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TV Oscilloscope Almost Done

I just converted a 5" B&W TV into a very basic oscilloscope. I now have a nice steady 60 Hz horizontal time base and can deflect the vertical trace full screen with just 200mA @12V. The last item I would like to improve is the keep the electron beam at a constant voltage. Right now, the beam intensity varies based on the received picture information. The little PCB on the back of the CRT neck has only 4 wires going to it and 2 of the wires would have to be for the heater. Could one of the other wires simply be tied to a constant voltage source through a pot so I can vary the brightness or is there a better way of doing this? Thank You - T.C.

Reply to
T.C. Mann
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Might be. Trace the circuit, there can't be much there. With only four wires it's probably amplified video going right to the grid or cathode.

Tim

-- Deep Fryer: A very philosophical monk. Website @

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Reply to
Tim Williams

On a sunny day (Thu, 22 May 2008 18:56:22 -0700 (PDT)) it happened "T.C. Mann" wrote in :

heater, heater, cathode, grid. Normally the video is fed into the cathode (wehnelt). Find the cathode, it should be at about 1/2 the supply for the video output tube. Best is perhaps to disconnect the grid of the video ouput tube and use pot there, form -10 to + 10 V or so, to test. This will cut the TV signal and create a nice brightnes control.

Reply to
Jan Panteltje

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there,

I should have mentioned this is a solid state chassis but I should be able to apply the same technique. If I understand correctly, I could simply cut the circuit supplying the video signal to the base of the video amp transistor (assuming a common-emitter configuration) and then hook it to a constant voltage source via a pot. Since I don't have a schematic for the set and it's kinda cramped working in that area of the PCB, it's probably easier just to vary the voltage directly at the cathode simply because it's easier to get to. Thanks for the suggestions, though - T.C.

Reply to
T.C. Mann

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