Wireless World 1947 television construction

[Phil215]

As a challenging retro project it looks quite promising until you discover the difficulty of constructing the LOPT. That's what would be required in order to do justice to the television design published in the Wireless World magazine in 1947.
Has anyone actually done this, or know anyone who has?

I was considering making the valve timebase circuits and power supplies to run a 9” CRT, as in the 1947 design. A convenient video input might be derived from the SCART connector on the back of a Freeview box.

Considering the WW design I can't see that the particular DIY LOPT described is a viable option, as the three concentric bobbins used in its construction appear to be a thing of the past. If I were to supply the relevant circuits, is anyone prepared to suggest an alternative LOPT solution and suitable impedance matching for the video input? The line-scan output valve is an EL38.

[Danny]

Hi I don't want to dampen your spirits but the set will be 405 line so you will need a standards converter.
The bobbin could be made with a little effort but if you want a starter project I would go for an electrostatic design as you can then choose what line standard you want more easily.
But if its got to be magnetic then your best bet will be to make it 625 standard and use a lopt from a thorn 1500 or similar set. Danny

[Phil215]

Thanks for your reply Danny, and the reminder! Yes, the line time-base must operate at 15,625 c/s in order to reproduce images on the 625 line standard.
At this stage I don't know enough about the Transitron-Miller integrator circuit to enable me to increase the saw-tooth frequency to 15,625 c/s. Are you familiar with this type of circuit by any chance?

The whole project hinges on finding a suitable CRT, which is a 9" monochrome type with a tetrode gun.
(Although a triode gun can easily be accommodated by the omission of a connection between +HT2 and a tube electrode.)
The CRT in the original is a Mullard MW22-7, which required a focus coil mounted on the tube neck, but if any 9" monochrome CRT exists outside a museum, I hope it will have an electrostatic focusing anode.
A friend might be able to get hold of a suitable donor set - the Pye cube, I believe.
Don't know anything about that one at the moment.
If it has electrostatic deflection plates as well, then so much the better.

I should have no problem building 67 year old electronics, but I want to be sure it stands a good chance of working first, else it's a huge waste of time and money.

At this stage, if the WW 1947 TV construction project (in 2014) manages to get off the drawing board and become a physical reality I shall be astonished!
Thanks for your interest. I look forward to your considered opinion on all this and any other snags you can think of.

Btw, Ed is looking into the LOPT problem, so there's some light at the end of the tunnel!
I'll try to upload some schematics to put you in the picture

[Phil215]

These are the three most relevant schematics:
Line Time-Base and EHT
Sync Separator and Frame Time-Base
CRT

[FERNSEH]

The frame scanning coils might be a problem. There is no output transformer so that means the scan coils will be of the high impedance type. There is no reason why a frame output transformer can't be incorporated into the design.
Otherwise it looks a very competent design, certainly no corner cutting.

DFWB.

[Ed_Dinning]

Hi Gents, the LOP is quite simple in this one as no boost and linearity windings are needed, but it is a class A stage so that might impact linearity; the use of the flyback EHT system will help reduce linearity problems, but I suspect it will still need to be a gapped core.
Was the original one laminated or a type of ferrite, it will almost certainly need to be ferrite for 15KHz.

Ed

[Phil215]

Thanks for your comments.

One little snag has occurred with regard to the power supply. It has a 4uF reservoir capacitor rated at least 750V working.

I've ordered a CV1264 full-wave rectifier which is a directly heated type. The transformer I plan to use has a 500-0-500V secondary rated at 180mA. That's underrated for the entire set, which requires 250mA, but I'm only planning on constructing time-base generators (TBG) at this stage. (The transformer will have to be upgraded later if more circuitry is built.)

As all the TBG valves are indirectly-heated types, the load on the HT supply will be negligible until they have warmed up. Using the directly-heated rectifier the initial voltage would rise to about 700V. This presents a problem in as much as 4uF electrolytic capacitors are no longer made with working voltages of 750V. The nearest I can find is a 600V rated part from www.studioelectronics.biz.
Some kind of time-delay switch might overcome the problem, but I'm not sure what form this would take.

This problem is not new, so could someone please direct me to an appropriate thread or offer some further advice.

Btw, I don't have the September 8th, 1938 issue of Wireless World which featured M.G. Scroggie's article “Choke v. Condenser Input”, else I might be able to dispense with the 4uF condenser altogether and adopt a choke input filter.
The choke will be on its way to me tomorrow: rated 20H @ 250mA.

[Sean Williams]

Put two 8uf caps in series - go for 500v working, and use a couple of 470K resistors to balance the voltage across them.

Problem solved - 4uf @1000v

HTH
Sean

[Ed_Dinning]

Hi Phil, these may well have been the big paper can type caps, non polarised and easily available as ex WD in 1947. Still occasionally seen for sale and usually still Ok after all these years
Ed

[Phil215]

Thanks Sean, that's good news. I take it that 940k in parallel with the smoothing circuit is acceptable. What about the power rating of the balancing (bleeder) resistors, 1W or less?

Yes Ed, the original was a non-polarized Dubilier Nitrogol, oil-filled unit. Actually, two 2uF capacitors of 900V rating were used in parallel since they were more readily available at the time.

Question: Where can I get 8uF, 500V working electrolytic capacitors in the UK?

[HamishBoxer]

Most likely they will be ex WD,or use 2 X 10uF500V or more easily available from many sources RS CPC etc Possibly Ebay.

If non polarised needed they may still be available as "motor Start" capacitors.

Here's one, a bit pricy but would look the part! http://uk.farnell.com/epcos/b25834f4...rew/dp/2291747

[julie_m]

Originally Posted by Phil215

What about the power rating of the balancing (bleeder) resistors, 1W or less?

Well, you know the voltage across them and the resistance; so you can calculate the current flowing through them using I = E / R. And then, knowing both the voltage and the current, you can determine the power dissipation using P = E * I.

In fact, unless you're desperately interested to know the current, you may as well just conflate the two steps into one: P = E ** 2 / R.

[Phil215]

Thanks for your comments. Luckily I found the correct part today (4uF / 750 VDC wkg) and have just ordered one!

However, until this point (with a nod to Hamish) I was considering the motor run capacitor as a reservoir capacitor. So, although it's now an academic point, it might be useful to discuss for the benefit of others. Apologies to moderator(s) if this is 'old hat' and can be found in an archived thread.

Is it possible to use a motor run capacitor (unpolarised) as a reservoir capacitor?
There's no essential reason why this component has to be of the electrolytic type.
So, these polypropylene capacitors, with their high AC voltage ratings (400V, 450V, 500V) look promising. However, they don't have a DC rating at all, maybe because their application is for use with induction motors only. They may well be viable components in a DC circuit but the makers simply don't bother to print a rating on them. So, are they safe to use as reservoir capacitors?
Considering their low cost I would imagine many constructors have been tempted to experiment with them. Does anyone with experience of this have any comments?

[Phil215]

After further musing it occurs to me that the basic question is:
Can 'motor run' capacitors cope with high ripple currents?

[Ed_Dinning]

Hi Phil, the answer is yes as they carry current all the time if a "run2 cap, not necessarily so if a "start" cap

Ed