Bush TV24 picture fade

[German Dalek]

Hi all!

And now there is something completly different:

ARE YOU ABLE TO RUN THIS SET (AND/OR SECTIONS OF IT) FROM A SEPARATE REGULATED POWER SUPPLY?

IS IT POSSIBLE, TO RUN THIS SET WITH THE HV FROM ANOTHER SOURCE?
MAYBE WITH A SIMILAR CONSTRUCTION.

DID YOU TRY THE HV-TRANSFORMER/CRT IN QUESTION WITH ANOTHER BUSH, MAYBE WITH ONE OF A COLLECTOR`S SET FROM YOUR "NEIGHBORHOOD"?


And what about the heater voltage/current during the crazy operation?

Happy weekend & good luck,
German Dalek

[Ian - G4JQT]

Originally Posted by murphyv310

Hi.
Surely the hotter the LOPT gets the resistance of the wire, especially the overwind will increase and the EHT drops accordingly, even on a LOPT that has no moisture ingress? I eventually gave up trying to get a stable EHT with even rewound LOPTs and fitted a Thorn Jellypot, which I stabilised at as little under 9kv which stays there no matter what the brightness is set too.

Did you add replace the LOPT on a TV22/24 with a Thorn Jellypot?

[Ian - G4JQT]

Originally Posted by German Dalek

Hi all!

And now there is something completly different:

ARE YOU ABLE TO RUN THIS SET (AND/OR SECTIONS OF IT) FROM A SEPARATE REGULATED POWER SUPPLY?

IS IT POSSIBLE, TO RUN THIS SET WITH THE HV FROM ANOTHER SOURCE?
MAYBE WITH A SIMILAR CONSTRUCTION.

DID YOU TRY THE HV-TRANSFORMER/CRT IN QUESTION WITH ANOTHER BUSH, MAYBE WITH ONE OF A COLLECTOR`S SET FROM YOUR "NEIGHBORHOOD"?


And what about the heater voltage/current during the crazy operation?

Happy weekend & good luck,
German Dalek

Good points, thanks. I have operated the set from a variac but didn't get the same effects. I have a TV22 and I guess I could swap the EHTs over as long as the chassis were both at the same potential!

The tube heather voltage was normal, so that would suggest (but not necessarily guarantee) the heaters are all behaving normally.

I have anther LOPT but I'm hoping to avoid swapping it out until it really shows a fault. Can't remember if this is possible without removing the chassis from the case which is a real faff on a TV24.

I suspect the erratic behaviour only occurs after some weeks of non-use, and then not consistently. At the moment it's behaving very well!

[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
I believe the reduction in EHT in the hot but otherwise dry LOPT is mostly due to the increase in resistive losses in the overwind mentioned by Murphyv310. The TV22 has no EHT regulation, so this effect will always be observed even in a LOPT free of additional losses due to water ingress or solvents. Higher temperatures increase the resistivity of the copper wire (hence the higher resistive losses) but also increase the ionic conductivity (water and/or solvents in the overwind insulation), which results in additional losses. The latter can be greatly reduced by driving out the moisture and/or solvents from the overwind. It doesn't really matter if the LOPT has been rewound or not since the losses occur mostly in the overwind and this part is not rewound anyway (it's usually ok but is also wavewound and hard to replicate).
- Helder

[Helder Crespo]

I just asked ChatGTP4 the following question:
"in a line output transformer without EHT (extra high voltage) regulation, should one expect a drop in EHT as the transformer heats up?"
This is what it replied:

"ChatGPT
In a line output transformer (often referred to as a "flyback transformer" especially in the context of CRT televisions and monitors), the EHT (Extra High Tension, also known as high voltage) output is the result of the transformer converting low voltage pulses from the line output stage into very high voltage required for the picture tube or other applications.

Whether the EHT voltage drops as the transformer heats up depends on several factors:

1. Core Saturation: As the core temperature rises, the magnetic properties of the core material can change, possibly leading to increased core saturation during each cycle. Core saturation can cause increased current draw and reduced efficiency, potentially leading to a drop in the EHT voltage.

2. Resistance Increase: The resistance of the windings in the transformer will increase with temperature. Copper winding resistance, for instance, can increase by about 0.39% per °C rise. This means that if the transformer gets significantly hotter, the increased resistance can lead to a drop in the EHT voltage.

3. Leakage Inductance: Temperature rise might also affect the leakage inductance of the transformer, which can further influence the EHT voltage.

4. Aging and Breakdown of Insulation: Continuous heating or excessive temperatures can age or break down the insulation materials within the transformer. This might not directly lead to a drop in the EHT, but it can result in short circuits or arcing which can eventually affect the performance and EHT.

5. External Circuits: Components connected to the transformer, such as capacitors and diodes in the EHT rectification circuit, can also be affected by temperature. Their behavior might indirectly influence the EHT voltage.

It's also worth noting that the design and control circuitry associated with the transformer plays a crucial role. Some circuits may try to maintain a consistent EHT by adjusting the drive to the transformer in response to variations, including those caused by temperature.

In general, while it's possible for the EHT to drop as the transformer heats up, the specific behavior will depend on the design of the transformer, the characteristics of the materials used, and the associated circuitry. If consistent EHT is critical, monitoring and regulation circuits should be implemented."

- I then asked for a more detailed explanation of the mechanism in point 2, together with some calculation examples (see attached pdf). Based on the reply I got, where I didn't even have to tell ChatGTP to use typical values for a Bush TV22 (which it knows, btw), the conclusion is that heating of the primary is responsible for the drop in voltage in the secondary, even in the absence of additional losses (moisture and/or solvent in the overwind insulation).

[Ian - G4JQT]

Originally Posted by Helder Crespo

I just asked ChatGTP4 the following question:
"in a line output transformer without EHT (extra high voltage) regulation, should one expect a drop in EHT as the transformer heats up?"
This is what it replied: [...]

Thanks. It's sort of frightening this AI isn't it? I've used it occasionally and it can be scarily imaginative and informative, but sometimes wildly inaccurate, although stated with authority!!

But it's come up with some interesting stuff here which I think mostly only confirms what we've already said. (Was it reading this thread...?)

[Helder Crespo]

I totally agree Ian. A key aspect is the increase in performance in each subsequent generation of GTP.
Going back to the topic, the conclusion above is more precise that the preceding discussion since the change in EHT is due to heating of the primary, and not the overwind. That the overwind alone could not be responsible for the drop in EHT had already been pointed out by Panrock in a previous thread.
I will also ask about core saturation but will need to use the actual parameters of the TV22. For the parameters used in the previous numerical examples this effect is negligible even at 75 Celsius.
- Helder

[murphyv310]

Originally Posted by Ian - G4JQT

Did you add replace the LOPT on a TV22/24 with a Thorn Jellypot?

Indeed I did.
Fernseh (David Boynes) has also done this in a slightly different way to myself with excellent results.

[Helder Crespo]

Originally Posted by Ian - G4JQT

Thanks. It's sort of frightening this AI isn't it? I've used it occasionally and it can be scarily imaginative and informative, but sometimes wildly inaccurate, although stated with authority!!

But it's come up with some interesting stuff here which I think mostly only confirms what we've already said. (Was it reading this thread...?)

Well, in this case the mistake was mine. Looking again at the numbers used, namely the turn ratio of 1000:1 (which is fine as an example, but is way off compared to the actual values in the TV22), it's clear that the decrease in EHT due to increased temperature in the primary and secondary is in fact way smaller that the ~2% shown in the example when one considers the actual values in the TV22, which has a turns ratio closer to 6:1. So the observed reduction in EHT (e.g., from 8kV to 7.7kV after warm-up, as reported by Ian) is probably associated with the additional losses due to moisture and/or residual solvent in the overwind, which do increase with temperature. The good news is that it should then be possible to have a TV22 LOPT with very good EHT stability, despite the lack of regulation, provided the additional losses (including those in the core) are effectively minimised. In a previous thread, Heatercathodeshort mentioned that he can turn the brightness all the way up on one of his TV22s with the EHT reducing from 9 to 7kV and without a significant change in image size, so in principle it can be done (see https://www.vintage-radio.net/forum/...80#post1471080)
Regardless, my main recommendation still holds: it's important to get rid of moisture and/or residual solvents in the overwind as much as possible.

[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.

[Panrock]

At long last, I have updated my pages on the TV22 to include current knowledge about this problem... see the bold text toward the bottom of THIS page.

Helder, I invite you to run your experienced eye over the paragraph in question. I've tried to keep it concise. Any clarifications or amendments you suggest will be incorporated.

Thanks, Steve

[Helder Crespo]

Originally Posted by Panrock

At long last, I have updated my pages on the TV22 to include current knowledge about this problem... see the bold text toward the bottom of THIS page.

Helder, I invite you to run your experienced eye over the paragraph in question. I've tried to keep it concise. Any clarifications or amendments you suggest will be incorporated.

Thanks, Steve

Hi Steve
Thanks for this update. You're very kind but I have a couple of suggestions:
You don't have to mention me explicitly in the revised text. I have not done more than (possibly many) others who have also passed a current through the overwind to drive out moisture, sometimes followed by a light coating of anti-corona spray - this last step is something you also describe in the section of your website devoted to treating the LOPT. The fact that the current heating technique also works with (poorly...) varnished LOPTs and with repitched LOPTs is of course a nice bonus and really saved my LOPTs, but again I merely applied a technique that was already being used by others. By also treating the somewhat rusty laminations in one of my LOPTs I was able to conclude, at least empirically, that the major contribution to the EHT reduction problem in LOPTs with cores in moderately good condition (some rust and deformation at the edges) still seems to come from losses in the overwind insulation, which fortunately respond very well to the current heating procedure even if the LOPT has been varnished or repitched.
Perhaps it would be important to add that old and cracked pitch can also be removed by immersing the LOPT in a white spirit bath over a period of several days, as advanced by Dr. Hugo Holden, which can then be followed by varnish or again a light coating of anti-corona spray.
The technique that you mention in your webpage, which involves placing the LOPT in a oven to soften it and then flake it off by hand, actually seems to perform both tasks, i.e., it allows removing the old pitch and it also drives away moisture - although it may pose higher risk of damage to the windings and fine wires due to its mechanical nature.
Kind regards
Helder

[Panrock]

Right you are. I've amended the text accordingly, and added a mention of Dr Hugo Holden's method on the appropriate page HERE.

Feel free to make further suggestions.

Steve

[Helder Crespo]

Originally Posted by Panrock

Right you are. I've amended the text accordingly, and added a mention of Dr Hugo Holden's method on the appropriate page HERE.

Feel free to make further suggestions.

Steve

Hi Steve
I must thank you for having produced so many great TV restoration guides - and not just the TV22 one. Your pages are a very important reference in the field and I really appreciate your efforts in reviewing and updating them with new/alternative techniques and findings.
Kind regards
Helder

[Helder Crespo]

Originally Posted by Panrock

Right you are. I've amended the text accordingly, and added a mention of Dr Hugo Holden's method on the appropriate page HERE.

Feel free to make further suggestions.

Steve

Hi again Steve

The text below about the EHT overwind losses is very clear and concise. I would just add a sentence similar to the one below (shown in bold for clarity), and perhaps replace 'windings' with 'overwind' where relevant, as the 50 mA will only result in significant dissipation in the 800 ohm overwind, and not in any other winding:

Further knowledge about this problem has since been gained. It has been established that, in the main, this is being caused by damp (or undried replacement varnish) trapped in the windings of the LOPT, reducing both the line scan amplitude and the EHT. This shows as a dimmer, softer picture and an increase in height. Standard practice nowadays is to pass a current of around 50mA through the overwinding for several days at the time of service, driving the moisture out (this technique can also improve the performance of LOPTs where the overwind has already been varnished or repitched). Then to seal the windings with an anti-corona aerosol (such as from RS) before more moisture can be absorbed.

Many thanks and kind regards
Helder

p.s. - should the text also mention that the EHT problem is often accompanied by an excessive dependence of ETH on beam current? - this becomes more evident when increasing the contrast and/or brightness or when displaying a bright scene. This was surely the case with both of my LOPTs and the current heating method has really made a difference here, but I'm not sure whether this occurs in general.

[Panrock]

Thanks Helder. Your suggestions have been added and the text split into two paragraphs. See what you think.

Best regards, Steve

[Helder Crespo]

Originally Posted by Panrock

Thanks Helder. Your suggestions have been added and the text split into two paragraphs. See what you think.

Best regards, Steve

Hi Steve. Looks great. Many thanks!
Kind regards
Helder

[Panrock]

You're welcome. 50mA through 800 ohms for several days equates to 40v and 2 watts. Things must get pretty hot...?

Steve

[Helder Crespo]

Originally Posted by Panrock

You're welcome. 50mA through 800 ohms for several days equates to 40v and 2 watts. Things must get pretty hot...?

Steve

Hi Steve
It's actually a bit more than that, since the resistance increases to a little over 1kohm, so for 50mA you need more than 50V and will be dissipating some 2.7W. So far I've successfully used up to 53mA, which gives around 3W, and there are reports mentioning 65mA and more for the TV22 LOPT. The overwind gets hot but not excessively so. I've measured around 65 celsius at the surface of the overwind with an IR thermometer. You can still touch it, and I believe it is still safe for the enamel and varnish: so far I've only seen improvement - and no degradation - in performance.
Kind regards
Helder

[Panrock]

Sounds like a hand warmer for the winter! I'm clocking off now. Thanks for all the help.

Best regards, Steve