LOPT design

[ppppenguin]

I've only skimmed the articles but it's clearly a good solid review of how to design an energy recovery LTB. The only thing that's missing (or I may have missed it) is harmonic tuning. In monochrome sets it was common to tune the LOPT leakage inductance (I think!) to the 3rd harmonic of the flyback pulse. This raised EHT and lowered voltages eleswhere. In colour sets it was usual to do 5th harmonic tuning which lowered the EHT a bit but gave better EHT regulation.

When that article was written I think 90 degree CRTs were the norm and 110 degree tubes were on the horizon. As the angle increased so did the scan power and the need for low losses in the LTB. Colour brought another increase in scan power. I think an A63-11X needed about 3500VA circulating in the LTB, of which rather more than 95% was recovered. The powere consumption would have been horrifying without efficient energy recovery.

[ppppenguin]

This guy certainly took an interesting approach and full credit is due to him.

http://www.genre.fsnet.co.uk/museum/tv/index.html

Surely he must be known to some of us.

[renato_enca]

ok...things are joining each other...now I managed to connect the CRT and gave it some power. I'm feeding it's heater with a 230V->12V transformer.
The LOPT has been wound with the thoughts given on the articles you have been posting. Many thanks again for your time. I've used the design that came on the "Practical Television" article, with the 750 turn primary LOPt winding, and I wound it on the former posted above, using layers (200 turns each layer) and "insulating" layers with paper tape(with glue on one side).
With some experimenting, I realized that the EHT rectifier heater winding just needs 1 turn on the LOPT core to lit up satisfactorly. The circuit I'm using is the direct drive circuit, as the LOPT was fairly easy to wind. After applying power, the "set" went on and gave a full line on the screen, albeit defocussed...I'll use a voltage divider on the rectified EHT to give the focus voltage. When I thought that was it, it must be working, the LOPT main winding started sparking(not the EHT winding...I'm using the old one that served the old dead Philips for more than 15 years), and the stage went down. I think that the glue of the tapes became conductive and whiped all the insulation of the wires. If I'm indeed correct, how can I solve this problem? Using paper without glue, less turns per layer? All thoughts are welcome.

PS: Surprisingly, the winding is NOT open-circuit. It measures the correct resistance. Must be the glue...
oh and sorry about any typos...it's 1 AM and I'm a bit tired...

[Ed_Dinning]

Hi Renato, there may have been moisture in your paper. The normal insulation on transformers is Type 56 polyester tape, usually a yellow colour, self adhesive and will take 2KV per layer.
You will probably have to re-wind it as the paper will now be carbonised.
I usually dip transformers like this into just melted paraffin wax that is uncoloured.

Ed

[kalee20]

Agree totally with Ed, you may have carbonised your paper interwinding insulation. When it breaks down, the energy of breakdown liberates free carbon (as graphite) from the cellulose fibres. And, microcrystalline graphite is conductive.

Type 56 tape is a good insulator (it is polyester film, [Mylar, Melinex]) with excellent breakdown capabilities. But it might be a little bulky in your application, if you have a layer of tape between each layer of winding.

If you are happy to take a chance, I might suggest grease-proof kitchen paper. It is not massively different from Kraft paper, which is used as thin interleaving. What is important with this is, if you use it, to thoroughly impregnate the winding. Ed's advice is good - although paraffin wax may start to melt at operating temperatures. I would also add, before impregnating, bake at 105 deg C for at least 4 hours to dry the winding, and then impregnate as soon as possible (before it has cooled right down).

It is important to leave an empty margin at each side of the winding. See below for a series of photos (totally different core shape, but you'll get the idea). I would suggest at least 6mm of margin for the line output anode, bearing in mind it will swing to 6kV plus.

[renato_enca]

ok...I used the direct-drive circuit as given in the "practical television" article, while refering also to the "electronic engineering" article. To achive maximum efficiency, I needed to use the vertical coil of the deflection yoke, as the proper horizontal coil has too little inductance. Of course, the yoke has been turned 90 degrees on his axis. The circuit did work, indeed, but the vertical coils started clapping, which may indicate that the coils can't sustain the high voltage flyback pulse. Am I correct? If yes, which starting values (eg. total turns number, "N15" as given in the Electronics Engineering" design) may I have to be able to calculate a new LOPT and drive the small horizontal yoke? There are no turns number in that article, only the turns ratio between taps.

[renato_enca]

ok...after some time waiting, I received from a friend a real LOPT from a VC208 chassis (I don't remember the brand). After copying the circuit using the service sheet (the valves the circuit uses are PY88-PL504...my output valve is a PL36), I've regarded with this "picture" on the screen:

NOTE: I'm using the vertical deflection coils rotated 90 degrees to achieve the horizontal line...the circuit doesn't work with the real horizontal deflection coils, because of the impedance...any ideas?