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Old 5th Oct 2020, 6:34 pm   #101
AdrianH
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Default Re: My first foray into Old model TV repairs.

Hello Peter. If you go to the first post there is a clip on top of the scan coils that goes to the CRT, when I have been measuring the aquadag and getting 3K Ohm it was between the aquadag and general chassis.

But there is nothing wrong with me taking the clip off and giving the underside a clean, I have some plastic green cleaner stuff that should get it shiny.

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Old 5th Oct 2020, 6:39 pm   #102
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Default Re: My first foray into Old model TV repairs.

It's a slightly low emission tube. The weaker electron stream is easily deflected by static charges, even the static build up on your body. Hence it moves about if you put your fingers on the screen.

The dag is earthed sufficiently. Damp and humid conditions aggravate the problem. No need to run it on a bright raster, just give it a couple of hours use a day for a few days and see if it improves. It's a very common symptom with slightly tired Mullard tetrodes. John.
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Old 5th Oct 2020, 6:43 pm   #103
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Default Re: My first foray into Old model TV repairs.

OK on it being a common issue, this I did not know.
The TV being in the house is certainly not in a damp or humid area so I can not blame that. I hope it improves, not sure if I would like to try a CRT rejuvenator to improve the tube or not that seems to be a kill or cure.

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Old 5th Oct 2020, 6:53 pm   #104
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Default Re: My first foray into Old model TV repairs.

I would if at all possible avoid the "rejuvenator" or you may end up with a 24 hour shine then good day.
Most of the above are pretty crude.
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Old 5th Oct 2020, 8:45 pm   #105
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Default Re: My first foray into Old model TV repairs.

Quote:
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Hello Peter. If you go to the first post there is a clip on top of the scan coils that goes to the CRT, when I have been measuring the aquadag and getting 3K Ohm it was between the aquadag and general chassis.

Adrian
Probably a waste of time but does it make it better or worse if you disconnect the clip with a little insulating tape?

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Old 5th Oct 2020, 8:54 pm   #106
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Default Re: My first foray into Old model TV repairs.

I will give it a try tomorrow and report back.

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Old 5th Oct 2020, 9:04 pm   #107
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Default Re: My first foray into Old model TV repairs.

Do not reactivate! As Hamish has stated. You may well ruin a tube that is capable of giving a good picture. The 991T is almost 70 years old and these deficiencies must be accepted.

You also have to take care with tube testers. Many I have seen are far too brutal and some are worse!

It is a fringe model designed to work on very low signals. The alignment is probably biased towards gain rather than definition. The 2mc/s bars should be clearly visible but don't expect much more. Quite incredible receivers. The 992T is the service area model with a wider bandwidth and lower gain. John.
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Old 5th Oct 2020, 9:41 pm   #108
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Default Re: My first foray into Old model TV repairs.

And with use the tube might well improve. My Pye FV1 if not used for a considerable length of time will display a very dim picture, but after a half hour so the picture brightens up and is very watchable.
The tube is the original Mullard MW31-16 made in 1951.

DFWB.
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Old 5th Oct 2020, 9:44 pm   #109
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Default Re: My first foray into Old model TV repairs.

This is a really curious condition. At first read through of this thread the raster distortion would probably be attributed to scan coil leakage line/field and vice versa. However, the fact that it goes bad again if the brightness is left low with the set switched on rather knocks that on the head as the coils will be warmed through.

With EHT of 14/15 kV, the beam should be pretty stiff - it's not some old 6 kV soft as putty situation and one starts to think about EHT smoothing due to poor aquadag, as has been surmised. However, if it's there and connected, can it be that bad?

If it was in my workshop, I'd hook up my stabilised EHT PSU instead of the internal supply. That would prove whether the EHT is constant and fit for purpose. Without access to an external lab EHT unit it's down to elimination and the EY51 might be on my hit list as a starter.

Does the image breathe badly with increasing contrast/brightness? That's a clue about the integrity of the EHT supply.

Reading back to the very beginning of this thread, I note the scratch on the CRT neck. I've known cases where such damage hasn't been enough to cause catastrophic failure, but enough to admit very, very small quantities of air which then cause havoc in all sorts of weird ways.

An interesting problem and back to the physics of CRT technology.

Great job so far!

Best regards,

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Old 5th Oct 2020, 10:56 pm   #110
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Default Re: My first foray into Old model TV repairs.

I have been expecting the issue to be down to things like leaky caps, possibly damp in the coils but it has had chance to dry out now and I also ran current through the LOPT over two days for about 12 hours.

I am hoping that, as people say the tube will improve with use, it has been only a couple of days I know so I will keep my fingers crossed. Perhaps I have issues with flyback blanking but if the brightness is turned up I see the flyback lines on the screen, but, and this is a biggy, this is my first foray into these old TV's so I really do not know what I should be expecting hence many, many questions.


One thing that I can possibly do to show the effects better would be to take a video of the TV as I have it now, perhaps an excuse to register a youtube channel.

That way I can show the screen when I switch on, turn down the brightness, turn it up etc. The only thoughts that I have that will be misleading will be the camera I use compensating for exposure as it is a digital bridge camera and typically tries to correct automatically. I am not sure this can be switched off.

I also need to check the tuning of the cans, I do have a test set that will do this sort of work with a tracking generator and spectrum analyser, just have to be very wary of earthing anything.

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Old 6th Oct 2020, 5:00 am   #111
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Default Re: My first foray into Old model TV repairs.

I’ve got a couple of sets that also have that strange distortion when first turned on, one is an Ekco that I fitted a Mullard tube into, with that one I just turn the brightness up full which clears it in a couple of seconds, then turn it back to a normal viewing brightness. I don’t see it as a problem, just a quirk of old CRT’s. I also read somewhere that using anti-static foam cleanser on just the tube face can help keep static charges from building up on the CRT, but I’ve tried it on the Ekco and it didn’t seem to make any difference.

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Old 6th Oct 2020, 6:29 am   #112
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Default Re: My first foray into Old model TV repairs.

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I just turn the brightness up full which clears it in a couple of seconds, then turn it back to a normal viewing brightness.
Regards
Lloyd
+1

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Old 6th Oct 2020, 7:10 am   #113
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Default Re: My first foray into Old model TV repairs.

This symptom has nothing to do with dampness in the scan coils, leakage and other suggestions.
It is purely caused by a low emission tube aggravated by electrostatic charges mainly on the screen faceplate.
It used to be a very common display with older receivers and looking back can only remember it occurring with Mullard tetrode tubes such as the 12" MW31-16/18/74 types, 14" MW36-24/44 and the 17" MW43-64.
It is interesting to note that all of these tubes are non aluminised and at a guess I would suggest that the aluminium backing probably prevents static build up. As Lloyd has mentioned it's just a matter of giving the tube full beam current for a couple of seconds to clear it. Very low emission tubes take a bit longer.

I don't know the type of glass employed in the early 50's tubes but it may or may not have contained a percentage of lead. This may have had a bearing on the fault. Regards, John.
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Old 6th Oct 2020, 7:49 am   #114
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Default Re: My first foray into Old model TV repairs.

Good morning,
Would the application of say an an anti static polish to the front of the CRT help stabilise the static effects on the faceplate?

Christopher Capener
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Old 6th Oct 2020, 4:23 pm   #115
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I've never tried it Chris. Possibly for a short time but back then all the TVs were cacked up with nicotine and open fire soot particles, most of which the customer never noticed or bothered the owners as the effect vanished after a very short period of time and did not return once it had vanished!

It's just a curious happening with these old and probably tired television receivers. I have seen some really weird effects during my 50+ years in the servicing trade including the appearance of test card C on the screen of a Mazda CRM141 that had it's neck broken off after having been standing on an open wood rack in the back yard... O.T. so you will have to guess the reason. J.
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Old 6th Oct 2020, 10:37 pm   #116
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Default Re: My first foray into Old model TV repairs.

I have not done very much with the TV today, take a few measurements, empty and re-stuff the main smoothing can, that was a job and a half, ended up using the lathe, not a very good job but it is in and working.
A few coats of Walnut stain to the cabinet.

I had a bit of luck on Ebay, got what I hope is the correct mains plug and a couple of again what appear to be the correct antenna plugs with connections for the outer screen.

Tried to do a video but deleted it as the camera was focus hunting all the time trying to keep on the front screen, will have to dig out another camera.
Another day tomorrow.

Adrian
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Old 8th Oct 2020, 4:46 pm   #117
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Default Re: My first foray into Old model TV repairs.

I recall the picture on my nan's DER rented Ferguson 204T coming on this way in its later years of service (mid 60's). The left and right top corners were cut off about 2 inches. This took about a minute to fill the screen. Never saw this on any other TV set, but remember reading about it in the Your Problems Solved section in Practical Television.

Alan.
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Old 8th Oct 2020, 7:58 pm   #118
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Default Re: My first foray into Old model TV repairs.

The TV has gone back into its cabinet tonight.
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The new plywood panel has been painted several times with walnut wood-stain and the other panels as well after a light sanding. I am still flattening out the back panel and it looks as though the pink paint that is on the back will have to stay, trying to rub it off started to remove the backboard finish. Perhaps use of walnut stain may tone it down somewhat.
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I fitted some new ECL80's into the sync circuits, but to be honest with very little change to the frame linearity.

Not saying the set will not be taken out of the case at some point soon, but need to move on to a few other unfinished projects soon.

Thanks for the comment Alan, if you could pin it down from memory to what year that would be good as there are a lot of mags to go through to find the article.

I think I have helped reduce the screen effect, or at least made it go away quicker! I had trouble trying with minimal raster to set the ion magnet so in the end resorted to doing it at full brightness to the point where all the screen is of close as possible to the same brightness. It seems to help and give a slightly brighter picture, but turning the brightness control full up does now shift it within 30 seconds and not the minutes previous.

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Old 8th Oct 2020, 9:27 pm   #119
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I'm afraid I can't remember the date of the Practical TV magazine I read that particular letter but it would have been in one of a load of 1950s editions that I was given as a young teenager in the late 60s which I no longer have. The actual set in the letter to the magazine may or may not have been a Ferguson but the problem was the same and the answer given by the experts was the same as in the posts above - uneven static charges on the CRT screen. No solution to a cure was given as far as I can remember. It just happened to answer why this was happening to my nan's set. I think you have done very well indeed with this set and the cabinet is looking good as well.

Alan.
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Old 8th Oct 2020, 9:34 pm   #120
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Default Re: My first foray into Old model TV repairs.

OK Alan, thanks for the comments. Now to try and get some 4:3 format videos to play on it.

Adrian
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