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Old 3rd Sep 2007, 6:39 pm   #1
dominicbeesley
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Default TV24 Still no picture - knackered tube?

I'm sorry if this is turning into an epic of hopelessness!

I've messed about with the ion trap until I'm fairly sure its not that - though now I can't remember which way round the arrow should point! (the gun points upward). The magnet is reasonably strong (before I was testing it by touching against the wrong bit!)

Going back to previous posts about possibly dodgy tube (blemish/scar) the cathode glows nicely would it burn out if the vacuum was shot?

I can't see any other signs of a shot vacuum.....any ideas please? this is driving me potty! Especially now I've got a signal from my Aurora, bought for this set, all the way through to the cathode showing up steady as a rock on my 'scope!

Please, any ideas from those more experienced, before I put this in the "I give up" pile!?

Thanks

Dom
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Old 3rd Sep 2007, 6:58 pm   #2
Steve_P
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Default Re: TV24 Still no picture - knackered tube?

Oh dear. Well, have you tried shorting the First Grid and Cathode together briefly to see if you get a full raster?

The problem could be there.

If that brings up anything, then check the connections between the chassis.

Cheers,

Steve P.
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Old 3rd Sep 2007, 7:13 pm   #3
dominicbeesley
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Default Re: TV24 Still no picture - knackered tube?

Thanks no, I hadn't tried that, have just now though - no joy. Again with io trap fiddling and tried the ion trap both ways round.

Cheers

Dom
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Old 3rd Sep 2007, 8:09 pm   #4
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Default Re: TV24 Still no picture - knackered tube?

Silly Question - is the tube heater lit?

(I'm clutching at straws now!)

Cheers,

Steve P
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Old 3rd Sep 2007, 8:29 pm   #5
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Default Re: TV24 Still no picture - knackered tube?

I had similar problem with Bush TV 62 a few months ago.Turned out to
be the ion trap magnet position though was initially confusing &
disheartning.

Does the EHT reduce a bit when you turn the brightness up even if you
can't see a picture? Mine did on the TV62.This is a good sign

You need to rotate the ion trap magnet quite slowly.Turn brightness up
quite high and SLOWLY rotate it plus you'll have to move it up and down the tube neck near where the "bend" is in the
electron gun..Make sure you can easily see the screen when moving
the magnet & a darkish room will help
the tube neck-Position is very very critical,if the ion trap magnet has an arrow on make sure it is pointing towards the front of the CRT.
My magnet wasn't in the correct position even though it seemed to have been fixed there for years & the correct place ended up being between the
electron gun bend and the base socket.

All very frustrating.You wouldn't have this problem with an LCD "TV" !!!

Good Luck
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Old 3rd Sep 2007, 8:33 pm   #6
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Default Re: TV24 Still no picture - knackered tube?

Quote:
Originally Posted by dominicbeesley View Post

Going back to previous posts about possibly dodgy tube (blemish/scar) the cathode glows nicely would it burn out if the vacuum was shot?

I can't see any other signs of a shot vacuum.....any ideas please?
Dom
If the vacuum had been lost, there would be a nice firework display in the neck of the CRT (assuming EHT is present). It is unlikely the cathode would be glowing.

Two things possible. Check the boost HT. Not sure what it is on this set but would think it's around 300 - 400v.

If this is OK...well it could be an o/c cathode within the CRT. You could try some careful tapping of the neck and see if you get flashes on the screen. Also put the set in a completely dark room and see if there is any form of faint picture. Sometimes even an O/C cathode will show a very faint smeary picture (assuming EHT and boost are OK).

Not much you can do except to look out for a replacement CRT in the last case.


Rich.
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Old 4th Sep 2007, 12:08 am   #7
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Default Re: TV24 Still no picture - knackered tube?

Hi all,

I'll answer all the questions in one post (20 questions style):

Does the filament light up: yes - bright enough to see what I'm doing whilst desperately trying to see something, anything on the screen.

Does EHT reduce with brightness: The EHT is a bit of a closed book as I''ve no idea how to measure, however the spark looks a bit less "fat" when brightness is up. Also when the EY51 anode was arcing to the can it only did it with the brightness turned down. So I guess the answer is probably - yes.

Ion trap magnet: I will persevere with this though I've tried both ways round. I turned the lights out and used a mirror last time!

No fireworks, or picture!

If by boost HT you mean second grid (please excuse my lack of knowledge) then this varies between about 250V up to 300V depending on settings of other controls (book value is 300V)

Thank you all again for your help, and please do consider my answers - I'm desperate to get this going, its driving me daft!

Cheers

Dom
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Old 4th Sep 2007, 7:53 am   #8
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Default Re: TV24 Still no picture - knackered tube?

O.K. Dom,
It is very unusual to get absolutely nothing in the way of a raster on the screen of a Mullard tube.
If the EY51 is lighting up you should have enough EHT to produce something, if only a glimmer on the CRT face.

First measure the heater volts across pins 1 and 12. It should read around 6volts AC of course but may be very SLIGHTLY under this.
If it only reads around 3-4volts the heater is partially short circuit and a temporary cure can be effected by tipping the set backwards whilst working and gently tap the tube neck three or four times to dislodge the short. It is not a permanent cure and the short will return.

If effective the tube heater will glow noticeably brighter.
Now brutal measures.
Short the control grid and cathode of the tube with a resistor around 2k and try again adjusting the ion trap. It needs to be about a 1/4 of an inch from the tube base. If you have got some eht this must produce a screen illumination. If not the tube may have a disconnected cathode or poor vacuum. Hope these notes are of help.
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Old 4th Sep 2007, 9:36 am   #9
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Default Re: TV24 Still no picture - knackered tube?

Morning Dom.

I see from your other thread that congratulations are in order. Excellent news! Now you'll have to post up some pictures and supply your girlfriend with earplugs for the 10kHz line whistle

Well done.
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Old 4th Sep 2007, 9:38 am   #10
dominicbeesley
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Talking Re: TV24 Still no picture - knackered tube?

Yes thanks to you all too, I would have thanked here too but my computer ate itself!

Now...I'll no doubt be back later this evening with a few more questions....

Cheers

Dom
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Old 7th Sep 2007, 12:36 am   #11
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Default Re: TV24 Still no picture - knackered tube?

As requested some pictures...

I'm still not sure this is as it should be, on the Test Card from the Aurora I can clearly make out the top second and middle grates, but the other two just look grey. Am I expecting too much?

If I slightly detune I can sort of make out another grate but only just. I've spent a good few hours trying and retrying the calibration process, now that I've fixed the deliberate mistake that stopped the sound rejection from working!

Any ideas anyone?

Anyways still amazing to see the thing working again!

Dom
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Old 7th Sep 2007, 8:28 am   #12
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Default Re: TV24 Still no picture - knackered tube?

Looks pretty good to me. Does that model have manual focus with lever or with a pot?
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Old 7th Sep 2007, 10:44 am   #13
dominicbeesley
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Default Re: TV24 Still no picture - knackered tube?

Its a manual focus. The collar is held on the chassis by two bolts.

I loosened the bolts, slid the collar about until the picture came on and get best results with the handle in mid position then tighten bolts and fix up with handle. Is that the right procedure?

Dom
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Old 7th Sep 2007, 7:26 pm   #14
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Default Re: TV24 Still no picture - knackered tube?

Hi Dom,
sounds like the right way to do it.

Cheers
Andy

Last edited by beery; 7th Sep 2007 at 7:26 pm. Reason: spelling
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