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Question Teltron Woes

6 years 4 months ago #31464 by beaky
Electron Diffraction. Newish tube (<8 years). We know it's been a bit damaged by overzealous use by teachers but up until now it was functioning correctly, if a little feint.

Now we can get the rings with the EHT but any attempts to focus it and it's turning off completely. This is using the HT supply at 50V.

The Senior Old Git and I are looking at it later (he is really good with them) but we're a bit stumped. Is it a tube issue, or an HT issue? We are wiring as per the diagrams. I am relatively inexperienced with them but up until now have just followed the very complex diagram and it's worked. None of us are doing anything differently.

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6 years 4 months ago - 6 years 4 months ago #31466 by D.B.Ferguson
Our electron diffraction tube does not have a focussing adjustment (unlike some other tubes -eg. fine beam).
It does, however, come with a clip arrangement to place a magnet at the cathode which deflects the beam to give a "better" result.

Edit: Ensure that none of the leads are earthed on either HT or EHT side. If they are "floating" this may remove the problem

Technicians: providing solutions and more.

David Ferguson B.Sc.
The following user(s) said Thank You: TecHKnow

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6 years 4 months ago #31467 by AndyG
This contains all the pertinent info about Teltron diffraction tubes.

If your tube is 'cutting out', is your EHT supply stable and your circuit correctly connected - eg the polarisation of the HT relative to the EHT is correct? As DBF says, if you've earthed both the EHT and HT supplies remove one link and make sure you have the correct configuration on *both* supplies otherwise you'll short out the EHT via the earth. You can run it floating (I prefer to) but it's usually recommended to earth the EHT for some reason.

Otherwise, are you having to drive the tube really hard to get rings? If so it's possibly about to expire and the HT is pushing the EHT current over the edge.

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6 years 4 months ago #31470 by beaky
[quote="AndyG" post=31467
Otherwise, are you having to drive the tube really hard to get rings? If so it's possibly about to expire and the HT is pushing the EHT current over the edge.[/quote]

After extensive testing with various different bits of kit we now think this is what is happening. I just needed a fresh pair of eyes, so to speak.

Instructions are being followed clearly as they have been since we bought it. It is looking like the HT voltage is tipping it over.

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6 years 4 months ago - 6 years 4 months ago #31472 by tom foxe
If indeed the tube is on its last legs, you might consider "rejuvenation". It's many years since I did this to the last remaining black & white CRT TVs of a TV rental co. If the picture was washed out and could not be focussed, we used to wait till the tube got nice & hot before we whacked the tube near the end with the rubber handle of a big screwdriver. Apparently the cathode can have a deposit of crud which may be removed so that more electrons can escape. We also had a setup for colour TVs, but all I can remember about that is we disconnected the cap feeding the CRT and fitted a new one which fed the heaters with a bigger voltage to get the cathodes hotter than usual. Some TVs were revived, some just got worse.
The sort of thing you only do if the other option is the bin.

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5 months 1 day ago #41722 by Andreas Tober
sorry if this doesn't belong here, I could not see where I can start a new thread.
I have looked at our electron diffraction tube again after not using it for a while, and am a bit puzzled by the manual which says I should not exceed an anode current of 0.2mA. Putting a multimeter between the anode and the EHT supply I am measuring 0.028mA. Unless something else is wrong here (tried different multimeters), it seems I am so far from the current limit, there seems little point in measuring it (as people on this forum have suggested)
My question is: do other people find that the anode current is so low? It starts even lower and gradually increases when the tube is on for several minutes.
I am using the orange Unilab EHT supply, the tube is very old, has been here longer than me.

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