Pye D18T Focus Problem

[DoctorWho]

I've been restoring a Pye D18T which works well and, when powered up initially it gives a very good picture.

When it's been on for a while the focus becomes poor, all of the condensers have been replaced, so any suggestions would be greatly appreciated. Would a new PL38 be advised, or something else first?

Thanks for any advice.

Peter.

[beery]

Hi Peter,
I wonder how long it lasts for?
These sets work ok for about an hour and then de-focus becoming un-watchable after about two hours. It is a well known problem and I have never heard of one that did not do it. My set does it as well.
Back in 1948 TV broadcasts were not very long and they got away with it.
Two things that people have done... Darrius has described how to cut down the screen current in the PL38 which should slow the heating of the lopty. Another collector fitted a BRC 1500 jelly pot in his!

Cheers
Andy

[DoctorWho]

Thanks for your input Andy,

Well I wouldn't say it stays good for an hour, perhaps 20 - 30 mins, then it starts to become quite defocussed. I know the transmissions were shorter back then, but I believe transmissions were usually 2-3 hours weren't they? I have a B18T which is fine for a couple of hours.

I have replaced the EY51 by the way.

Is replacing the PL38 likely to improve matters?

Thanks again,

Peter.

[beery]

Hi Peter,
well yes 1/2 an hour is not good.
PL38 valves are odd things and sometimes I have to find one that best fits a set, so do try another. Check the effieciency diode and the boost cap. Also see if the screen voltage on the PL38 varies with time.

Cheers
Andy

[DoctorWho]

The set now has a new PL38 and a new EY51, the focus control is at the far left of its travel ans is quite acceptable for up to half an hour, but it still looses focus after a while. With the focus assembly already at the end of its travel it's not possible to get a better focus on the picture.

Any other suggestions greatly appreciated.

[Heatercathodeshort]

Hello Peter,
Slight focus drift on these early flyback EHT sets is not uncommon.
The B18T is in fact Britains first.
Mine drifts slightly but settles down after about 20 mins. The full warm up period for these sets is around 15 mins from cold.

I would suggest you set the user focus control halfway when slackening the securing screws to the complete focus assembly.
Switch the set on and wait 15 mins. Move the assembly back and forth to obtain reasonable focus and tighten the screws.

The set will be out of focus from cold [slightly] but should hold ok for at least 3 hours. If yours does not I would suggest you check the EHT voltage as this is no doubt dropping.
As suggested the screen current of the PL38 can be quite high and aggravate this. There are some mods that will improve this.

Hope this is something to go on. The Coronation broadcast was seven hours in total.....Regards John.

[Duke_Nukem]

I displayed my D18T at the last NVCF. After a couple of hours it was definitely not happy, being somewhat defocussed, and after five hours I switched it off as it looked very poor (a shame as it is capable of a very good picture). The fact that it was also very hot in the display area probably didn’t help too much either.

BTW, the D/B18T was not the first British set to derive EHT from the flyback – but it was the first to use the combination of flyback EHT plus a boost diode.


TTFN,
Jon

[DoctorWho]

Thanks John & Jon for your advice.

I have actually moved the focus assembly, by moving it nearer to the CRT the problem worsens, it is now moved as far towards the back of the set as it will go, this gives a superb picture when powered up but, as mentioned previously, after about 20 - 30 mins it does lose focus.

Do I perhaps need to elongate the screw slots with a fils so that the focus assembly can be moved further back down the neck of the CRT?

Any advice appreciated.

Good point about the coronation broadcast, I would personally have thought that this set should have performed well for a couple of hours at least.

Thanks again,

Peter.

[Sean Williams]

Hi Peter,

I would not modify the focus assembly - as others have mentioned the set ought to be ok for a couple of hours - so you must have a fault in there somewhere.

Have you looked at the screen supplies to the PL38? - it might be worth doing the modification that Darius has worked out - see if this improves the situation.

Try running the set with a meter on the HT line and one on the PL38 screen line to see if there is any correlation between poor focus and changing voltages.

Are there any parts in the set that are getting noticably warmer than perhaps they should?

Mechanical modifications are the absolute last straw and are non reversable if something goes wrong.

Are the focusing magnets as strong as they used to be?

Lots of other areas to look at first....

Cheers
Sean

[mickjjo]

There was a thread about this problem last year:-

https://www.vintage-radio.net/forum/...light=Pye+D18T

Regards, Mick.

[DoctorWho]

Thanks for the input guys. With regard to the magnets, I have no way of telling if they are as strong as they were originally, but I guess that is a possibility, any suggestions as to what to try adding on to make an improvement.

Thanks also for the link to the other thread, I'll have a look at that.

I too like to keep sets as original as possible, but I am willing to make modifications if necessary to get a good picture. Does anyone have details of the modification that Darius has designed or the one/s you mention John?

The only part of the set which seemed to get very warm quickly was the resistor in parallel with the Thyristor, but I presumed that was not a big problem and maybe even normal? The CRT heater does glow much quicker than the rest of the valves, but I had put this down to being an issue with an AC/DC circuit of this nature.

I have experimented with moving the focus assembly further back than it is designed to go, but this has not made any noticable improvement.

Any other suggestions appreciated.

Thanks again,

Peter.

[BRC3000]

Peter, Does this help?

here is my experiance with the problem which proved to be a reliable repair and could be easly hidden inside the LOPT can.

this is an extract from th thread on the subject..,...............


It seems that the drifting focus problem with these Pye’s can be tamed.

I obtained a solid state rectifier with a PIV of 12KV from CPC.

Then I completely removed the Ey51, then connected the anode end of the diode to the overwind O/P on the LOPT and the cathode to the EHT lead.

The picture came up much more quickly than normal due to not having to wait for the EY51 heater to warm up.

With the EHT meter connected the immediate improvement was that the EHT was now showing at just over 7KV rather than 6KV with the old valve rec. This ment the picture was brighter and sharper straight away.

I soak tested the set for 3 hours and the focus moved very little requiring only one small adjustment in the 3 hour period. The EHT reduced by around 700v over the 3 hours rather the 2.5KV drop that had been experienced before.

So in summing up I am unsure if removing the EY and substituting a solid state rec has cured the fault or just lightened the load on a tired LOPT but either way the set is now very usable where before it was not, so I am well pleased.

I have also applied the same fix to the Ferguson set with similar success.

I have attached some pictures of the the PYE after it had been on for a couple of hours.

This weekend passed I ran the PYE for 8 hours for a vintage 405 line display at a local radio rally and it worked well all day maintaining its bright well focused picture.

so thanks to all for the suggestions on this problem and hopefully this thread may help others who experience the same problem.

[DoctorWho]

Thanks for this information Dave, that sounds well worth trying. Could you possibly tell me the part number at CPC which you used for this, I will then try the same myself.

Thanks again,

Peter.

[DoctorWho]

You can find the unit at:

www.rswww.com

and enter the part number 2508956490

Can anyone advise if this would be suitable, or is there anything better anyone knows of? I can't find anything suitable at CPC which hasn't been discontinued. Any help greatly appreciated.

All the best,

Peter.

[Duke_Nukem]

(Posts to do with early flyback EHT split to a new thread :-
https://www.vintage-radio.net/forum/...ad.php?t=11592)

[DoctorWho]

Thanks for that Jon, if anyone can give me advice on a suitable solid state diode/rectifier I would be grateful as this is the point at which I am now stuck.

Peter.

[Sean Williams]

Hi Peter,

Not a nice thing to do, but if you are convinced that this will solve the problem then you need to source a diode that will handle 12KV PIV (peak inverse voltage) - if you can find one that is good for high speed use so much the better - I am sure that RS will stock something suitable....

[DoctorWho]

Well I have trawled RS with no success I'm afraid, except for the one I detailed on the previous page. I know this isn't an original solution, but Dave seemed to have good success with this and so it seems at least worth a try.

If you can suggest anything suitable from RS I'd be grateful as it seems very difficult to find something with a high enough operating voltage.

[Sean Williams]

Well, you could give it a go - it will handle what you propose to do with it.

I still think there must be a problem elsewhere though.....

Cheers
Sean

[beery]

Hi Peter,
something like the BY8410. This is rated at 12KV. BY8412 is rated at 14KV. The RS part no. for this is 294-3886. I notice though that it has been discontinued and I can't find any other suitable types in RS.

Cheers
Andy