Pye VT4 circuit differences? - UK Vintage Radio Repair and Restoration Discussion Forum

thermionic · 28th Dec 2007, 10:31 pm

Hello All.

I obtained this set a few months ago and whilst trying to re organise my workshop, thought I'd have a peek inside to evaluate it's condition.

All looked original and untouched - good! However there seems to be an irregularity with the A1 supply. According to the diagram in the 'red book' the A1 supply is fed from the boosted HT supply as expected, but the value of the series resistor (R130 - 10 Meg) and the smoothing capacitor (C108 - 0.1uF) are considerably different in my set - R130 - 1.2Meg, C108 - 1.0uF.

All these components are original - the tube also appears original but lacks the makers/type label.

Any ideas?

Thanks. Simon T.

David_Robinson · 29th Dec 2007, 1:48 pm

Excerpt from the original Pye datasheet attached - looks like it's an official mod.

thermionic · 29th Dec 2007, 3:37 pm

Thanks for that David.

My one thought was that it may have been due to the fitting of the poorer 'Cathodeon' tube. But it seems that it was just a general mod.

Many thanks for clearing that one up!

Simon.

Heatercathodeshort · 12th Jan 2008, 9:47 pm

The Cathodeon tube was used in the V4 series and was actually made by PYE, I believe. It was a very long life tube but the screen phosphor was very coarse, rather like the Eastern bloc monochrome tubes of the late 70's.

The tube can be identified by its very rough dag coating. [rather like a 'concrete' finish]. Original versions had an odd cut-off and required a slightly lower 1st anode voltage to centre the brightness control. That is probably why the 10Mohm resistor was used. Great sets. J.

thermionic · 12th Jan 2008, 11:49 pm

Thanks John.
The dag seems to be smooth, so can assume a Mullard tube perhaps. I don't have a great deal of time at the moment to get down to fixing a few sets. But The V4 looks to be a bit of a challenge, with it's under chassis 'rats nest'!!

Cheers. Simon.

murphyv310 · 15th Jan 2008, 9:34 pm

Quote:

Originally Posted by thermionic

The V4 looks to be a bit of a challenge, with it's under chassis 'rats nest'!! Cheers. Simon.

Don't be put off by Pye's wiring; these sets can be truly amazing giving superb pictures. There is only one weakness - the scan coils.
I've just completed a V4 using a VT4 for parts and the results are totally amazing: although EVERY cap has been changed, well worth it, though.
Best of luck.
Trevor

rob77 · 16th Jan 2008, 9:25 pm

I agree about these sets.
When they are running in tip top state they really do give a great picture. Well worth the cap-changing marathon. I converted one to 625 lines a few years ago and I know that its still giving great service (early digibox as a tuner).
Look out for the 2 little M3 (i think) selenium diodes! Thay do give a little trouble, but there is more info regarding this elsewhere on the forum so I won't expand any more.
Have fun with her!

Cheers
Rob

thermionic · 18th Jan 2008, 8:20 pm

Hi all.

I have just been looking at the photos of Trevor's V4, shown in another thread, and am very impressed with the definition! Digging out all those duff caps is obviously worth it. I just hope my tube is ok!

Cheers. Simon.

murphyv310 · 19th Jan 2008, 11:51 am

Quote:

Originally Posted by thermionic

I just hope my tube is ok!

Cheers. Simon.

Hi Simon.
The tubes fitted to the V4/VT4 were generally reliable. Three different types were fitted: Mullard MW36-24, Mullard MW36-44 & Cathodeon MW36-24.
The Mullard ones can get a partial short on the heater giving 3.6v across the heater pins, this gives a very flat picture but sometimes a tap on the neck effects a cure (dont hit it with a hammer, though!) they can also go a little soft; not only does the picture go soft but you get strange bending on the picture when the set first comes on - this is due to static on the tube screen charging up the perspex screen, due to lack of beam current with the soft tube, turning up the brightness to max will take it away: a soft Mullard will boost fine.
So far I haven't come across a Cathodeon so I'm not too sure, but I would say if "heatercathodeshort" says they are good they will be.
Cheers.
Trevor

thermionic · 19th Jan 2008, 9:59 pm

Thanks for that Trevor.
I remember reading somewhere, that Cathodeon tubes tended to be fitted to sets destined for dealers whose clientele were not so affluent. Also because Mullard were sometimes unable to provide continuity of supply. I don't know if this is based on fact, or whether it is an 'old wives tale'!
However, I can remember servicing a few 70's colour sets during my time in the trade in the late 1980's with Cathodeon tubes and IIRC they did not fare so well as other main brands at the time.

Cheers. Simon.

28th Dec 2007, 10:31 pm	#1
thermionic Heptode Join Date: Apr 2003 Location: Cornwall, UK. Posts: 989	Pye VT4 circuit differences? Hello All. I obtained this set a few months ago and whilst trying to re organise my workshop, thought I'd have a peek inside to evaluate it's condition. All looked original and untouched - good! However there seems to be an irregularity with the A1 supply. According to the diagram in the 'red book' the A1 supply is fed from the boosted HT supply as expected, but the value of the series resistor (R130 - 10 Meg) and the smoothing capacitor (C108 - 0.1uF) are considerably different in my set - R130 - 1.2Meg, C108 - 1.0uF. All these components are original - the tube also appears original but lacks the makers/type label. Any ideas? Thanks. Simon T. __________________ The honesty of imperfection..........

29th Dec 2007, 3:37 pm	#3
thermionic Heptode Join Date: Apr 2003 Location: Cornwall, UK. Posts: 989	Re: Pye VT4 abnormality? Thanks for that David. My one thought was that it may have been due to the fitting of the poorer 'Cathodeon' tube. But it seems that it was just a general mod. Many thanks for clearing that one up! Simon. __________________ The honesty of imperfection..........

12th Jan 2008, 9:47 pm	#4
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: Pye VT4 abnormality? The Cathodeon tube was used in the V4 series and was actually made by PYE, I believe. It was a very long life tube but the screen phosphor was very coarse, rather like the Eastern bloc monochrome tubes of the late 70's. The tube can be identified by its very rough dag coating. [rather like a 'concrete' finish]. Original versions had an odd cut-off and required a slightly lower 1st anode voltage to centre the brightness control. That is probably why the 10Mohm resistor was used. Great sets. J.

12th Jan 2008, 11:49 pm	#5
thermionic Heptode Join Date: Apr 2003 Location: Cornwall, UK. Posts: 989	Re: Pye VT4 abnormality? Thanks John. The dag seems to be smooth, so can assume a Mullard tube perhaps. I don't have a great deal of time at the moment to get down to fixing a few sets. But The V4 looks to be a bit of a challenge, with it's under chassis 'rats nest'!! Cheers. Simon. __________________ The honesty of imperfection..........

16th Jan 2008, 9:25 pm	#7
rob77 Tetrode Join Date: May 2005 Location: Norwich Posts: 87	Re: Pye VT4 abnormality? I agree about these sets. When they are running in tip top state they really do give a great picture. Well worth the cap-changing marathon. I converted one to 625 lines a few years ago and I know that its still giving great service (early digibox as a tuner). Look out for the 2 little M3 (i think) selenium diodes! Thay do give a little trouble, but there is more info regarding this elsewhere on the forum so I won't expand any more. Have fun with her! Cheers Rob

18th Jan 2008, 8:20 pm	#8
thermionic Heptode Join Date: Apr 2003 Location: Cornwall, UK. Posts: 989	Re: Pye VT4 abnormality? Hi all. I have just been looking at the photos of Trevor's V4, shown in another thread, and am very impressed with the definition! Digging out all those duff caps is obviously worth it. I just hope my tube is ok! Cheers. Simon. __________________ The honesty of imperfection..........

19th Jan 2008, 9:59 pm	#10
thermionic Heptode Join Date: Apr 2003 Location: Cornwall, UK. Posts: 989	Re: Pye VT4 abnormality? Thanks for that Trevor. I remember reading somewhere, that Cathodeon tubes tended to be fitted to sets destined for dealers whose clientele were not so affluent. Also because Mullard were sometimes unable to provide continuity of supply. I don't know if this is based on fact, or whether it is an 'old wives tale'! However, I can remember servicing a few 70's colour sets during my time in the trade in the late 1980's with Cathodeon tubes and IIRC they did not fare so well as other main brands at the time. Cheers. Simon. __________________ The honesty of imperfection..........