5FP7 project starting with the focusing coil.

[AdrianPH]

I wonder if the Dutch used electrostatic deflection tubes, do yo know of any articles?

Adrian

[AdrianPH]

The only thing I have found for the Dutch mechanic TV is from the early TV museum site, 90 line receiver using a DG 7-1. Anyway back at it.

Adrian

[Mike Watterson]

All the early CRTs were electrostatic.

I did read an article, but I can't remember where.

[AdrianPH]

I am still in the planning stage for the next test, I have been working through some used EL84's I have on my 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.  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. 

With a few pot's in the cathode resistance of the EL84, I should 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.

I also received the iron wire and hop to wrap the focusing coil and see how much of a difference that makes.

Adrian

[AdrianPH]

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

[`FRANK.C]

I managed to completely fill the bobbin which either means I have gone way OTT or that I am crap at winding a bobbin.

Hi Adrian
If you have completely filled the bobbin and got all the turns you need in it I would say you got it spot on. Always best to keep the bobbin as full as possible. It helps to keep the copper losses down.

Frank

[Murphyv310]

Hi
I use the larger version of these CRTs for test purposes.
A very useful tube. Can I ask why you are using a coil for focusing?
I've only ever used a permanent magnet for focus and they work fine.
I had one of these in the Frankenbush TV22 for some time.


https://youtu.be/TP9wwzmpNgA

[AdrianPH]

Hello Frank.
When I worked out how many turns and layers I was putting on I thought I would have a couple of mm spare, either my layers of tape between the layers is more than I guessed or my winding is not side by side and even, I tend to know that is the case as with a hand winder I feed the wire across by hand. I will see how it works at the end, that will be the main thing, it's all guesswork by me magnetism and Inductance is magic!

Trevor.
The reason for using a coil was when I started this I did not have a magnetic focusing unit, at the time i was trying replicate a focusing coil as mentioned in the spec sheets. I have got a magnet one since, but now like the idea of using the coil. It could means I can have a simple 'Pot' on the front panel to tweak focus on the monitor if I get that far down the line that stage. Possibly wind a small 50 volt winding on a transformer at 100mA and drive the focus coil with that its only 5 watts. If I do need more space for the deflection coils then I may need to go magnetic as it is a shallower unit.

Adrian

[Jac Janssen]

Adrian,

Concerning the focus coil: please see this drawing of a Philips focus coil for a 1949 receiver.
The shaded parts 1 & 3 are metal (iron) and the cross-shaded part 2 is the coil.
The idea is that the metal concentrates the magnetic field towards the CRT-neck.

   

Hope this helps a bit.
Good luck with your project!

Jac

[AdrianPH]

Thanks Jac