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Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc. |
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7th May 2019, 1:38 am | #21 |
Tetrode
Join Date: Oct 2009
Location: Chepstow, Monmouthshire, UK.
Posts: 60
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Re: Emiscope 3/1 Testing, and value?
Update on this thread...
I offered Sean what I thought was a reasonable amount for the CRT which was accepted. I then had a 5 hour drive after work on Thursday to collect it. Over the weekend my brother (Catkins) dug out his HMV 904 to test the tube. This was last turned on in 2015, and in the meantime it had developed a fault (which I'll let Catkins explain). With work looming up, we finally got to test the CRT around midnight. Short story, the tube works well, and shows no ion burn (yet), which implies it is unused. See attached pictures. Thanks to Sean, this now means I can resume the restoration of my Marconi 706 which I put on hold back in 2010 |
7th May 2019, 3:48 am | #22 | |
Pentode
Join Date: Feb 2015
Location: Chepstow, Monmouthshire, UK.
Posts: 234
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Re: Emiscope 3/1 Testing, and value?
Quote:
The screen-grid feed wire was some of the original wire in the set. In this particular case I thought it was OK but borderline back in 2015. Obviously it wasn't any good anymore as it had shorted through the insulation. This set was my fist pre-war television set restoration, and I was constantly worried about "over doing it" and needlessly destroying originality, and in hindsight I obviously kept some wire which I should have replaced (as mentioned elsewhere, in the A56V restoration, I wasn't taking any chances and replaced all of the wiring). |
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7th May 2019, 4:28 am | #23 |
Pentode
Join Date: Feb 2015
Location: Chepstow, Monmouthshire, UK.
Posts: 234
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Re: Emiscope 3/1 Testing, and value?
Probably an off-topic aside, but one which I only realised today, is I found going back to working live on the HMV 904 chassis far more stressful than working on the Murphy A56V. This is obviously because so much of the underside of the HMV 904 chassis is at EHT potential and unshielded. Discretion being the better part of valour once I discovered where the fault was, I switched the set to radio, to turn off EHT.
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7th May 2019, 8:13 am | #24 |
Nonode
Join Date: Feb 2005
Location: Worcestershire, UK.
Posts: 2,525
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Re: Emiscope 3/1 Testing, and value?
Well done!
Yes, in particular there are some connections on the EHT transformer rather too close for comfort to the earthy rear cans of some concentric pots. Check the gaps here or insulate. The underneath is a potentially lethal environment. Everything is rather squeezed in. Steve |
8th May 2019, 1:19 pm | #25 |
No Longer a Member
Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,679
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Re: Emiscope 3/1 Testing, and value?
Actually, I don't think so.
It possibly is a shame that my advice in post #6 was ignored, though I cannot be 100% sure. I have spent many hours experimenting with non-aluminised phosphor CRT's. The P4 white phosphor (of which there are a number of chemical types) is particularly sensitive to degradation in sensitivity. Either from intense electron bombardment, or negative ion bombardment. This surprised me initially because I'm used to damaging chemical reactions taking place in a gaseous or liquid medium and I had some fantasy it would not happen in a vacuum. Ions have a very large mass compared to electrons. Their velocity in the CRT beam is lower. Because of that the magnetic field associated with them is weak so if they are deflected by a magnetic field the deflection is a lot less than for an electron where the velocity is high. In electrostatic deflection, the ions and electrons are deflected equally on the basis of their charge and the deflection is independent of the mass of the deflected particle. The above of course leads to the typical ion burns seen in magnetically deflected TV's, you end up with a central spot or zone of ion burn. Sometimes it can be sharply delineated if an electrode structure impinges on the ion path. If you power a magnetically deflected CRT though (as suggested by some on this thread) to "test the CRT" without any magnetic deflection field applied, you can end up with both electron and ion bombardments, of equal deflection size (because the beam is only being electrostatically deflected with this "test"). This can desensitize the fragile P4 phosphor and the zone of that area, looking at the photos posted, matches the desensitized area on the screen of the CRT, which is an off center elliptical zone on the CRT face, where if you look you can see that the light output of the Phosphor is lower. If this was a used CRT , most likely what you would have seen if any ion burn was there would be a circular disc, not an off center ellipse. So as I suggested, its better not to perform that test and run the CRT in an actual TV set for testing. Even then, its hazardous. I had to modify my 904 for beam spot suppression at turn off to help protect the fragile phosphor. |
8th May 2019, 10:19 pm | #26 |
Dekatron
Join Date: May 2003
Location: Edinburgh, UK.
Posts: 3,273
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Re: Emiscope 3/1 Testing, and value?
Argus,
I think you are getting confused with the images. Sean's test shows the defocused beam in the centre of the tube. The image with the offset beam is Jac's test on a completely different CRT. I would also suggest that exposing the screen to a few seconds of ion stream is highly unlikely to produce perceptible damage. My 6/6 does have a darkened area approximately corresponding with the Test Card C circle but this is only after many hundreds of hours exposure. Peter |
8th May 2019, 10:29 pm | #27 | |
No Longer a Member
Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,679
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Re: Emiscope 3/1 Testing, and value?
Quote:
Normally is does take a while to get ion burn, but desensitizing of the phosphor can be very rapid sometimes, so it still does pay not to energize the CRT with the scanning field from the yoke to keep the energy as spread out as possible and to always keep the grid in the negative region with respect to the cathode. |
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9th May 2019, 8:20 am | #28 |
Heptode
Join Date: Jun 2006
Location: Eindhoven, Netherlands.
Posts: 640
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Re: Emiscope 3/1 Testing, and value?
Hugo,
As indicated, the photo I made of my tube was infinitely less bright than it looks. I had to do the test in almost darkness to see it. I enhanced the photo to make it all visible, but it goes without saying that I took extreme caution and increased the brightness from not visible to very faint. Certainly less bright than it would be in normal operation. You are absolutely right to caution everyone to be very careful. If you don't exactly know what you are doing, proceed with extreme care, or test carefully in a set. Damage done can not be corrected. Unfortunately many of the tubes I have in my old sets have a serious ion burn (like the 3/1 in my Marconiphone 706) or a switch-off burn (like the 6/6 in my HMV 900). The ion burn makes watching a movie on the set most unpleasant. A switch-off burn is also quite irritating, but less so than a pronounced ion burn. dreanog, the newly found 3/1(?) looks to give an impressive picture! (I'm envious!) Jac |
9th May 2019, 8:50 am | #29 | |
No Longer a Member
Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,679
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Re: Emiscope 3/1 Testing, and value?
Quote:
If phosphor is aluminized it is much more robust, negative ions barely effect it and its much harder to burn quickly , but it still can happen. Non aluminized coatings are very thin and fragile. I do have one aluminized screen TV CRT that does have a visible ion burn, so it still can happen to a much lesser extent. I seem forever to be putting turn off spot killers into video monitors. One easy way is to simply increase the charge storage in the circuit supplying the CRT grid and it hold it up more negative for a longer period at turn off. Even at normal brightness though the phosphor browns and ages over time, especially visible on CRT's used for text and graphics displays. |
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9th May 2019, 9:40 am | #30 |
Nonode
Join Date: Feb 2005
Location: Worcestershire, UK.
Posts: 2,525
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Re: Emiscope 3/1 Testing, and value?
One thing I learned during a visit to RACS in France a few years ago was that a mild 'ion burn-like' effect can affect the screen glass itself over a long period.
We had some CRM121s rebuilt with new phosphor + added aluminising and there was sometimes still a patch of slight yellowing in the centre of the screen. Steve |
9th May 2019, 12:20 pm | #31 |
No Longer a Member
Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,679
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Re: Emiscope 3/1 Testing, and value?
Well I guess the ions that hit the phosphor & glass cannot magically disappear, unlike electrons that can bleed away to external locations via the anode connection. So its not surprising they can also "stain" the glass. If you look at beam tubes that have holes or spaces in the anodes, the glass often gets a brown stain there, it might be the same mechanism.
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10th May 2019, 10:37 pm | #32 | |
Dekatron
Join Date: Feb 2005
Location: St.Ippolyts, Hitchin, Hertfordshire QRA IO91UW
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Re: Emiscope 3/1 Testing, and value?
Quote:
Cheers Sean
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12th May 2019, 5:25 pm | #33 |
Dekatron
Join Date: May 2010
Location: Greater Manchester, UK.
Posts: 18,676
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Re: Emiscope 3/1 Testing, and value?
Is there a mechanism where by a CRT with a non-straight beam path, and a correctly adjusted ion-trap magnet can still suffer ion burn, albeit slight?
I ask, because I've witnessed it myself.
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13th May 2019, 8:18 am | #34 |
Dekatron
Join Date: May 2003
Location: Edinburgh, UK.
Posts: 3,273
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Re: Emiscope 3/1 Testing, and value?
Hi Graham,
Is that slot cut at an angle or physically bent gun? Peter |