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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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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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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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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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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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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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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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 Post subject: Re: Quick CRT test - if you don't have a CRT tester
PostPosted: Jun Mon 25, 2018 2:17 am 
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I just picked up a 49 GE model 830 today. Is this test accurate for the 12KP4? If I understand the specs it is less than 70 degrees deflection.Thanks in advance for the help!

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 Post subject: Re: Quick CRT test - if you don't have a CRT tester
PostPosted: Jun Mon 25, 2018 3:23 am 
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I just looked this tube up and the connections are the same. The deflection angle is unimportant for this test.

Let us know what you get!


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 Post subject: Re: Quick CRT test - if you don't have a CRT tester
PostPosted: Jun Mon 25, 2018 4:43 am 
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Well it doesn't look good. The most I got on my 12KP4 after 15 minutes was .5 volts DC. This set has most likely sat for a long time. I can rig up a bit more volts to the heater but have no other equipment to try to rejuvenate it with.

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 Post subject: Re: Quick CRT test - if you don't have a CRT tester
PostPosted: Jun Tue 26, 2018 2:56 am 
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Well 0.5v is enough to get a picture.

This tube has been in hibernation for a long time but it shows promise. I'd try a few hours of 9 volts on the heater.

Also have a look at the getter flash under the yoke, close to the cone. Is it shiny (good) or patchy and dull (not so good)?
As mentioned though, I have a TV running daily in a museum with a 50's tube that has a patchy getter and, although it's slow to warm up, it shows a good picture once it does.
I know the provenance of this TV and it was run 16 hours a day for 20+ years.


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 Post subject: Re: Quick CRT test - if you don't have a CRT tester
PostPosted: Jul Mon 02, 2018 12:44 am 
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I'll have to admit that I wasn't very interested in this idea since I have a very serviceable CRT tester.
Also I was doubtful such high currents could come from a bias of only 6 volts. However I did have cause to try it last night. I used a CRT that I had tested on my B&K 465 as good. I did get a high current using 6V between the cathode and G1. In the B&K (like most other testers) bias is applied between cathode and G2 and is usually several hundred volts.
However I happened to know due to research I did to restore the B&K tester that the full scale reading on the meter is with a 1.5 ma current and the Good/ Bad line corresponds to about 300 ua. The tester also has a switch (higher sensitivity) for some CRT which are intended to operate with much lower current.
I think that your test technique should be used with caution. Pulling 4 ma or so from the cathode is far more current than what it is in normal operation. It may exhaust (use up) the cathode at a high rate.
Common sense would indicate that the more complex CRT tester circuits would not be used if a circuit as simple as the one you suggest would suffice.

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 Post subject: Re: Quick CRT test - if you don't have a CRT tester
PostPosted: Jul Mon 02, 2018 2:46 am 
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Quote:
Common sense would indicate that the more complex CRT tester circuits would not be used if a circuit as simple as the one you suggest would suffice.


Engineering history is littered with examples of the opposite being true! I only did this in the first place because I didn't have a CRT tester or (at the time) a suitable working chassis to test a CRT and I needed to determine if it was worth starting a restoration. It was a quick experiment that worked. I passed it on to a friend who had a similar need, it worked for him, so I thought "why not publish it?" It ONLY tests emission, not shorts etc which need more complexity. But then, what is the thing you need to know most about a CRT?

I'll readily concede that measuring the perveance of the G1 - K diode in a gun (which is basically what this simple circuit does) will give different results with different gun structures. Perveance is a measure of a cathode's ability to create a space charge and hence its emission performance. However I can't see the tiny currents involved (limited by the low voltage and the 1k resistor) damaging the cathode in any way.

But hey, if you have a working CRT tester, you don't need this!!


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 Post subject: Re: Quick CRT test - if you don't have a CRT tester
PostPosted: Jul Mon 02, 2018 7:18 pm 
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I was mainly pointing out the test can result in much higher cathode current than what the cathode was probably designed for. 4 ma might not be a lot compared to typical receiving tube current but it may be risky for a CRT. It is a relative thing. The caution I advised was that the test not be done for an extended period of time. I actually was going to mention the test to someone that probably didn't have a tester, however after I tried it it occurred to me that the current level could be a problem.

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 Post subject: Re: Quick CRT test - if you don't have a CRT tester
PostPosted: Jul Tue 03, 2018 4:10 am 
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OK, let's assume worst case maximum power transfer occurs at 3 volts across the G1-K path. That's a dissipation for that part of the gun structure of 3mW.
Not a problem for a cathode that is being heated by typically 1.8 or 3.6 watts of energy. It will add 0.1% to the power dissipation, and then only for a short time.

I can't see a problem here. Certainly, running this test has had no detrimental effect on any of the 20+ CRTs I've tried to far.


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 Post subject: Re: Quick CRT test - if you don't have a CRT tester
PostPosted: Jul Wed 04, 2018 7:52 pm 
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It wasn't additional heating I was referring to. I am thinking of current density on the emission material. The effect that I would expect is decreased life, which would be hard to determine.

I do have a suggestion. From your tests it seems a current of over 0.5 ma. (0.5V) would just qualify as acceptable to try to use the CRT in a TV. If you insert a 5K potentiometer into the line to the cathode. The test is started with the pot at maximum resistance. After a period of time so that the heater is up to temperature, the pot resistance is decreased until the current rises to 1.5 ma (1.5V) or zero pot resistance whichever comes first.
If the current is between 0.5 and 1.5 ma (0.5 to 1.5V) then the tube is worth a try.

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 Post subject: Re: Quick CRT test - if you don't have a CRT tester
PostPosted: Sep Sun 02, 2018 9:20 pm 
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If anyone has the time and is good at explaining things orally, this test should be made into a youtube video. just a thought.


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 Post subject: Re: Quick CRT test - if you don't have a CRT tester
PostPosted: Sep Tue 04, 2018 1:59 pm 
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Time is a problem for me....
Basically what I have discovered is, if you get any reading at all from the test, the tube will show a picture. Whether you need a darkened room to see it or not, is a matter of what numbers you get.


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 Post subject: Re: Quick CRT test - if you don't have a CRT tester
PostPosted: Sep Wed 12, 2018 12:37 am 
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Hello Guys,
what nifty little unit anyway it would be a easy device to make


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