Thread: Bush TV22 Mk 1
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Old 11th Jan 2017, 3:17 pm   #41
Argus25
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Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
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Default Re: Bush TV22 Mk 1

Quote:
Originally Posted by KeithsTV View Post
Once you have EHT if there is nothing visible on the screen briefly connect the CRT cathode to ground via a resistor of around 10k to 47k. If all is well the screen should light up.
Liam,

The reason for above is that the polarity of the video detector diode drives the video output tube grid positive, that is when the TV station RF carrier is received. This increases the video output tube's plate current and so the plate voltage drops, which takes the CRT's cathode more negative with respect to its grid (as does adding the temporary shunt resistor) This increases the CRT's beam current. So with no RF carrier the CRT beam can be cut off, out of range of the brightness control which takes the grid in the positive direction to get beam current and light up the CRT.

The raster looks compressed at the bottom, usually a sign of poor emission in the vertical output tube, or out of spec R's and C's in that stage. Or it might have a combination of off center beam alignment and low amplitude (height). Check the vertical running frequency.

(I think I mentioned that if the set is powered before the full restoration of componentry R's and C's etc and a set of trial nos tubes, there can be a few faults to chase down).

It is good to see the CRT light up, well done.

Many vintage CRT's have ion spots sometimes called ion burns. This is really ion contamination & de-activation of the phosphor. Non-aluminized phosphor is vulnerable to this effect. The reason the ion spot is in the center is that the negative ions have a higher mass and are deflected less than the electrons in magnetic deflection systems. Ion trap magnets help to minimize ions in the electron stream but they are not 100% effective. Aluminized screens solved this problem.

(In electrostatically deflected tubes ion damage to the phosphor wasn't as noticeable because ions are deflected in that case on the basis of their charge, not mass, so they get deflected the same as electrons).
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