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


Adrian

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].

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.

Adrian

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.

Re: 5FP7 project starting with the focusing coil.
 

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 .

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.
Attachment 243038Attachment 243039

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

Adrian

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


Adrian

Re: 5FP7 project starting with the focusing coil.
 

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

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.

Adrian

Re: 5FP7 project starting with the focusing coil.
 

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 .

Re: 5FP7 project starting with the focusing coil.
 

I came across this may be helpful
https://geeseang.wordpress.com/exper...radar-crt2014/

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.

Adrian

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

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.

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.

Adrian

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

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.

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.

Adrian

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.

Re: 5FP7 project starting with the focusing coil.
 

OK thanks for the info.

Adrian

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.

Attachment 243709

Short video

https://youtu.be/el3mzvLhkHc

Be safe.

Adrian

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.
Attachment 243929

Adrian

Re: 5FP7 project starting with the focusing coil.
 

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 .

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

Re: 5FP7 project starting with the focusing coil.
 

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

John.

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

Adrian

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.

Re: 5FP7 project starting with the focusing coil.
 

Thank you John appreciated.

Adrian

Re: 5FP7 project starting with the focusing coil.
 

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 .

Re: 5FP7 project starting with the focusing coil.
 

I wound the Iron wire around the focus coil just to see if it improved things, the result is, basically not a lot.

Previously with a EHT of 3K5 Volts, it took 82 mA to focus the beam now at 3K6 volts EHT it is now taking 81 mA to do the same. So I guess that there has been little to no improvement down the axis of the coil, perhaps I have stopped some tray field but nothing to really have much of a gain.

I used 1.6mm iron wire which was probably to thick, it was certainly a pain to wrap the coil with and I have done two layers. Oh well.

Adrian

Re: 5FP7 project starting with the focusing coil.
 

Thats a pain ! have a look at the link below
Richard lab guy looks to have found better way in this link
https://labguysworld.com/5FPn_CRT_Tester.htm

Re: 5FP7 project starting with the focusing coil.
 

I do have a magnetic unit, obtained when I got the chassis which was after the project started. If I need to use it I will, but still like the idea of using the focusing coil. Basically I have as per 'a' in the page an open field, unless I can machine some iron to fit the coil I have, it will have to stay that way.

I have been working through some used EL84's I have on my home made static tester, varying the screen volts from 250 down to 150 volts and mapping the anode current against grid voltage with Va at 250 volts and plotting the results.
Attachment 244451

I have also wound a single ended transformer to try and drive the scan coil. I had a spare EI core and bobbin that quick calcs showed it should be capable of approx 25VA and EI-60 with a stack height of 30mm. I have assumed the ra of the EL84 to be 4k5 Ohm and done a 9:1 n ratio to drive the existing wound scan coil. If I have it correct approx 23mA should give me the 204 mA required in the coil with approx 14 Volts across the scan coil and 126 Volts sawtooth.
Attachment 244452

With a few pot's in the cathode resistance of the EL84, I may be able to vary the shape of the scan or even add a bit of negative feedback?

I used a figure of 14 TPV for the windings and stacked all the E's together and used a bit of High temp masking tape used for powder coating as an insulator between the E's and the I's on the end. I managed to completely fill the bobbin which either means I have gone way OTT or that I am crap at winding a bobbin.

Next is to work building up a small test board.

Adrian

Re: 5FP7 project starting with the focusing coil.
 

Been pottering with components again, just using copper clad, just the EF91 as a Miller-Transitron, or what ever it is called, saw tooth generator. At present the sweep would be around 2.8mS and the flyback 0.22mS. The next stage is to try and drive the EL84 and drive the transformer, that for another day as fast running out of time for today.

Attachment 244503

Attachment 244504

Have to search my junk bits for a suitable variable Pot. Fingers crossed.

Adrian

Re: 5FP7 project starting with the focusing coil.
 

Just had a thought on this, I believe I am going to need to reverse the waveform if I am driving the grid of the EL84. The grid needs to be higher at the end of the trace to make the current through the transformer and anode highest before I cut back to have any chance of fly-back moving the trace to the opposite side of the screen. Does that seem correct?

Adrian

Re: 5FP7 project starting with the focusing coil.
 

I have added a second stage EF91 as a reversal of the original sawtooth, basically using the circuit that would normally drive an electrostatic CRO symmetrically. But with a change of cap values I have reduced the output of the waveform to approx a 50 Volt peak, it may also provide a little bit less loading on the first stage.#

Next step is to add the EL84 driver for the transformer and keep my fingers crossed it will work.

Attachment 244641Attachment 244642Attachment 244643

Adrian

Re: 5FP7 project starting with the focusing coil.
 

I stuck the EHT section and added a mains power supply to an old chassis from a forum member, the chassis was part of a BC1151 display section off a radar unit.

With the CRT and line transformer fitted, I could progress with more tests on the line deflection, It meant a bit more planning and plotting response curves with a used EL84. The project is certainly bringing up topics I have probably never dealt with before with electromagnetic deflection and is definitely interesting to see things work even if not perfect. Just takes me time to cram things on the chassis.

https://youtu.be/zOtp6PBUKA8

Just wondering if I can find a CRT with a similar footprint to fit the chassis, one that is electrostatic deflection for later so I do not have to listen to the transformer, perhaps it will end up getting varnished.


Adrian

Re: 5FP7 project starting with the focusing coil.
 

The project has been tidied up slightly, fitted the scan pcb within the front of the case, protected fingers by using some aluminium panels to keep finger tips out of HT. A small 50Volt mains transformer, bridge rectifier and cap used to supply the focus coil, fed via a variable resistor.

Lastly started with some horizontal scan coils for the frame side of NBTV and a bit of testing deflection with the bench power supply.

5FP7 Part 5 video

Still a long way to go.


Adrian