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Brightening beam in CRT television picture tube ?


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Jan 29, 2004
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Hello all.
What does the 'screen' control on a flyback transformer does to the beam ? Changes brightness ?
Does the 'focus' control on the flyback transformer affects only focusing the beam ?

Am feeding HV anode+, ground, screen, focus and proper filament heater to a lensed projection picture tube (as attached) with a canibalized plain-color-television-complete-circuit-board supplying
those voltages/connections. It works. Would like to increase brightness greatly, what should be tweaked ?
-There is no deflection coils, no video signal, no raster- Just the beam spot/dot on phosphor. Yes, It can burn it but not part of this thread nor care to be addressed.

Screenshot 2022-11-23 at 17-55-20 Sony KP-46WT520 Rear Projection TV Blue CRT Picture Tube ass...png
Changes the screen grid voltage. It does also change brightness but more importantly the linearity of the brightness. Normal procedure is to set it so the raster is only just visible with no video present.

Brightness is normally controlled by adjusting the cathode voltage relative to the control grid. Less voltage difference means more brightness. The control grid must always be negative relative to the cathode or it will be damaged almost instantly.

Correct. G1 is nearest to the cathode and called the 'control grid' since it has greatest influence on the electron flow, G2 is the 'screen grid'.

In the schematic you can see G1 is held at ground potential by R9221 and the cathode has some positive DC and the video waveform on it from pin 12 of the IC. That effectively means G1 is negative with respect to the cathode and can therefore control the overall current and hence brightness. G2 has some positive DC potential on it with respect to the cathode and that voltage comes from the 'screen' control on the flyback transformer because it is probably higher than any of the other PSU rails can supply.


Would it be a good place to start tweaking by feeding
+150V to cathode;
GND to G1 control grid;
'screen' voltage from the high tension transformer to the G2 grid;
'focus' voltage from the high tension transformer to the G3 grid;
Whatever ~20KV is outputted by HT transformer to CRT anode ?

What gets damaged almost instantaneously if G1 control grid is not negative respect to cathode ?

Am trying not to ask to hold my hand on teaching me how to make this work but the voids on my knowledge keep pushing...
+150V is probably too much on the cathode , a more realistic value would be about 75V to 100V but it would normally carry the video waveform sitting on a DC level set by the brightness control.

If G1 goes positive with respect to the cathode it works as though it was the anode of a diode and conducts current. Although I seriously wouldn't advise doing it, if the current is kept low enough, (less than about 10mA) no harm will come to the G1 electrode or cathode themselves but the overall current through the CRT might damage it.
Your other assumptions are correct, I do not have a specification for the CRT but typically, measured from ground there would be about 500V on G2, 3-4KV on G3 and at least 20KV on the anode.

Hi. Little progress...
Fed +26VDC picked from the repurposed TV board to CRT cathode, all other connections (control gridG1, screenG2, focusG3, anode, GND) unaltered.
Beam projected a spot two metres+ to a wall as desired; but a microcontroller on the canibalized TV guts board shuts off supply as senses some flaw in its power supply. There is some sort of X-ray protection feature I do not understand, probable derived from incorrect voltages events if happen.
Tried with a 0.5MOhm in series to cathode and delays the protection state a few seconds more but still shuts off. Open cathode does not trigger shut-off.

Cannot tell if there is something that got 'instantly damaged' previously; do not know what to check that could got instantly damaged if the cathode was not positive respect to grounded G1 grid.

The little board is hacksaw canibalized, supplies regulated heater 6.3VDC voltage, fed from the audio stage 12VDC supply which is not operating; the horizontal and vertical stages are also out of commission by having no deflection coils load connection. There should be surplus power supply with those stages not consuming.

Any guidance is welcome.
Projection TV always has protection against scan failure. It only takes a few seconds of stationary spot to permanently burn the phosphor on the faceplate so they have circuits that kill the supplies if the deflection fails.

Thanks, Brian.
The protection state happens in the plain TV board shown on left, which did not come out from a projection unit. It does not go into protection mode with deflection disconnected. It goes into protection mode when tapping from it some +DC to drive the cathode.
Should I try with a separate standalone supply for the cathode ?

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