm   #7
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Default Re: 5FP7 project starting with the focusing coil.

Done some testing and was very pleased with the results. The coils in series are 600 turns each in total give 54mH inductance and 63 Ohms resistance and running the EHT at 3.5 kV it needs approx 95mA for 50mm deflection. So for a first go I a pleased with that.


https://youtu.be/5KjOY8nr0j4://

Cheers


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Old 9th Oct 2021, 9:44 pm   #8
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Default Re: 5FP7 project starting with the focusing coil.

Quote:
Originally Posted by AdrianH View Post
A bit of lathe turning making two formers from Nylon bar and winding two deflection coils for DC testing. the next step would be to get all the gear out to obtain the voltages again. I need to come up with a simple means to get variable EHT, 2 to 6 KV. 250 Volts and -50 volts. But until I figure things out that is a way off.
Attachment 243038Attachment 243039

I will give it a try and see how the deflection goes.

Adrian
Adrian on my sstv build i used a Chinese bug zapper it out puts about 7kv clean DC with a lm317 variable HV worked for me
Another idea if you are using mains transformer for the ac side of it put one the transformers Ac windings in series with a tiny mains ac transformers primary in series with it ..you can control the ac output by the tiny transformers secondary as a magnetic amplifier so varying the ac to the multiplier.....when the secondary is open you get a lower voltage out of the tiny transformers primary ...if shorted full voltage out so very safe ...using high wattage resistors across the secondary you can control what you want the ac out put to be you could also use a light globe to with a resistor
a lot of video and information show dc controlling the transformer but this case you can get a nice range without it ,

https://www.youtube.com/watch?v=DBX1-POuJMw

Btw i used the bug zapper idea on this monitor as well
http://www.taswegian.com/NBTV/forum/...hp?f=15&t=2436
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Old 9th Oct 2021, 9:58 pm   #9
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Default Re: 5FP7 project starting with the focusing coil.

Thanks for the suggestion Dalek I will have a look at them, the magnetic control I seem to remember using in an old welding set many years ago, at least I think it used that principle.

I do have a Velleman kit PWM controller that has a blown output device. I was going to get that fixed and see if the frequency is fast enough to drive a LOPT and get HT that way.

But I would really like to build it all with a valve if I can using one as a RF osc placing it all in a screened box. Well that's the plan at present, it just takes time, patience and brains, I am starting to get in short supply of two.

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Old 10th Oct 2021, 2:57 am   #10
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Default Re: 5FP7 project starting with the focusing coil.

Quote:
Originally Posted by AdrianH View Post
Thanks for the suggestion Dalek I will have a look at them, the magnetic control I seem to remember using in an old welding set many years ago, at least I think it used that principle.

I do have a Velleman kit PWM controller that has a blown output device. I was going to get that fixed and see if the frequency is fast enough to drive a LOPT and get HT that way.

But I would really like to build it all with a valve if I can using one as a RF osc placing it all in a screened box. Well that's the plan at present, it just takes time, patience and brains, I am starting to get in short supply of two.

Adrian
Its Harry BTW ; ) i like to do the HV with mains transformer up to a point as it is cleaner than a high frequency inverter as you can get a dirty DC result in the display as i found out on the Beast monitor i swapped to a mains multiplier got 3kV which was fine for that one.

I had the same idea to do the DC side of it all with Valves too but i got talked out of it for now ..if any thing i can see its needs with the power supply i have made .

Any case perhaps because i used the high frequency inverter on a electrostatic tube this was the problem i have to admit never have had this problem with a magneticlly deflected tube so you should be right ...with your idea of a valve inverter and a magnetic amplifier varying the supply this would think work ..i was meaning to ask why do you need it variable ? if you know the tubes data you aim for its normal operating range voltage.
But does help running it up how it works on lower voltages used the idea on CRT i had no idea no data .

I am using a 3bp1 the supply for it is - 1200v at the moment which should be workable on that .

Do you have any links to any of your circuits as i am still a baby with doing all this with valves i found your page and interesting seeing your past CRT work ...i am glad you pointed out the CRT dot limit size is a problem on higher line rates on small CRTS reason for use of larger CRTS and most people have no idea of this problem .
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Old 10th Oct 2021, 7:17 am   #11
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Default Re: 5FP7 project starting with the focusing coil.

I came across this may be helpful
https://geeseang.wordpress.com/exper...radar-crt2014/
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Old 10th Oct 2021, 9:15 am   #12
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Default Re: 5FP7 project starting with the focusing coil.

It was Geesang's posts that convinced me to try the 5FP7. I came across his work when I was on Facebook last year as I was building up the home made TV, I later left Facebook as a platform.

I have a small selection of CRT's from 2 inch to 6 inch mainly ex-scope or RADAR tubes and at times swap tube sizes. So having things that are variable can help a lot when changes are made.

I have used mains transformers to get up to the 1100 Volt range using a simple Cockcroft-Walton multiplier, but for higher EHT I prefer something that is not mains derived.

As to circuits, the Worldradiohistory site and Practical Wireless and Practical Television publications have been my main source of inspiration. Many of the TV articles made it into a couple of books which I eventually picked up "Practical Television Circuits" and "Television Principles and Practice", both by F.J.Camm.

Then there is Foundations of Wireless by M.G.Scroggie, the older versions are a must have to me.

I copied many of the relevant magazines topics from various months publications and saved them on the PC as complete pdf articles. They include home made TV, explanations of Sync separators etc, these I can print off and then scrawl over with my notes.

If you, or anyone wants these pdf's, pm me you email and I will send them.

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Old 13th Oct 2021, 3:53 am   #13
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Default Re: 5FP7 project starting with the focusing coil.

Hi Adrian my email address is dalekmoore2007@gmail.com i would email you but i don't know yours .
Yes any thing you think of interest do email always nice to add to the information.
BTW what was Geesang's FB page called wouldn't mind having a look ?
This also a good site on this fellows CRT work lots to look at .
http://www.vk6fh.com/?fbclid=IwAR1jL...4d5ai2niLN1CJg
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Old 13th Oct 2021, 5:04 pm   #14
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Default Re: 5FP7 project starting with the focusing coil.

The RSGB-manual SSTV monitor using a 5FP7 has this circuit to generatre HT for the tube, using a LOPT with the feed to the scan-coils removed.

The potentiometer varies the mark:space ratio of the oscillator and hence the HT.

These days I'd look to doing it using a 555 and a switching transistor scarfed from a SMPS.
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Old 13th Oct 2021, 6:32 pm   #15
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Default Re: 5FP7 project starting with the focusing coil.

Unfortunately I can not find on the Internet the book. It is not on the internet archive or in Worldradiohistory as both have removed RSGB publications.

So thanks very much for the small circuit section.

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Old 13th Oct 2021, 6:47 pm   #16
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Default Re: 5FP7 project starting with the focusing coil.

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
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Old 14th Oct 2021, 9:42 pm   #17
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Default Re: 5FP7 project starting with the focusing coil.

Hi Adrian , I built an SSTV monitor based on a 5FP7 45 years ago.
If I remember correctly the EHT generator was EF80 osc into a 6BW6 driving a Thorn 1500 Jelypot with an EHT tray that I think was a doubler. The rest of the circuit was solid-state, the time-bases were 741 driving a TIP41/TIP42 pair. I'm sure I over did the EHT voltage and flashovers regularly did for one or both 741s!

The scan coils came out of a 1950s TV and as there was no focus coil, I must have used a permanent magnet, probably out of the same TV, the focus was very good.

Those 5FP7s are versatile things, with a blue filter over the screen, its raster has a short enough persistence to make make a passable flying-spot scanner, I did some SSTV and fast scan experiments with a 921A PM tube.
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Old 14th Oct 2021, 10:26 pm   #18
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Default Re: 5FP7 project starting with the focusing coil.

Hello Graham. I am going to try using a version of the circuit provided in post #14.

I spent a few hours in the garage/workshop making a small chassis and drilling holes for the valve sockets. I will admit that I am initially trying ECC82 & EL91 valve, just because I have the EL91's. I think I should be OK within the valve ratings it does fine in the home made TV.

I am not sure if the SSTV unit in the 1970 RSGB book was all valve or just the HT unit?

I am pondering if I can scan the full width of the tube with valves rather than using a transistorised push pull circuit. Just not sure if with a very slow frame rate, 6Hz, I will get the required negative going flyback in the coils to take the trace to the left of centre? With a transistorised and split power rails it would be easier. Just something for me to ponder.

I used my little component tester TC1 on the LOPT I have, and measured a whopping 7 Henry from the overwind to the other end of the coil. A lot higher than I expected? It is getting 20+mA passed through it to get rid of any moisture that may be in it.

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Old 15th Oct 2021, 6:26 pm   #19
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Default Re: 5FP7 project starting with the focusing coil.

For info: the RSGB SSTV circuit is all solid-state [silicon transistors in the separator/trigger/sync-circuits and AD161/162 Germanium transistors driving the sweep-coils] apart from the CRT and the HT-supply.
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Old 15th Oct 2021, 7:18 pm   #20
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Default Re: 5FP7 project starting with the focusing coil.

OK thanks for the info.

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