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Old 1st Apr 2021, 8:56 pm   #1
murphyv310
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Default Pye VT4 Jelly-pot mod

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https://www.vintage-radio.net/forum/...d.php?t=178363


Hi.
The interesting thing is when using a Jellypot the inherent available EHT current is higher so the usual issues on some sets running at high brightness and contrast you don't get the usual defocusing and picture size changes.
I'll still try to use the Jellypot if possible as everything else is 100%, it neat and efficient.

Comparing the circuits between the 1500 & VT4 only small changes will be required to the existing circuit. I'll have a go over the next few days.
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Last edited by murphyv310; 1st Apr 2021 at 9:19 pm.
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Old 1st Apr 2021, 9:26 pm   #2
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Default Re: Charlton TV Tubes

Hi Trevor,
how about a diode biased to a stabile reference level to reduce the AGC keying pulse to the correct amplitude?

DFWB.
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Old 1st Apr 2021, 9:44 pm   #3
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Default Re: Charlton TV Tubes

Hi David.
Indeed it should be easy enough to reduce the AGC pulse. The other issue is the flywheel, the VT4 has a transformer after the winding on the LOPT its centre tapped for phase reversal to the discriminator diodes, I could dispense with the transformer and change the circuitry so we don't need this, later flywheel circuits omit the tranny.
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Old 2nd Apr 2021, 10:28 am   #4
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Default Re: Charlton TV Tubes

Circuit diagram of the line timebase in the Pye VT4.
The phase splitter transformer T11 supplies the 320V negative gating pulse to the vision AGC amplifier.

DFWB.
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Old 2nd Apr 2021, 4:27 pm   #5
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Default Re: Charlton TV Tubes

Hi.
Well the 1500 Jellypot works. Spend some time today on this project and have come up with a solution although I may modify it later. The idea of using an intermediate transformer between the jelly pot and the existing transformer was still in my mind after yesterdays failure.
I found another this time with a ferrite core. This proved to be the answer and now I have spot on waveforms at the correct level. The result is not bad, there is a bit of foldover on the left hand side and some cramping on the RHS, also the line is jittery and drifts a lot during warm up which is not valve related. A quick look around shows me some way off resistors and a couple of caps that are notoriously bad in these sets. I'll no doubt change these over the next few days and see if we can get an improvement.
The sync is low too both line and fame sync is rather weak so I'll need to scope the sync seperator and see whats going on there as well.

David the voltage on test point 18 is 220v P-P not 320.
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Last edited by murphyv310; 2nd Apr 2021 at 4:47 pm.
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Old 2nd Apr 2021, 4:51 pm   #6
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Default Re: Charlton TV Tubes

Some more pictures
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Old 4th Apr 2021, 9:16 pm   #7
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Default Re: Charlton TV Tubes

Hi.
I realise this is off topic for the thread so perhaps the mods could split this please to (Pye VT4 Jelly-pot mod) If they think its best.

There are some issues with the modification. The main issue is the flyback time is too long so the picture information appears in the flyback giving foldover on both sides. I think this is due to the 400pf tuning capacitance. On previous mods to 70 degree CRTs using a jellypot I reduce the HT to the line stage and this gives a good compromise between EHT and width without foldover. This issue here is I'm using a 625 line TX on 405 so it needs more tuning but that is counterproductive.
So I have rewired the scan coils from series connections to parallel, this gives more width, removing the tuning brings the width in but still with overscan and high EHT, reducing the HT to the line stage gives correct width without foldover but only 11kv EHT, the idea then is to fit the width control back into the set and increase EHT for 12-14Kv and then adjust the width.

The other issue to show up is those dodgy height and linearity pots, the slider can short to the centre reducing scan which I need to attend to.
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Old 4th Apr 2021, 9:44 pm   #8
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Default Re: Pye VT4 Jelly-pot mod

Hi.
The idea tomorrow is to complete the mod and fix the control.
The other issue is line jitter, I initially thought it was was the line hold control but that's not the case. Looking at the HT on the scope it is fluctuating in sympathy with the jitter. It's worse on the feed to the line oscillator, I'm wondering if one of the smoothing caps are breaking down. So there are a few things to sort out on this set. I'm confident though It'll be good once completed.
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Old 5th Apr 2021, 3:05 pm   #9
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Default Re: Pye VT4 Jelly-pot mod

Hi.
Well today I completed the mod to the line output stage.
I now have the 1500, 625 line Jelly-pot installed. The Tuning capacitor is 100pf and the HT to the line stage is dropped by a 500r resistor. This gives me full width, 14Kv EHT and 510v boost. the G2 of the CRT is at 330v WRT Chassis and the focus around 180v WRT Chassis. A supplementary transformer is between the LOPT and the reference phase splitter transformer which is part of the VT4 circuitry.
The Frame linearity pot was leaky and has been replaced. This is of importance to anyone restoring a V4 or VT4 as this fault is hard to diagnose, you get very little control over linearity and the height control just makes the picture stretch badly, If you get stuck with that problem then loosen the controls securing nut lifting the control from the chassis will make the height jump up if its faulty. the leak can be a full short circuit to up to a few meg ohms.
I still need to do other things to the set but so far I am well pleased.
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Old 5th Apr 2021, 6:08 pm   #10
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Default Re: Pye VT4 Jelly-pot mod

Good work, picture looks great!
I have a V4 in storage, it more or less works, but yet to be restored.
I hope the pots in mine are OK when I finally get it on the bench!


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Old 10th Apr 2021, 5:47 pm   #11
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Default Re: Pye VT4 Jelly-pot mod

Very nice job. I recently had a tricky 'Auto Picture Control' fault. Basically there was excessive IF gain. But if I turned the RF sensitivity control down far enough to get the right contrast, then there was not enough RF gain and noise appeared on the picture. Voltages and component values were ok. I only found the fault by scoping the line pulse feeding the APC circuit from the little transformer which in turn is fed from the LOPT. It was way down in amplitude and weird in shape. The little pulse shaping cap on the small transformer was faulty (the type that looks like a half watt resistor). I don't know who makes them, but I've had problems with them before when fitted as IF de-couplers.
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Old 10th Apr 2021, 7:34 pm   #12
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Default Re: Pye VT4 Jelly-pot mod

They are poor caps and there are still a few in this set. I'll need to replace them soon.
I'm waylaid by a paying job......Pye D16T... That's another story.
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Old 13th Apr 2021, 6:14 am   #13
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Default Re: Pye VT4 Jelly-pot mod

Quote:
Originally Posted by Niechcial,Steve View Post
........ The little pulse shaping cap on the small transformer was faulty (the type that looks like a half watt resistor). I don't know who makes them, but I've had problems with them before when fitted as IF de-couplers.
I’m pretty sure these were made by EGEN, an Ekco subsidiary. They’re a change on sight part now, I’ve never come across a good’un!

Cheers. SimonT.
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Old 13th Apr 2021, 7:59 am   #14
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Default Re: Pye VT4 Jelly-pot mod

My word, what a courageous man you are, Trevor. That's a hell of a job. I'm very impressed by the picture on that set, especially the almost perfect geometry. It always annoyed me to see that most manufacturers couldn't get straight vertical edges to the picture, even on their most modern sets.
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Old 24th Apr 2021, 9:26 am   #15
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Default Re: Pye VT4 Jelly-pot mod

The Pye CTM4 is electrically similar to the later version of the VT4 having BREMA intermediate frequencies and an electrostatic focus CRT.
The rewound line output transformer has failed and I'm considering using as a replacement the transformer that was used in the 1953 Plessey TV chassis.
Apart from concerns matching the scanning coils the real issue is the fact the Plessy transformer does not have a winding to to supply the AGC gating pulse and the flywheel sync diodes.

DFWB.
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