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Old 22nd Jul 2014, 5:08 pm   #1
Ian - G4JQT
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Default Bush TV24 picture width

I've been struggling with this for some weeks now, on and off. But I just can't get the picture to reach the edges.

Vertical height is easily adjusted to well beyond what's needed.

Even with the width control at max, TC1 (800pF in line osc) adjusted for max width, T5 (horizontal form) adjusted, deflection coils up against the tube (and substituted), focussing magnets moved about and substituted, along with various ion traps, the picture is still not wide enough by at least 3/4 each side.

All voltages seem more or less correct, including those around the metrosil (also substituted).

As far as I can tell the EHT seems OK, drawing a 1/4 inch spark. If I detune the LOPT by changing C20 470p, I can reduce EHT and increase picture size, but of course that reduces brightness considerably, so not the way it's done I guess.

All relevant valves tested good and substituted.

(Incidentally, won't a too-high EHT brighten the picture but reduce the size since the electrons are pulled faster towards the screen and therefore under the influence of deflection for a shorter time?)

The picture is fairly bright and well defined when everything is adjusted, just too narrow and I've now run out of ideas.

I get the impression looking through various threads that there never was much width in hand in these sets even when new...

Thanks.

Ian

Last edited by Ian - G4JQT; 22nd Jul 2014 at 5:30 pm.
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Old 22nd Jul 2014, 5:31 pm   #2
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Default Re: Bush TV24 picture width

I have never had a problem with the width on a TV22/24. Have you replaced the scan coupling electrolytic to the scan coils, scan coupling capacitor to pin 5 of the PL38, boost capacitor and the general condition of the PZ30. Low H.T. will make it difficult to obtain full scan. Scope the line drive to pin 5 and if low check the ECL80 line oscillator valve for low emission and it's anode load resistor that may have gone high. It is unlikely to be the transformer. John.
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Old 22nd Jul 2014, 6:55 pm   #3
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Default Re: Bush TV24 picture width

Thanks for your suggestions.

All wax caps have been changed (even those lurking under tagstrips), and all mica caps checked. Resistors also checked.

Valves all NOS and checked/swapped.

Ian
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Old 22nd Jul 2014, 10:03 pm   #4
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Default Re: Bush TV24 picture width

Are the scan coils pushed right up to the tube flare?

Oops sorry just re-read post.
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Old 22nd Jul 2014, 11:45 pm   #5
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Default Re: Bush TV24 picture width

Quote:
All wax caps have been changed (even those lurking under tagstrips), and all mica caps checked. Resistors also checked.
How about the two electrolytics that John mentioned (boost capacitor and the one on the scan coils - both 2 uF)?

Barry
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Old 23rd Jul 2014, 9:58 pm   #6
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Default Re: Bush TV24 picture width

Here's a photo of the screen. It's a bit 'on the wonk' yet, but you can see it's a long way from meeting the edges.

Is it possible something's not quite right with the LOPT? It's got windings connected to the Horizontal Form, and Width variable inductors.

Horizontal Form behaves as expected, but Width is fully extended.

Any tips on filling the flexible cracks in the rubber mask?

There's a bonus point for anyone who recognises the two L-brackets. They were in my TV24 hardware box, but the project has gone on for over a year now and I don't recognise them! And I can't see an obvious location for them in the temporary assembly of the chassis in the un-restored case.

Thanks.

Ian
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Old 23rd Jul 2014, 10:38 pm   #7
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Default Re: Bush TV24 picture width

If it is just the paint cracked then you can dip it in paint stripper and repaint it. It won't affect the rubber.

I'm not sure but I might still have a good TV24 mask which you would be welcome to (if I do still have it) if it could be collected. In fact I am pretty sure it is around here somewhere.

Just a thought. It might be worth looking closely at the Lin coil at the back of the chassis. Is it working?

Last edited by AC/HL; 24th Jul 2014 at 2:12 am. Reason: PS merged
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Old 23rd Jul 2014, 11:59 pm   #8
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Default Re: Bush TV24 picture width

Quote:
Originally Posted by Boom View Post
Just a thought. It might be worth looking closely at the Lin coil at the back of the chassis. Is it working?
Yes, the picture linearity changes as the linearity core is adjusted and widens and narrows as the width slider is moved.

I've also peaked the capacitor under the chassis on the tag board.

Thanks for the offer of the mask - I may well take you up on that. The one I have is cracked/crazed and although I've found a paint that is about the right colour and adheres to the rubber, of course it cracks too as the mask flexes.

Ian
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Old 25th Jul 2014, 3:04 pm   #9
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Default Re: Bush TV24 picture width

I think I've sorted it. I swapped the LOPT and now the picture fills the screen with some to spare.

If I remove the LOPT tuning cap (470p in TV22 and 300p in TV24) the picture is smaller but brighter. With a variable cap I can change picture size (and brightness) so guess maybe I should put the width control near the centre and select a capacitor that gets the picture the right size - probably less than 300p - then if there ever needs to be width adjustment, I have some to spare from the width control.

Regards,

Ian

PS: Actually, on second thoughts, I'm not entirely convinced that this rewound LOPT just provides slightly less EHT so the picture 'balloons' out more to fill the screen...

(Give me a TV22 any day - so much easier to work on!)

Last edited by Ian - G4JQT; 25th Jul 2014 at 3:18 pm.
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Old 25th Jul 2014, 6:36 pm   #10
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Default Re: Bush TV24 picture width

A bit of a shock there Ian when you mentioned that the LOPT had been rewound. I thought it was the original one. It's possible that it needs a little tinkering with the third harmonic tuning cap to provide a balance with EHT/scan. This is sometimes required bacause it is almost impossible to rewind a flyback transformer to the EXACT specification of the original component. Modern wires and interleaving tissues are a lot different to those of 60+ years ago. I would suggest you clip your EHT meter to the CRT final anode, set the width adjuster to around 3/4 of it's travel and experiment with the tuning. Try a few caps and note the EHT voltage when the width is correct. Just my way of experimenting when all else fails! John.
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Old 25th Jul 2014, 9:53 pm   #11
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Default Re: Bush TV24 picture width

The project seems to be coming together. The ballooning was a bad EY51, but the good replacement tested virtually the same on the CT160.

I tested the tuning of the LOPT with a 1000pF variable capacitor. I was expecting a peak at some point, but EHT was max at fully open* (6.6kV) and min when fully closed (4.7kV).

The compromise between width to the edges and brightness is about 5.7kV. This seems far too low (assuming my EHT probe is accurate) as the TV22 service data claim 8kV and the TV24 8.5kV, though as the circuits are almost identical, it's not obvious to me where the extra 500V comes from.

*Incidentally, the Bush supplementary fault sheet states that an O/C C20 (The 'tuning' cap across the LOPT) will cause high EHT, but doesn't describe the symptoms; presumably a bright but too small picture.

Ian
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Old 26th Jul 2014, 9:27 am   #12
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Default Re: Bush TV24 picture width

Hello Ian,
You cannot test EHT rectifiers on a valve tester other than the obvious faults such as O/C heater. This also goes for most other TV valves on any type of valve tester.
They only give an indication and must not be regarded as a guarantee of actual goodness in a working chassis. You may find that the original EY51 that blooms will work OK in another receiver. Your transformer may be producing slightly low heater volts due to being slightly out of spec. The only way to check this is to visibly compare the brilliance of the heater with the valve run from a 6v battery or transformer. Readings with general test meters are unreliable due to the frequency and character of the flyback pulse. The EHT voltage with the width just making the edges should be no less than 1kv below the manual on this chassis
It would be a good test to substitute a known good transformer and compare the results. You may have a problem with your rewind.
If Mike Barker carried out your transformer rebuild I know he extensively checks his TV22 rewinds in an actual chassis displaying a test card.
What is so difficult repairing the TV24 when compared with the TV22? Other than a few minor mechanical differences, it is the same chassis.
Next move is to try another transformer if you can locate one. Regards, John.
[PS I took this picture this morning from the screen of one of my TV22's. With the width set slightly low the EHT is 7.5kv. Increasing the width to just touch the edges of the mask results in a fraction under 8kv and it remains at this level over an extended period.]
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Old 26th Jul 2014, 9:55 am   #13
Ian - G4JQT
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Default Re: Bush TV24 picture width

Thanks for the info HCS.

Yes, the rewind is from Mike Barker so should be fine. I've collected quite a number of inductance and resistance measurements from a selection of original and rewound TV22/24 LOPTs and I'm pleased to report most are all within about 5% of each other. However I can't easily compare the mutual inductance between various windings nor their self capacity, and as you say it will be almost impossible to match every parameter precisely during a rewind, but the measurable parameters are remarkably close.

The CT160 doesn't give any current readings for the EY51, just wheel settings and a heater voltage, but I connected the leads and compared meter readings. Both looked very close, but the new 'Zaerux' (?) make didn't work properly in my TV24. I was told there were two types of EY51 each having different heater currents...

I guess the main difference in working on the TV22 and TV24 is the awkwardness of supporting the tube. The front wooden panel has holes presumably to string it up temporarily, but it does seem to depend on being in the case and having the mask give some proper support.

My TV22s have a nice linear picture, but the TV24 has a slight kink at the top as shown in your image!

Thanks for you help and suggestions.

Ian
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Old 27th Jul 2014, 1:00 pm   #14
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Default Re: Bush TV24 picture width

Have you tried feeding a different set of test cards through the Aurora. I spent ages messing with the width components on a VT4 only to find out that the default test card that was on my Aurora was too narrow : https://www.vintage-radio.net/forum/...t=53921&page=7

Just a thought...
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Old 27th Jul 2014, 1:30 pm   #15
Ian - G4JQT
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Default Re: Bush TV24 picture width

Oh, thanks Dom. That wasn't a possibility I had considered. I bought the Aurora just last year, so it should be one of the corrected ones.

I have an older Aurora which I use with my TV22 which seems ok, although I haven't used it with the TV24.

I also have a Domino converter which needs the testcard DVD which I will try.

Regards,

Ian
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Old 27th Jul 2014, 7:35 pm   #16
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Default Re: Bush TV24 picture width

If you 'scope the baseband out signal you should be able to tell - there was a fairly long blank either side of the line sync...
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Old 27th Jul 2014, 11:25 pm   #17
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Default Re: Bush TV24 picture width

Ian,

When checking the EHT voltages on the TV22/TV24, the stated EHT voltage in the manuals are only correct with zero beam current (Brightness at minimum).

You should expect to see between 1Kv to 1.5Kv less with the picture brightness set so that no flyback lines are visible on a plain blank screen. This is the correct setting of the brightness control, and if you need more brightness with flyback lines showing, that's a different problem.

Another experiment is to drop the HT tap on the ballast resistor down one voltage setting to bring the set's HT line up.
It only takes a few volts to make a big difference on these sets. Then just check the HT is not excessive.
Also, you can try 3x PL38 valves (all new and fully tested) and each one will give a different result in some TV22/24 chassis.
There is no regulation on this basic economy TV. You will find if you bridge out the PZ30 diodes with Silicones and limiting resistors a reluctant valve will be exposed immediately.
The PZ30 is another one of those valves where several excellent tested ones will give strangely differing results.

Mike...
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Old 28th Jul 2014, 7:12 am   #18
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Default Re: Bush TV24 picture width

The transformer fitted to the TV22 in post 13 was indeed rewound by Mike. My EHT readings were taken with the screen just blanked out. The transformer has been run for 24hrs + and remains stable. Cheers, John.
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Old 28th Jul 2014, 8:49 am   #19
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Default Re: Bush TV24 picture width

Quote:
you can try 3x PL38 valves (all new and fully tested) and each one will give a different result in some TV22/24 chassis.
The PZ30 also makes a big difference, these need to be top notch in these sets.

I have a superb picture on my TV22, I am hoping my TV24 will be as good once restored, I do have a NOS PZ30 & PL38 ready to fit.

Mark
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Old 28th Jul 2014, 9:08 am   #20
Ian - G4JQT
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Default Re: Bush TV24 picture width

Hi Mike.

Thanks for the info. Yes I remember you telling me about those tips which I tried and they did make a difference.

I didn't realise the EHT was quoted with no beam current, but it makes complete sense - obvious when you think about it! I'll re-measure it.

I'll be away from the workshop for a couple of weeks (in Reading now) so hope to get the project wrapped up later in the month. As far as I can tell, I'm pretty sure I'm almost there.

I'm not a TV expert, but considering the parts count, the TV22 and TV24 are remarkably good!

I think a write-up for the Bulletin from me is in order...

Regards,

Ian
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