Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives :: Books
Support This Site: Contributors :: Advertise


It is currently Dec Mon 11, 2017 5:09 am


All times are UTC [ DST ]





Post New Topic Post Reply  [ 19 posts ] 
Author Message
 Post subject: Quick CRT test - if you don't have a CRT tester
PostPosted: Nov Tue 28, 2017 3:03 am 
New Member

Joined: May Sun 07, 2017 11:35 am
Posts: 21
I posted a link to this in another Topic but then after I found the jpeg I thought it deserves its own Topic.

Let's say you are wondering if you should start a restoration of an old TV... The tube might be bad and impossible to obtain. If so, recapping that chassis could be a big waste of time.

You can perform emission testing of a CRT using just a 6 volt battery or DC power supply, a 1k resistor and a voltmeter.
The picture shows how:

This works for almost all B&W CRTs, from 70 degree to 110 degree. Use clip leads to connect to the CRT pins.
If the CRT is in a series heater string, it may need a different heater voltage, maybe 12 volts. I haven't tried this though, I don't have any such CRTs.

What to expect:

A 100% good CRT will give you about 4 volts and the reading will come up quickly, about 10 or 15 seconds after you connect the voltage. The speed with which it comes up, once it starts to rise, will give you a good idea of the health of the CRT.
A well-worn but still watchable CRT might only give you 1 volt or so - equating to about 25% emission. I have a TV with a 60 year old Thomas CRT that reads 1 volt on this test but still displays a good picture under subdued room lighting.
Any reading at all generally equates to being able to see some light on the screen!

If you get 6 volts immediately, you have either left out the 1k resistor or your CRT has a G1 to cathode short.

If you get a very low reading and the CRT has been sitting around unused for 50+ years, try increasing the voltage to 9 volts (max) and leaving it for a few hours. The tube might have shipped a little gas and the extra heat will often clean off the cathode surface. After this, your tube might well give you a respectable reading at normal heater voltage.


Attachments:
CRT emission tester.jpg
CRT emission tester.jpg [ 13.04 KiB | Viewed 669 times ]
Top
 Profile  
 
 Post subject: Re: Quick CRT test - if you don't have a CRT tester
PostPosted: Nov Tue 28, 2017 3:48 am 
Member

Joined: Sep Tue 30, 2014 6:08 am
Posts: 2066
Location: The Old Dominion VA 23518
Full write-up, and pins identified:

http://www.americanradiohistory.com/hd2 ... e-0073.pdf

_________________
Brian
"Capacitor Cosmetologist since 1979"
USN Retired 1984-2006 (Avionics/Cal)


Top
 Profile  
 
 Post subject: Re: Quick CRT test - if you don't have a CRT tester
PostPosted: Nov Tue 28, 2017 8:30 am 
New Member

Joined: May Sun 07, 2017 11:35 am
Posts: 21
Thanks Brian, that's very interesting. I hadn't seen it but it sure looks similar. Not many new ideas left!

Actually I came up with this idea for a friend who needed to decide which TVs he'd keep and which he'd scrap, without a functioning chassis.
That's the difference, my version doesn't need the chassis to be powered up, or even present.


Top
 Profile  
 
 Post subject: Re: Quick CRT test - if you don't have a CRT tester
PostPosted: Nov Wed 29, 2017 2:05 pm 
Member

Joined: Jul Wed 22, 2009 3:07 pm
Posts: 656
Hi irob,

Am I correct with the interpretation of the pins?

Peter


Attachments:
CRT emission tester_.jpg
CRT emission tester_.jpg [ 18.85 KiB | Viewed 277 times ]


Last edited by orbanp on Dec Mon 04, 2017 3:35 pm, edited 1 time in total.
Top
 Profile  
 
 Post subject: Re: Quick CRT test - if you don't have a CRT tester
PostPosted: Nov Wed 29, 2017 6:32 pm 
Member
User avatar

Joined: Mar Sun 01, 2009 10:27 pm
Posts: 4232
Location: Ann Arbor, MI
orbanp wrote:
Hi irob,

Am I correct with the interpretation of the pins?

Peter

Pin 2 is G1.

_________________
Tom


Top
 Profile  
 
 Post subject: Re: Quick CRT test - if you don't have a CRT tester
PostPosted: Nov Wed 29, 2017 6:43 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 16805
Location: Albuquerque, NM 87123
If you are going to carry that piece of equipment, why not carry a CRT tester?


Top
 Profile  
 
 Post subject: Re: Quick CRT test - if you don't have a CRT tester
PostPosted: Nov Wed 29, 2017 8:05 pm 
Member
User avatar

Joined: Mar Sun 01, 2009 10:27 pm
Posts: 4232
Location: Ann Arbor, MI
Johnnysan wrote:
If you are going to carry that piece of equipment, why not carry a CRT tester?


You would only use this circuit if you do not have a CRT tester but do have all of the parts laying around. This test is essentially the same as what cheap emission tube testers use on ordinary tubes.

Since I do have an emission tube tester but no CRT tester, I made an adapter cable to convert from a CRT socket to an octal tube socket. I set the switches on the tester to connect up the heater, cathode and grid so that the tester saw the CRT the same way it saw other tubes. I connected up a good strong CRT and set to load switch to show a good tube. I can now do an emission test on other CRTs and get an indication of how strong they are.

_________________
Tom


Top
 Profile  
 
 Post subject: Re: Quick CRT test - if you don't have a CRT tester
PostPosted: Nov Wed 29, 2017 9:18 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 3763
Location: WI 54812
I suppose you could go portable using a 2 cell Li-ion pack.
Good for those that like scrounging around TV junkyards. :)


Top
 Profile  
 
 Post subject: Re: Quick CRT test - if you don't have a CRT tester
PostPosted: Nov Thu 30, 2017 5:27 am 
New Member

Joined: May Sun 07, 2017 11:35 am
Posts: 21
Yes, + to G1, not G2.

That was part of the idea, to be able to go portable and check a pile of old TVs.
And not everyone can get hold of a proper CRT tester these days, they are mostly all in landfill.. :(

When I was a callow youth, servicing TVs in the 60s, I made myself a CRT tester - rejuvenator.
Tickling the cathode with a couple of hundred volts of half-wave rectified AC used to be very effective most of the time.


Top
 Profile  
 
 Post subject: Re: Quick CRT test - if you don't have a CRT tester
PostPosted: Dec Fri 01, 2017 6:15 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 17953
Location: Warner Robins, GA
Very neat idea and quite useful.

Could make it up with several individual rechargable cells to handle multiple CRT voltages with a selector switch to select the voltages.

A battery pack such as a laptop battery that has a charging connector built in would be perfect as all one has to then do is open the pack and add wires to each cell going to the switch. The pack could then be charged with the regular charger used for the battery.


Top
 Profile  
 
 Post subject: Re: Quick CRT test - if you don't have a CRT tester
PostPosted: Dec Sat 02, 2017 5:02 am 
New Member

Joined: May Sun 07, 2017 11:35 am
Posts: 21
For the length of time it has to run, 4 AA alkaline cells is all you need for 6.3v heaters.
D cells would last longer tho...
Saves dragging a power cable around when you are climbing a stack of TVs, looking for a good CRT


Top
 Profile  
 
 Post subject: Re: Quick CRT test - if you don't have a CRT tester
PostPosted: Dec Mon 04, 2017 3:34 pm 
Member

Joined: Jul Wed 22, 2009 3:07 pm
Posts: 656
irob2345 wrote:
Yes, + to G1, not G2.
...

Interesting!
You suggested to use that "Tube Analysis" paper to identify the pins.
The first paragraph of that paper clearly mentions to use "g2" and to disconnect "g1".

PS:
Updated the drawing to "g1".

Peter


Attachments:
CRT tester text.jpg
CRT tester text.jpg [ 38.01 KiB | Viewed 278 times ]
Top
 Profile  
 
 Post subject: Re: Quick CRT test - if you don't have a CRT tester
PostPosted: Dec Mon 04, 2017 6:27 pm 
Member
User avatar

Joined: Mar Sun 01, 2009 10:27 pm
Posts: 4232
Location: Ann Arbor, MI
orbanp wrote:
You suggested to use that "Tube Analysis" paper to identify the pins.
The first paragraph of that paper clearly mentions to use "g2" and to disconnect "g1".

In that paper you are applying the full normal voltage to g2. In your quick circuit you are using only 6 volts and you need to use a grid that is closer to the cathode to get the current to flow. If you look at the circuit for most emission tube testers, you will see that they use the cathode and g1. That is why you do not need to connect anything to the top cap of a tube if the top cap is the plate but you do need to connect it if the top cap is the grid. They also use a lower voltage than you would normally put on the plate of a tube.

_________________
Tom


Top
 Profile  
 
 Post subject: Re: Quick CRT test - if you don't have a CRT tester
PostPosted: Dec Mon 04, 2017 10:26 pm 
Member

Joined: Jul Wed 22, 2009 3:07 pm
Posts: 656
Hi Tom,

Thanks for the explanation, it makes sense!

Regards, Peter


Top
 Profile  
 
 Post subject: Re: Quick CRT test - if you don't have a CRT tester
PostPosted: Dec Tue 05, 2017 1:11 am 
New Member

Joined: May Sun 07, 2017 11:35 am
Posts: 21
Yes, that is correct.
To get a meaningful current reading from only 6 volts you need a high perveance diode, which is pretty much what a CRT cathode and G1 is (or should be if it's in good condition).
It tests the ability of the cathode to boil off electrons, which is mostly what you want to know.
If it can't do this, any other test is a waste of time.....

In Oz we had very few non aluminised CRTs (TV started in 1956 and all local CRT makers made exclusively aluminised screens from day 1). Tube faults other than low emission were rare, mostly O/C cathodes which this test picks up. A screen burn that's bad enough to be an issue is normally visible on a dark CRT anyway.


Top
 Profile  
 
 Post subject: Re: Quick CRT test - if you don't have a CRT tester
PostPosted: Dec Thu 07, 2017 11:33 pm 
Member

Joined: Oct Thu 23, 2014 5:15 pm
Posts: 45
I tried this with a 7JP4 which has a different pin out, 1&14 heaters, 2 cathode, and 3 G1. Voltage came up very quickly to 6.4 volts. I used a 6 volt lantern battery. I get a raster on this tube but it is faint and you can hardly see it in a well lit room. Set also has low high voltage I suspect. It takes about 2 minutes for the tube to light up. Do I have a short, or is this test no good for the 7JP4?

Tried it on a 10BP4 and voltage came up slowly, about 15 seconds and I got 1.4 volts. This set has a pretty good picture. Wondering how much an improvement I might see with a better tube?

Thanks for this info!


Top
 Profile  
 
 Post subject: Re: Quick CRT test - if you don't have a CRT tester
PostPosted: Dec Sat 09, 2017 6:22 am 
New Member

Joined: May Sun 07, 2017 11:35 am
Posts: 21
Sounds like you have a short or some mis-wiring in the 7JP4, 6.4 volts is just too good to be true!

I just checked a 17BP4 (70 degree, mag focus, ion trap) and it reads 1.3 volts after 30 seconds. This tube shows a reasonable picture, a bit of white crushing but a good, watchable picture in subdued lighting. It was made by Thomas in Australia in 1957 and is a replacement tube in this TV, looks like its original tube failed under warranty.

The best tube I've tested gives me 4 volts in 15 seconds. It was a replacement (rebuild) 21CBP4 made in 1974, so it probably did very little work before its TV was relegated to a back room in 1976 by color. . This tube shows a VERY bright, sharp picture, much better than the 17BP4 with plenty of headroom on the peak whites..

Other respondents have commented that you need to test your tester with a number of tubes in working TVs to know what to expect from it. I'd concur with that.


Top
 Profile  
 
 Post subject: Re: Quick CRT test - if you don't have a CRT tester
PostPosted: Dec Sat 09, 2017 4:21 pm 
Member

Joined: Oct Thu 23, 2014 5:15 pm
Posts: 45
Thanks irob2345!

I must have made a mistake. Today I repeated the test and was certain I had everything hooked up right. I used a 6 volt lantern battery that measured 6.4 V. This tube only read 250 millivolts and it took several minutes to get there! I guess it's a dud! I do get a raster but it takes a few minutes to see it in a dimly lit room, but am unable to get a picture.

I have read about rejuvenating a picture tube. Could it possibly work on this tube?


Top
 Profile  
 
 Post subject: Re: Quick CRT test - if you don't have a CRT tester
PostPosted: Dec Sun 10, 2017 4:06 am 
Member

Joined: Apr Thu 14, 2016 8:25 pm
Posts: 282
Location: pensacola fl
A company called REM made a great crt tester and rejuvenator the circuit was simple and it had several levels of recovery.


Top
 Profile  
 
Post New Topic Post Reply  [ 19 posts ]  Moderators: 7jp4-guy, Mr. Detrola

All times are UTC [ DST ]


Who is online

Users browsing this forum: No registered users and 9 guests



Search for:
Jump to:  


















Privacy Policy :: Powered by phpBB