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 Post subject: Gassy 14 inch picture tube
PostPosted: Nov Mon 05, 2018 3:23 pm 
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Joined: Sep Tue 08, 2009 5:37 pm
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Location: Southington, ct
I just had a 14 inch 1956 admiral portable electrically restored and the CRT is " a bit gassy" according to the person that restored it. I haven't picked up the set as of yet but was curious what that actually means and if it means the CRT is bad. The picture he sent shows it slightly "fuzzy", but he said as it warms up it would improve. Any information for me on this?


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 Post subject: Re: Gassy 14 inch picture tube
PostPosted: Nov Mon 05, 2018 4:15 pm 
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Location: S. Dartmouth MA 02748-1225 USA
I have never seen a gassy CRT improve when the set "warms-up" Gas is gas, it will not go away.

Gas, generally effects the image by de-focusing the electron beam. The result is not only a blurry image but odd high-lights, gas will also cause a certain amount of blooming when changing the brightness.

However, poor focus voltage could also be the issue. Both the gas in the CRT can be determined with a CRT tester and focus voltage with a meter. If the CRT does improve with "warm-up" then it is more likely to be a focus voltage problem.

IMHO I would would ask for more clear evidence the CRT is at fault. Put the chassis on a jig and see if it is a chassis issue...

YMMV

Chas

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 Post subject: Re: Gassy 14 inch picture tube
PostPosted: Nov Mon 05, 2018 4:37 pm 
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Location: Lafayette, CO
I worked on a GE portable from the mid-fifties and had much the same result. Either those picture tubes weren't great or just didn't last. It tested okay....not new, not bad. Craig


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 Post subject: Re: Gassy 14 inch picture tube
PostPosted: Nov Mon 05, 2018 6:23 pm 
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Joined: Apr Sat 06, 2013 2:18 pm
Posts: 506
Location: New Hampshire
We used the term for a tube that showed a streaky, negative & poor pic.
Its different looking than just a dark raster, they can be still bright.
Any old jug will improve with warm up. To make it better looking keep
the bright & contrast down & light off the screen. As dark a room as possible so
you dont have to drive it hard.
The CRT is on the way out but time is unpredictable. If you like the set start looking
for a replacement. Regular use seems to stretch life out.

73 Zeno 8)
LFOD !


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 Post subject: Re: Gassy 14 inch picture tube
PostPosted: Nov Mon 05, 2018 6:36 pm 
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Location: Ann Arbor, MI
zeno wrote:
We used the term for a tube that showed a streaky, negative & poor pic.
Its different looking than just a dark raster, they can be still bright.
Any old jug will improve with warm up. To make it better looking keep
the bright & contrast down & light off the screen. As dark a room as possible so
you dont have to drive it hard.
The CRT is on the way out but time is unpredictable. If you like the set start looking
for a replacement. Regular use seems to stretch life out.

73 Zeno 8)
LFOD !

What you describe is usually not a gassy tube. It is a problem with the cathode's inability to supply enough current. It can supply current up to a point with no problem and then above that point can supply no more. And if is has been supplying current near it's maximum in one part of the picture, the maximum current may drop to a lower value in other parts of the picture. You can usually get a much better picture by turning down the brightness and contrast as mentioned above. You would then have a dim but otherwise reasonable picture.

A gassy CRT will not improve with warm up.

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 Post subject: Re: Gassy 14 inch picture tube
PostPosted: Nov Mon 05, 2018 11:20 pm 
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Location: Belrose, NSW, Australia
If it's gassy, the getter flash will be murky grey and fading at the edges, not silvery.

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 Post subject: Re: Gassy 14 inch picture tube
PostPosted: Nov Tue 06, 2018 2:38 am 
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Joined: Apr Thu 14, 2016 8:25 pm
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Location: pensacola fl
I would also check the high voltage at the tube ultor and see if it is changing too much with scene changes and or control settings. If it is varying too much then it could be a bad high voltage rectifier tube or a resistor feeding the high voltage to the crt. this resistor is usually under the rectifier socket. Also if there is a resistor feeding the rectifier filament it could be off value causing the rectifier to be running too cool. If your set uses a 1B3 or similar then take the tube out and hook up a 1.5V battery c cell to pins 2 and 7 and look down into the plate. You will see it lit up. Now put the tube back in the tv and look at the tube and see if it is the same brightness. If it is a lot dimmer then something is causing the heater not to get enough voltage like a resistor gone up in value in the socket. Those resistors are low value only a few ohms or so.


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 Post subject: Re: Gassy 14 inch picture tube
PostPosted: Nov Sat 10, 2018 10:15 am 
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Location: Lincoln City, OR 97367
Greetings to the Forum:

It is hard to know what "fuzzy" means. When you see the actual picture, you will have a better idea.

Low emission from the CRT gun usually results in luminance clipping... the best way I can describe it is that the picture looks "silvery". Inadequate high voltage will usually cause "blooming" (change of picture size - larger) and loss of focus with increasing brightness.

Hopefully, you will find a problem not CRT related.

Regards,

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 Post subject: Re: Gassy 14 inch picture tube
PostPosted: Nov Sat 10, 2018 5:54 pm 
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Location: WI 54812
I've been noticing that in general, a low emissions testing CRT will have dirty or oxidized looking elements compared to one that tests strong. Maybe not a very scientific way to judge a tube, but lately I've been avoiding ones with very dark elements. The 10FP4 shown below actually has a very bright picture, but there's very little contrast control, and exhibits that silvery effect mentioned.


Attachments:
CRT Cathodes 004.jpg
CRT Cathodes 004.jpg [ 54.26 KiB | Viewed 1185 times ]
CRT Cathodes 003.jpg
CRT Cathodes 003.jpg [ 55.17 KiB | Viewed 1185 times ]
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 Post subject: Re: Gassy 14 inch picture tube
PostPosted: Nov Sat 10, 2018 6:20 pm 
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Location: Ann Arbor, MI
The first order of business is to see what the picture looks like. You may find that the picture looks good if you dim the lights in the room and run the contrast down a little. The brightness should be adjusted so that parts of the picture that should be black are in fact black. The contrast is adjusted so that peak whites are the brightness you want.

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 Post subject: Re: Gassy 14 inch picture tube
PostPosted: Nov Thu 15, 2018 9:07 am 
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I think a small amount of gas can cause astigmatism such that you can adjust focus in one dimension or the other but not both simultaneously. With extended use I think you may get a small improvement.

Peter

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