Bush TV24 picture fade

[Helder Crespo]

Originally Posted by theredhouseinn

Back in the 60's I Cobbled together a load of bits to build my first colour set using a 14" Toshiba tube. I added some extra windings to a GEC monochrome lopt to drive the convergence circuits.
After about 2 hours the picture stated to bloom and the width came in slightly.
What I had done was to tighten the clamp holding the two halves of the ferrite core to much, loosening the clamp so that the copper springs exerted less pressure on the core cured the problem. Presumably the core has to vibrate at 15625 c/s.
John.

Hi John
The problem in your GEC - increased height and reduced width as the transformer heats up - is similar to the one in the Bush TV22/24 sets and others in the sense that it results from a reduction in the efficiency of the transformer (in your case probably due to losses in the magnetic circuit) although your solution is not directly applicable here since the TV22/24 LOPT is built around a single core made of iron-based laminations.
- Helder

[Helder Crespo]

Originally Posted by Ian - G4JQT

After a few days of more use that the TV24 usually gets, the EHT seems to have settled down. With brightness right down and after five minutes it's hovering around 8kV, then slowly falls to about 7.7kV. Normal viewing brightness brings it down to about 6.5 to 6.8kV.

All very odd to me, but maybe it makes more sense to you Helder!

I've put the back on now and will hope for the best, but I've been here before...

Hi Ian
I was wondering how your set has been behaving. If the EHT is still somewhat erratic it may be a good idea to try the current heating method as it is way more efficient in driving moisture out than simply running the set or heating the LOPT from the outside in.
Kind regards
Helder

[Ian - G4JQT]

Hi Helder,

It's been off for a few days but I used it last night for a few hours watching Dad's Army. It behaved perfectly...

[Helder Crespo]

Hi Ian
I'm glad it's all sorted out and leaving the set on was enough to drive out the moisture from the LOPT. If you come across similar problems again after having the set off for a while, you know what to try first.
Kind regards
Helder

[Ian - G4JQT]

Yes. Thanks for your help here. I'd assumed that once the LOPT has been rewound, re-dipped in new pitch and appeared to be working OK that any EHT problems were caused by something else.

The set does get left unused for months sometimes and the first few viewings tend to be fine. It's only later that the the problem starts to re-emerge. I would have expected that the problem would be the other way around; starts off bad then gets better. I was also put off the scent by switching it off after it had had a 'bad turn', only to find that after an hour or two it was temporarily OK again.

Am I right in thinking that once the LOPT is working well and 'dried out' that it would be a good idea to coat the entire pitch dipping in some sort of lacquer to 'seal the dryness in' for good? Maybe the complete assembly and not just the pitch covering? Is there any particular type of lacquer you'd recommend?

Thanks again for your assistance and advice. I hope this has helped everyone with a TV22/24 whether the LOPT has been rewound/pitch and dipped or not.

Ian

[Helder Crespo]

Originally Posted by Ian - G4JQT

Yes. Thanks for your help here. I'd assumed that once the LOPT has been rewound, re-dipped in new pitch and appeared to be working OK that any EHT problems were caused by something else.

The set does get left unused for months sometimes and the first few viewings tend to be fine. It's only later that the the problem starts to re-emerge. I would have expected that the problem would be the other way around; starts off bad then gets better. I was also put off the scent by switching it off after it had had a 'bad turn', only to find that after an hour or two it was temporarily OK again.

Am I right in thinking that once the LOPT is working well and 'dried out' that it would be a good idea to coat the entire pitch dipping in some sort of lacquer to 'seal the dryness in' for good? Maybe the complete assembly and not just the pitch covering? Is there any particular type of lacquer you'd recommend?

Thanks again for your assistance and advice. I hope this has helped everyone with a TV22/24 whether the LOPT has been rewound/pitch and dipped or not.

Ian

Hi Ian

The behaviour you mention does seem strange, but I believe the combination of the LOPT and its insulation with the environmental and operating conditions are possibly complex enough to produce it. Nevertheless, the performance always seem to improve after drying out the LOPT and the behaviour is much more consistent after doing that.

As for a more permanent sealing, I haven't yet done anything other than the initial varnishing, which despite the thick coat (my varnished LOPT was immersed some 10 times in marine spar varnish) doesn't seem to have resulted in a perfect seal. There are other posts in the forum (I've quoted a couple of them in a previous reply within this thread) that state good lasting results after coating with anti-corona spray. According to other posts, anti-corona spray has been used successfully with LOPTs that have been stripped of pitch by immersing in a solvent (white spirit). Also, Panrock's (Steve Ostler's) TV22 restoration pages mention its use after heating the LOPT in an oven and flacking the pitch off by hand. I have a couple of cans of anti-corona spray purchased from RS a few years ago that I didn't yet try.

I'm not aware of explicit reports where anti-corona spray has been used to (re)seal a LOPT that still has its original pitch on, or has been repitched.

Kind regards
Helder

[beery]

Hi Ian,
Interesting to hear about your set.
I have a TV22 that does a very similar thing.
In my set the EHT remains fairly stable, but of course as the picture dims, the EHT goes up slightly.
I've done and extensive soak test, checking the video amplitude, the A1 voltage, the EHT voltage and the CRT heater voltage.
The thing is that with the RF unit pulled out of the chassis, it doesn't really do it.
What I have since found out is that it is actually the contrast that is changing. I have had TV22s in the past where a dodgy valve socket for the local oscillator is a problem, but in this set I am suspecting the cathode resistors of the IF valves.
I need to carry out more testing...

[Ian - G4JQT]

Originally Posted by beery

Hi Ian,
Interesting to hear about your set.
I have a TV22 that does a very similar thing.
In my set the EHT remains fairly stable, but of course as the picture dims, the EHT goes up slightly.
I've done and extensive soak test, checking the video amplitude, the A1 voltage, the EHT voltage and the CRT heater voltage.
The thing is that with the RF unit pulled out of the chassis, it doesn't really do it.
What I have since found out is that it is actually the contrast that is changing. I have had TV22s in the past where a dodgy valve socket for the local oscillator is a problem, but in this set I am suspecting the cathode resistors of the IF valves.
I need to carry out more testing...

I have a spare MK1 set which this set is, and swapped over the RF chassis after replacing all the wax paper caps of course, just to see if it was not EHT related in the earlier tests. But results were inconclusive, maybe because I did the swap test on one of the set's bad days - can't remember now. But if it happens again I'll swap over and see if it makes a difference or not. If it is the LOPT (which seems most likely) it won't make any difference.

Ian

[Helder Crespo]

Originally Posted by beery

Hi Ian,
Interesting to hear about your set.
I have a TV22 that does a very similar thing.
In my set the EHT remains fairly stable, but of course as the picture dims, the EHT goes up slightly.
I've done and extensive soak test, checking the video amplitude, the A1 voltage, the EHT voltage and the CRT heater voltage.
The thing is that with the RF unit pulled out of the chassis, it doesn't really do it.
What I have since found out is that it is actually the contrast that is changing. I have had TV22s in the past where a dodgy valve socket for the local oscillator is a problem, but in this set I am suspecting the cathode resistors of the IF valves.
I need to carry out more testing...

Hi
This is very interesting as a couple of years ago I had a similar problem with my TV22 Mk2. I also tracked it down to the RF deck, not by doing any careful analysis, but by noticing that tapping the deck would give a sudden flash in brightness and this was not caused by the contrast pot, which was working fine. The effect was indeed equivalent to a change in contrast (gain) and in my case was due to poor contacts from residual dirt and rust in the valve sockets and pins. Contact cleaner didn't solve this entirely - and I tried it some 2-3 times - as there were still some persistent scratchy connections, but gentle brushing and wiping with WD40 produced a cure. I did not measure the EHT when this occurred but I imagine that, like in your case, it would go up a bit due to the reduced beam current. This is still different from Ian's problem as he was measuring a clear reduction in EHT.
I believe that problems with the metrosil and heater cathode partial shorts can also result in a dim image, which can also be intermittent.
Helder

[Helder Crespo]

Hi again
Regarding the RF deck, I just remembered this post from a little over two years ago. The thread is again about a dimming image in one of Ian's sets, which he said has been a recurrent problem:
https://www.vintage-radio.net/forum/...14&postcount=6

[Ian - G4JQT]

Thanks Helder. I know this has happened before but couldn't remember if I'd posted about it on here or not. Seems I did a valve change that time too which appeared to do the trick, but now looks more like the same 'cure' as seems to be the case this time; i.e. the fault goes away after it's been on-off for a couple of hours a few times.

[Helder Crespo]

Originally Posted by Ian - G4JQT

Thanks Helder. I know this has happened before but couldn't remember if I'd posted about it on here or not. Seems I did a valve change that time too which appeared to do the trick, but now looks more like the same 'cure' as seems to be the case this time; i.e. the fault goes away after it's been on-off for a couple of hours a few times.

Hi Ian. Have you tried replacing the metrosil? See attached picture for two possible alternatives (taken from Dr Hugo Holden's report Bush TV22 Restoration).
Helder

[Ian - G4JQT]

Hi Ian. Have you tried replacing the metrosil? See attached picture for two possible alternatives (taken from Dr Hugo Holden's report Bush TV22 Restoration).
Helder

Yes, thanks. I remember swapping a metrosil over from another Bush TV22 set, and also trying that equivalent last time I had this problem, but it didn't make much difference.

Ian

[Helder Crespo]

Originally Posted by Ian - G4JQT

Yes, thanks. I remember swapping a metrosil over from another Bush TV22 set, and also trying that equivalent last time I had this problem, but it didn't make much difference.

Ian

Hi Ian. I think that if the metrosil is ok and if you don't have heater-cathode intermittent shorts (although I think these are not always easy to spot), this brings us back to the LOPT. I still think you should try passing a current through the overwind for a few days to drive away moisture from within as this will be very effective (and possibly longer lasting, even without further sealing) compared to just running the set for several hours. And it's really easy to do if you have a suitable PS available as you don't have to remove the chassis from the cabinet.
Kind regards
- Helder

[Ian - G4JQT]

Thanks again Helder, I will try that.

BTW, I looked at your deep analysis of the TV22 and you have correctly spotted a typo in my table of LOPT inductances. You are of course correct, 30uH should read 300uH.

Regards,

Ian

[Helder Crespo]

Hi Ian
Thanks. The analysis you mention is probably that of Dr Hugo Holden, and not mine. He is way more knowledgeable and experienced in electronics that I am. I did measure the inductance of all windings in my LOPTs and for the e-winding the value I got is close to 400uH, so very close to the 300mH you mention.
Kind regards
Helder

[Ian - G4JQT]

Originally Posted by Helder Crespo

Hi Ian
Thanks. The analysis you mention is probably that of Dr Hugo Holden, and not mine. He is way more knowledgeable and experienced in electronics that I am. I did measure the inductance of all windings in my LOPTs and for the e-winding the value I got is close to 400uH, so very close to the 300mH you mention.
Kind regards
Helder

Oh yes, sorry, you are quite right! It was indeed Dr Hugo Holden!