UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Television and Video

Notices

Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc.

Closed Thread
 
Thread Tools
Old 11th Feb 2017, 11:30 pm   #1
1100 man
Octode
 
1100 man's Avatar
 
Join Date: Apr 2016
Location: Ventnor, Isle of Wight, & Great Dunmow, Essex, UK.
Posts: 1,377
Default Testing emission of CRT's- questions

Evening everyone,
I was lucky enough to purchase a B&K 467 CRT tester/ restorer last year and have been having a play with it. Many years ago in my youth, I built a basic tester/ restorer which served me well for many years but it is great to have such a sophisticated unit as the B&K.
Before proceeding to the emission test, one has to set up the cut-off. This entails setting the grid volts to -50V and then increasing the G2 volts until the meter just shows some current flow. So my first basic question is what is the purpose of this?
I tested a Mazda CRM1703 of unknown condition. It proved to have almost no emission so I proceeded to rejuvinate it and it seemed to respond pretty well.
However, if I go through the 'set cut -off' procedure with the grid set to -50V, the tube can't be brought out of cut-off even with max setting of G2 (about 500V). I have to reduce the grid volts to -30V to get the meter to move and that's still with max volts on G2.
The emission at these settings reads well into the 'good' and pushing the life test doesn't change the readings much so would imply a fairly good tube.
So my second question is what does this actually mean? Is it likely to give a good picture when used in a set?
Am on a bit of a learning curve with this so would be grateful for any light that can be shed my way.
Many thanks
Nick
1100 man is offline  
Old 12th Feb 2017, 2:12 am   #2
Argus25
No Longer a Member
 
Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,679
Default Re: Testing emission of CRT's- questions

Setting up the cut-off:

It is very wise when testing any CRT initially, that the tube at least is biased such that the grid is significantly negative with respect to the cathode at least -50V. Depending on the applied voltages and other possible accidents of incorrect settings/mis-wiring if the grid to cathode voltage is too low, approaching zero, there could be a very high emission current. So its best to start from cut-off or beyond and reduce the negative bias to attain a beam current after that.

One thing to watch out for is, ideally; never take the grid voltage of a CRT higher (more positive) than the cathode.

One way to prevent this for test purposes, is to connect a 1N4007 diode across the grid to cathode, with the diode cathode (line on the diode package) connected to the CRT's cathode. That way the grid can never be taken more than about 0.7V positive with respect to the cathode. However this may inhibit some of the rejuvination functions of the tester, which are thought to be questionable by some, but they can sometimes result in improved electron emission, for some guns, in the short term.

Once the cathode coating is sufficiently aged and damaged the electron emission will fall to a low value despite any rejuvination attempts. In this case, often the TV user will advance the brightness control so that the CRT's grid does go more positive than the cathode and this induces beam de-focus. So a CRT which only produces a dim and de-focused image at full or above full brightness is a sure sign of low gun emission (assuming all voltages correct). Also when the CRT grid draws current in this mode, it clips the video signal peaks and the image loses contrast and gets a silvery washed out look to it, like many shades of grey are missing.

To conclude that a CRT is "worn out" though the EHT heater and electrode voltages and video drive voltages need to be checked.

With regards to the actual picture/image on the CRT face, not only is the electron gun emission important, but other factors affect the final result. With time & use the phosphor degrades and for any amount of electron bombardment, the light output is lower from the phosphor itself. Often as everyone knows, for electromagnetically deflected CRT's there is ion contamination in the screen center region too. This problem was solved with aluminized phozphors, but you will still see it all the time on non aluminized CRT's. Also gas in the CRT can de-focus the beam and make sharp focus impossible even if the gun & phosphor are perfect, so the CRT can go "soft".

So in a nutshell, a CRT tester looking at gun emission can only give a rough guide as to whether a CRT is likely to be able to have a chance of working well in an actual TV. Obviously if there is very low gun emission it won't work. But after a reasonable gun emission is established, the only real way to check the CRT properly is in a TV set looking at the scanning raster & image.

So the answer to your question if your tube is likely to give a good picture in a set, you will have to try it. The tube tester can't give you enough data.

However, the above statement is not necessarily the case, testing CRT's for emission, if they have another application like an oscilloscope tube.

To give an example of why this is the case; a CRT gun with some average emission, could for example, produce a bright single line trace, as on an oscilloscope and appear to be normal in every way. But a television scanning raster is much more demanding in that the beam energy available is spread over the entire tube face. In TV sets, like the HMV904 where the EHT (final anode voltage) is relatively low, it takes a CRT with exceptional gun emission, high vacuum and a good phosphor to attain a satisfactory image. I have a number of CRT's which have what look like satisfactory gun emission on a CRT tester, and they would work for just fine for a single line oscilloscope display, but won't cut it and are hopeless for a TV image.

So the bottom line for a TV CRT; They are better assessed for performance in the actual TV set for image reproduction, light output, focus etc and that includes the before and after results, if rejuvination is attempted on them, by a CRT tester/Rejuvinator. Just because it may look like more emission in the tester, after rejuvunation, doesn't necessarily correspond to an improved screen image, due to the other factors.
Argus25 is offline  
Old 12th Feb 2017, 8:54 am   #3
TonyDuell
Dekatron
 
Join Date: Jun 2015
Location: Biggin Hill, London, UK.
Posts: 5,190
Default Re: Testing emission of CRT's- questions

I am sure you know this, but according to the BK467 manual :

There is a 'high G2' switch which removes a couple of zeners from the G2 feed. You are supposed to use that if you can't get any emission with it at 'norm'

With some CRT types (incidated by a '*' in the type number lists) you set the G1 voltage to something other than -50V. Easy to miss....
TonyDuell is online now  
Old 12th Feb 2017, 10:53 am   #4
PaulM
Hexode
 
Join Date: Nov 2006
Location: Near Lincoln, UK.
Posts: 481
Default Re: Testing emission of CRT's- questions

As has been said, the emission tester is only a guide and I've had tubes that test OK that look fine on the tester which look awful and vice-versa. Partly, this is due to imperfect focussing with age which does tend to show up as the need for higher G2 and/or less grid bias. This comes about because an electromagnetically deflected CRT images the cross-over spot formed between the grid and the first anode onto the tube face and this relies on a 'sweet spot' of emission from the cathode. As the CRT ages, the sweet spot is no longer able to provide enough emission due to localised cathode deterioration and the user turns the brightness and contrast up to compensate. This works, but the emission is increasingly from an area outside the sweet spot and so the focus is impaired. Thus, the bottom line is that frigging the operating voltages creates a brighter picture but the focus will be off. This could be what's happening in the case reported, but then again, until you try it 'on picture' you really can't be sure . . .

Best regards,

Paul M
PaulM is offline  
Old 13th Feb 2017, 10:42 pm   #5
1100 man
Octode
 
1100 man's Avatar
 
Join Date: Apr 2016
Location: Ventnor, Isle of Wight, & Great Dunmow, Essex, UK.
Posts: 1,377
Default Re: Testing emission of CRT's- questions

Hi all,
Many thanks for the interesting replies. The CRT in question is from an Ekco T344. I have not done anything with the chassis yet so can't test it with an actual picture. I am about to start an overhaul so give me a while and I will be in a position to test it.
Many thanks
Nick
1100 man is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 9:30 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.