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 Post subject: Re: Quick CRT test - if you don't have a CRT tester
PostPosted: Dec Tue 12, 2017 6:00 am 
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Location: Ann Arbor, MI
Radioroslyn wrote:
Would this work on a 2v Predicta crt??

Terry

You would need to put 2 volts on the heater but you would still need 6 volts on the grid.

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 Post subject: Re: Quick CRT test - if you don't have a CRT tester
PostPosted: Dec Tue 12, 2017 8:27 am 
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Joined: May Sun 07, 2017 11:35 am
Posts: 281
Living in Australia I'm not familiar with the Predicta except in pictures.

Are you saying there was a model with a CRT that had a 2 volt heater? How strange! 12 volts I could believe if it was part of a series heater string.
If so, you would need to substitute a 2 volt power supply for the heater circuit but leave the 6 volts DC in place for the G1 to cathode current test. Or maybe use a wirewound pot to set the heater voltage from the 6 volt supply.
Other than that, there's no reason it shouldn't work just as well.


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 Post subject: Re: Quick CRT test - if you don't have a CRT tester
PostPosted: Dec Wed 13, 2017 1:19 am 
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Joined: May Fri 01, 2015 2:33 am
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Location: Roslyn,Pa 19001
Tnx guys. That's what I was thinking but wanted to check.
The set is needy and wanted check the crt before I dig in to it.

Terry


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 Post subject: Re: Quick CRT test - if you don't have a CRT tester
PostPosted: Feb Mon 12, 2018 7:11 am 
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Joined: May Sun 07, 2017 11:35 am
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The original 21" Predicta CRT was a 21EAP4 and its heater is 2.34 volts @ 0.6A.

To use my tester and have the correct heater voltage, insert a 6.6 ohm resistor (two 3.3 ohm in series will do nicely) in series with Pin 1.


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 Post subject: Re: Quick CRT test - if you don't have a CRT tester
PostPosted: Feb Thu 22, 2018 10:33 pm 
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Joined: May Sun 07, 2017 11:35 am
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I've since tested about 20 CRTs with this method, 70 degree, 90 degree, 110 degree, sizes from 10" prewar Mazda to 23" late 60s.
I was able to run the tube up on a working chassis and observe its actual performance. The numbers you get from it seem to correlate quite well.

So, here are some actual results from my measurements:

4 volts = a very sharp, bright picture. (21" 90 degree Miniwatt rebuild from 1974)
2.5 volts = still a good picture, not quite as much headroom as a "4". This one got a lot better over an hour or so running (23" 110 degree original Thomas from 1967)
1.5 volts = OK in a room with subdued light. (This TV is now in daily use in a museum and seems to be improving with time. CRT is a 17" 90 degree US made RCA from 1957)
1.4 volts = Surprisingly bright for the reading but this is an 11" 110 degree unknown brand CRT from 1964. A bit of flare on highlights. I use this tube on the bench as a chassis test CRT.
1.2 volts = OK in a room with subdued light. (17" 70 degree mag focus Thomas rebuilt CRT from 1958)
0.5 volts = you can see a good picture with the room lights off. Whites start to invert when driven hard (Miniwatt AW53-88 same as 21CEP4 from 1959)

Since I first ran these tests, I did some more tests on the last one, which I would regard as effectively dead,
I ran the heater up to 9 volts for an hour and the reading came up to 2.2 volts. I left it off for another hour, re-tested a 6 volts and got just over 1 volt. The picture was much improved. Maybe a little more time and it would improve further.

I should explain, my "6 volt" power supply has a switch on it for 6, 7.5, 9 and 12 volts. Perfect to this application!


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 Post subject: Re: Quick CRT test - if you don't have a CRT tester
PostPosted: May Sun 27, 2018 4:25 am 
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Joined: May Sun 07, 2017 11:35 am
Posts: 281
It occurs to me that it might be useful for old TV enthusiasts to add test results vs observed picture quality to this post so that potential users can get a better idea of what they will get from a CRT and how it compares with the numbers obtained when they test using this method. So we can have a bigger sample.

I have noticed that the results can be a bit non-linear (a 1 gives a picture nearly as good as a 3.5) and that ANY reading pretty much guarantees you will get SOME picture from that tube. I have a TV here at the moment where the 1956 vintage Thomas CRT only gives a 0.5 but the picture is still quite watchable.


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