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Old 28th Feb 2007, 10:36 am   #1
DoctorWho
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Default Hypothetical EHT question.

This question is purely hypothetical and asked out of curiosity, it's not something I am planning on trying.

With all TV sets, either mains EHT, flyback or RF eht, the EHT is rectified and then smoothed, with the earlier mains EHT sets this usually involves a 4-pin rectifier valve and then a rather large condenser, such as a 0.1 @ 5,000 V DC, and then connected to the CRT.

I am just curious as to what would happen if the EHT were AC instead of DC? Would the picture roll all over the place in sympathy with the 50 c/s frequency, would the CRT implode, or something else?

I'm just curious?

Peter.
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Old 28th Feb 2007, 10:50 am   #2
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Default Re: Hypothetical EHT Question.

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Originally Posted by DoctorWho View Post
I am just curious as to what would happen if the EHT were AC instead of DC?
If the EHT was genuine AC sine wave, the screen would be blank for the -ve half cycles and varying wildly in brightness and scan size on the +ve half cycles. Might also be some odd space charge effects causing even more havoc. I doubt you'd damage the CRT.

With flyback EHT there isn't really a -ve half cycle to be blocked, the rectifier is only there to stop the smoothing cap discharging back into the LOPT.

Note on scan size. In electrostatic deflection the defelction sensitivity is inversely proportional to EHT voltage. In magentic deflection it's inversely proportional to the square root of EHT. This is a good reason to use magentic deflection at high EHT voltages since the scan sensitivity doesn't drop as fast when EHT is raised.
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Old 28th Feb 2007, 11:25 am   #3
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Default Re: Hypothetical EHT Question.

Thanks for explaining Jeffrey, that's very interesting. So what would happen with flyback EHT if that was not rectified and smoothed at all, would this be the same, no picture?
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Old 28th Feb 2007, 11:57 am   #4
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Default Re: Hypothetical EHT Question.

Unrectified flyback EHT is a big peak during flyback and essentially nothing at other times. This is just a guess (don't try it at home folks since it might wreck the LOPT) but you would probably get a virtually blank screen and a suffering line output stage. Would depend on the amount of stray smoothing capacitance. More than a gnats gnadger and the whole line output stage won't be at all happy.
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Old 28th Feb 2007, 11:59 am   #5
Duke_Nukem
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Default Re: Hypothetical EHT Question.

The unrectified flyback EHT would be negative during the scan and +ve during flyback - so no picture. However the capacitance of an aquadag'd CRT loading the LOPTY would no doubt wreak havoc.

As for the AC EHT, at lower +ve EHTs the scanning angle would be so wide that at some point the electron beam might well hit part of the gun structure and cause heating, but whether that'd damage it I've no idea.

TTFN,
Jon
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Old 28th Feb 2007, 12:32 pm   #6
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Default Re: Hypothetical EHT Question.

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The unrectified flyback EHT would be negative during the scan....
Only a few hundred volts -ve at most. The voltage across a coil is proportional to the rate of change of current. That's only large during flyback.
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Old 28th Feb 2007, 12:39 pm   #7
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Default Re: Hypothetical EHT Question.

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Only a few hundred volts -ve at most...
Doesn't matter how -ve, no electrons will be going there so no pic.
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