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Old 05-31-2018, 11:10 PM
EdKozk2 EdKozk2 is offline
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12UP4 condition

I finished recapping a Zenith 23G24 chassis this week. I used a 5AXP4 test crt to check everything out. Brightness, contrast,and volume controls all work correctly. The set came with a 12UP4A metal cone crt, Rauland. The 12UP4 tested zero for emissions, but the filament works fine. I decided to give the 12UP4 a try in the Zenith chassis. After adjusting the ion trap I was able to get a very dark picture. Only viewable in a dark room. The contrast control worked fine, but the brightness had no effect on the image. I decided to try a brightener. The brightener increased the brightness level IMO 100% from what it was at. The picture is bright enough to have a small lamp lit in the room. Even with the brightener I still have the issue with the brightness control.It has no effect,it is static. This chassis has the brightness control connected to the cathode, pin 11. The set has 9KV on the anode. Has anyone else ever had total loss of brightness control from an extremely weak crt? I was going to use the old 12UP4 with a brightener till it dies. The first picture is with a brightener being used. The second with the camera set for increased exposure duration. The actual image brightness is between the first and second photo.
Ed
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Last edited by EdKozk2; 11-20-2018 at 12:13 PM. Reason: spelling
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Old 06-01-2018, 01:44 AM
old_coot88 old_coot88 is offline
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Did the brightness control work OK with the test CRT?
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Old 06-01-2018, 10:54 AM
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Electronic M Electronic M is offline
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I wonder if that tube is failing cutoff?...

ED I've got a good spare 12" CRT that I don't need* it is yours for $100 if you want it.

*Bought it as a spare for the weak one in the Dumont I sold just Tim at the ETF...Tim did not want the good spare CRT.
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Old 06-01-2018, 11:11 AM
EdKozk2 EdKozk2 is offline
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Quote:
Originally Posted by old_coot88 View Post
Did the brightness control work OK with the test CRT?
Yes, the brightness control works OK with the test crt. I checked the DC voltage range ( 11 to 99 volts ) from the brightness pot to cathode pin 11. That was correct also. I even tried re-flowing solder to pin 11, just for good measure. The schematic is in Sams' 91A.
Ed
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Old 06-01-2018, 11:28 AM
EdKozk2 EdKozk2 is offline
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Quote:
Originally Posted by Electronic M View Post
I wonder if that tube is failing cutoff?...

ED I've got a good spare 12" CRT that I don't need* it is yours for $100 if you want it.

*Bought it as a spare for the weak one in the Dumont I sold just Tim at the ETF...Tim did not want the good spare CRT.
Hi Tom,
Thanks for the crt offer. I do have some other usable 12LP4's . I was just wanted to try and not have to modify the original crt installation if possible.
When I do the cutoff check, it the needle goes from good to bad instantly.
Ed
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Old 06-01-2018, 02:06 PM
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jr_tech jr_tech is offline
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I'm guessing that the internal weld to the cathode may be bad....if you gently tap the neck of the tube, you might get some bright flashes. Some tube checkers might be able to re-weld the lead. Does the video connect to the grid?

jr
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Old 06-01-2018, 09:17 PM
EdKozk2 EdKozk2 is offline
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[QUOTE=jr_tech;3200240]I'm guessing that the internal weld to the cathode may be bad....if you gently tap the neck of the tube, you might get some bright flashes. Some tube checkers might be able to re-weld the lead. Does the video connect to the grid?

jr[/QUOTE
Yes,the video does connect to the grid. I think you may be correct about an internal connection problem. I already tried the tap on the neck idea, no luck. I'm wondering if there is any current being drawn through the cathode. I'll have to check that.
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Old 06-01-2018, 09:34 PM
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irext irext is offline
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Have you got access to a tube rejuvenator? Might be worth giving it a blast with one. You've got nothing to lose.
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Old 06-02-2018, 12:28 AM
EdKozk2 EdKozk2 is offline
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Originally Posted by irext View Post
Have you got access to a tube rejuvenator? Might be worth giving it a blast with one. You've got nothing to lose.
I already tried the rejuvenator when I first tested the crt. I was able to replicate the same lack of brightness control using the test crt. I removed the cathode lead from the crt socket. I turned on the set and it produced the same kind of dim image as the defective 12LUP4.
Ed
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Old 06-03-2018, 09:42 PM
EdKozk2 EdKozk2 is offline
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I was able to get the brightness control to vary the crt's brightness. I disconnected the existing crt filament leads from pins 1 and 12. I then taped them off. Then connected a 6.3 volt ac @ 600ma transformer to the filament pins. The existing brightness wire on crt pin 11 is unsoldered, then connected to pin 1 or 12. This was an easy way to enable brightness variation of a most likely broken internal cathode connection. Thanks again jr_tech. The filament transformer output leads can not be grounded, it should float. I tried this with a 6 volt battery, a 6 volt dc wall wart and even a 7.2 volt dc wall wart. All of the filament sources worked. The higher voltage produced a brighter picture of course.
Ed

Last edited by EdKozk2; 11-20-2018 at 12:28 PM.
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Old 06-21-2018, 09:53 PM
cluelessgame cluelessgame is offline
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Quote:
Originally Posted by EdKozk2 View Post
I was able to get the brightness control to vary the crt's brightness. I disconnected the existing crt filament leads from pins 1 and 12. I then taped them off. Then connected a 6.3 volt ac @ 600ma transformer to the filament pins. The existing brightness wire on crt pin 11 is unsoldered, then connected to pin 1 or 12. This was an easy to be able to vary the brightness on a most likely broken internal cathode connection. Thanks again jr_tech. The filament transformer output leads can not be grounded, it should float. I tried this with a 6 volt battery, a 6 volt dc wall wart and even a 7.2 volt dc wall wart. All of the filament sources worked. The higher voltage produced a brighter picture of course.
Ed
does this mean you're turning the filament into the cathode itself?
I may have to try this with the 21ZP4 in my Admiral. I think it has the same issue
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Old 06-22-2018, 02:32 AM
EdKozk2 EdKozk2 is offline
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Originally Posted by cluelessgame View Post
does this mean you're turning the filament into the cathode itself?
I may have to try this with the 21ZP4 in my Admiral. I think it has the same issue
I would have to say yes to your question. The method I used is basically the way a battery operated tube radio from the old days was wired. Only this method must be isolated or you will burn the crt's filament. Also note that if you have your video and brightness being input on your cathode this may not work. The Zenith set I have has the video on grid #1 of the crt.
Ed
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