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Old 6th Apr 2021, 1:55 am   #41
bionicmerlin
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Default Re: Bush TV 12B To restore or not to restore advice please

Just a slight update. Not much of a change. I have reduced the voltage to 230 .
It makes only marginal difference to the hight.
One other thing I found is on the Aurora set up I have a attenuator as I normally find the output to high. In this case I turned the gain up and it makes the whole picture larger before distorting if I go to high.
I will try and get some replacement valve to try next Andy
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Old 7th Apr 2021, 3:26 pm   #42
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Default Re: Bush TV 12B To restore or not to restore advice please

Quote:
I will try and get some replacement valve to try next Andy
I would get a few EF50's when you get to your valve stash, I had to replace two in my set.
I think the EF50's in the top deck are worth checking, also the EL42 frame output may be causing your linearity problems.


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Old 24th Apr 2021, 8:01 pm   #43
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Default Re: Bush TV 12B To restore or not to restore advice please

I’m not making any progress. After changing the EL42 and then PZ30. For known good replacements it's made no difference exactly the same. I have checked my work over and over and sure I have made no mistakes.

Hopefully the people who know more than me can give me some new leads.
Andy
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Old 25th Apr 2021, 1:41 am   #44
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Default Re: Bush TV 12B To restore or not to restore advice please

Hi Andy,
On the subject of the picture getting bigger when you turn up the contrast. Have you tried backing off the sound sensitivity a bit? On my set if the contrast is high and the sound sensitivity is too high as well, it results in the sound RF section oscillating which causes the picture to become excessively bright and balloon out.

On the frame issue, I would now suspect the frame output transformer, but you can only check it by substitution. I've rewound the one from my set and plan to put it back (It currently has a transformer from a GEC BT2147 in it. I might be able to offer to send you the GEC transformer when I have replaced it, but I'm not sure how soon this will be. Alternatively I could lend you the rewound Bush one so you can at least see if that is the problem.

Cheers
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Old 25th Apr 2021, 10:11 am   #45
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Default Re: Bush TV 12B To restore or not to restore advice please

Regarding the vertical shape of the test card. What is the effect of the vertical form [linearity] control? Can you reduce the stretched bottom of the circle? If you can even out top and bottom of the circle with the frame lin pot does this leave you with a straightforward lack of height display?
The components to check are in the linearity feedback part of the circuit and the cathode bias resistor.
The cathode bias resistor, R4 should be 510 ohms. The display looks to be over biased so check that it is close to this value. R4 Bush circuit. Check C7 .022uf and R9 1.5M.
The TV12 does not have 100% EHT regulation. The EHT voltage is much lower than the TV22 with a completely different LOPT. Your example looks a good one.
The height control operates in a similar style to a standard volume control in a radio. The output from the oscillator is coupled via the control by C5 .5uf. Very unlikely to be faulty but check anode load and screen feed resistors of frame oscillator R12 39K and R13 22k just to eliminate.
My own thoughts verge towards an incorrect component value, particularly C8 .05 [.047uf] C9.02uf .022uf] and C7 .022uf. I know you have checked these but we have all done it..
I will get my TV12 from the high shelf and have a twiddle with the height and lin controls to see the extent of their adjustments.
Just a last thought. If your EHT is too high you may be extending the height control to overcome the stiffer beam causing the EL42 to bottom out. I will check my EHT. John.
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Old 25th Apr 2021, 11:08 am   #46
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Default Re: Bush TV 12B To restore or not to restore advice please

I have taken these pictures a few moments ago. This Tv12 was one of the first receivers when I started to collect old junk tellies. It was repaired back around 1971 and as far as I can remember, has not been touched since.

The pictures show what is displayed on TCC with various settings of the height and lin controls. The lin/form does not do a lot but back then I would have just 'repaired' it rather than carried out a restoration overhaul so I suspect it may have a few wax caps lurking.

Remember that the TV12 has the old aspect ratio mask that was changed in 1950 so you will never be able to get a perfect circle without overscanning the width. This is demonstrated in one of the screen shots.

The EHT is exactly 5kv but I seem to remember that around 4.8kv was about normal with the many TV12s I obtained from jumble sales when I was a kid. I did have the Bush notes regarding the low EHT and the details of the modified LOPT that gave a guaranteed 5.5kv but I loaned them out and alas... Hope this may help.
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Old 25th Apr 2021, 11:10 am   #47
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Default Re: Bush TV 12B To restore or not to restore advice please

The final pics. The first shows the effect of the poor EHT regulation when the brightness control is advanced beyond normal viewing levels at 5kv EHT.. J.
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Old 25th Apr 2021, 2:02 pm   #48
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Default Re: Bush TV 12B To restore or not to restore advice please

I restored a tv12 along time ago ( about 15 years ) and was never happy with the eht regulation.
It would be in focus on dark scenes and out of focus on bright ones.
I intend to have another go one day.

Robin
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Old 26th Apr 2021, 12:59 am   #49
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Default Re: Bush TV 12B To restore or not to restore advice please

Andy
I have tried what you said is makes no difference to the picture ballooning becoming distorted and off the screen. It has the same affect on the contrast and the brightness.
It did make things slightly better when I turned the gain down on the Aurora. I found that out a little while ago.
John
After taking note of what you said and going over things again and not finding anything wrong .
I double checked R9 it was about 15% high. I thought that not too bad but changed it anyway. I also changed C5. I assumed it was ok as it was in a metal can and tested ok but changed it anyway. Well that’s done the trick on that side of things .
So the problem left is the ballooning . That’s not to bad but it makes it impossible to set the contract correctly. Also focus is not ideal.
Thank you both Andy
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Old 26th Apr 2021, 8:42 am   #50
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Default Re: Bush TV 12B To restore or not to restore advice please

That looks better!
You cannot reliably test capacitors that are 70+ years old. They may be OK in a beaten up radio but it's no go with television timebase circuits. They have to be 100%. Treat all test equipment no matter how sophisticated with caution and assess the readings as a guide only.
I presume your focus control is the electromagnetic coil? Later versions have a permanent magnet similar to the TV22. If yours is the earlier coil, is the focus control approximately in the center? If one ended the current through the coil maybe too high/low but focus is very dependant on the EHT voltage.

My example comes into focus with the pot in the middle with 5kv EHT. These do drift rather badly for the first 15 mins as the coil warms up and it's resistance increases. Once warm, focus should hold. Instruction books often stated a warm up time of 15mins. You simply turned it on 15 mins before the programme started.

This is a very early flyback model working on the edge of technology at that time [1949] It all has to be 'right' to work efficiently.
Points to watch are the scan coupling capacitor C19 .01uf. You have probably replaced it but essential. 2. Ensure that the EHT bleeder resistors are 10M each. Very easy to get a colour wrong. The EHT smoothing capacitor is a .001uf. I presume you have replaced/rebuilt this? The EHT voltage is little effected by this cap and in the majority of chassis can be disconnected without ill effects but not in the TV12. The slight drop in EHT voltage will effect it's performance. Make sure you are using a new EY51.
The picture should not bloom at normal viewing levels of brightness and contrast.
Can you measure the EHT? John.
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Old 26th Apr 2021, 9:14 pm   #51
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Default Re: Bush TV 12B To restore or not to restore advice please

John .
I remember you saying before about capacitor testers lie.
I have replaced the bleed resistors with 4 - 47M i Tate it that works out the same
I don’t have a EHT voltage tester. Looks like I should. I have changed the visconol and the EY51
I will leave the set on for a reasonable amount of time later and check the focus I’m sure I changed C 19 but will double check .
Thanks Andy
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Old 26th Apr 2021, 9:21 pm   #52
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Default Re: Bush TV 12B To restore or not to restore advice please

Just incase you think I done anything stupid hers a pic
I didn’t take the cap off the CRT incase I pulled it off so soldered on existing wire.
Andy
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Old 27th Apr 2021, 9:42 am   #53
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Default Re: Bush TV 12B To restore or not to restore advice please

Better to be safe than sorry. It looks OK to me. maybe a length of sleeving over the joint. Little chance of corona as it is only 5kv.
Make sure the EHT winding is 'clean' It is very delicate and best blown clean with a moderate air line. Take great care not to damage the winding. The slightest bit of damping here does have an effect with the TV12. John.
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Old 28th Apr 2021, 2:05 am   #54
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Default Re: Bush TV 12B To restore or not to restore advice please

I have had the set on for some time. After changing a couple more resistors it’s looking pretty good. The hight pot wants a bit of switch cleaner . Focus is much better now. The picture is slightly low but the adjustment on the tube is at maximum tilt. I’m not worried it’s plenty good enough. I will try it again for a hour or so and hopefully it can be put back together.
John I have now slid some sleeving over that EHT wire Andy
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Old 28th Apr 2021, 9:07 am   #55
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Default Re: Bush TV 12B To restore or not to restore advice please

Just a point that has not been mentioned is the poor condition of the Paxolin panel carrying the EY51 and the bleed resistors.
It is quite possible that this panel has become 'leaky' and is draining precious EHT. It certainly looks black in places and may well be worth making up a new panel. You will not be able to measure the leak with a meter. John.
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Old 28th Apr 2021, 11:29 am   #56
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Default Re: Bush TV 12B To restore or not to restore advice please

Quote:
Originally Posted by Heatercathodeshort View Post
Just a point that has not been mentioned is the poor condition of the Paxolin panel carrying the EY51 and the bleed resistors.
It is quite possible that this panel has become 'leaky' and is draining precious EHT. It certainly looks black in places and may well be worth making up a new panel. You will not be able to measure the leak with a meter. John.
The panel was conductively leaky on my TV11, with no visible sign of blackening, cleaning with IPA made no difference either.
With the resistor network disconnected, a neon screwdriver lit if touched on all the upper terminals. (know it will light in the vicinity of EHT anyway), new one didnt.
Made a new one using modelmaking fibreglass board. EHT and focus improved straight away.
This is the type used https://www.ebay.co.uk/itm/174290253781
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Old 28th Apr 2021, 12:49 pm   #57
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Default Re: Bush TV 12B To restore or not to restore advice please

If you feel like having a go I have some NOS sheets of hi grade Paxolin panels. If you would like one foc just send me your details via PM. John.
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Old 28th Apr 2021, 2:20 pm   #58
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Default Re: Bush TV 12B To restore or not to restore advice please

Well . At the moment it’s working quite good . If you both think it’s worth doing I will do it. I do have some paxlin but not sure it’s up to the job .
Having said that the outer EHT windings don’t look to great . What’s the opinion on that. I don’t want to go over kill but I will take your advice . The TVhas been in ideal storage conditions for the last 45 years
Thank you both Andy
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Old 28th Apr 2021, 6:25 pm   #59
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Default Re: Bush TV 12B To restore or not to restore advice please

I think the wax on that winding means you have a good chance it is fine.
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Old 28th Apr 2021, 6:44 pm   #60
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Default Re: Bush TV 12B To restore or not to restore advice please

I would leave the transformer itself alone and just remake the Paxolin panel housing the EY51 and the bleed resistors. Leave the transformer windings as they are. The less disturbance the better. If you are happy with the results I would be tempted to fabricate a new panel if the focus begins to deteriorate. John.
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