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TV Oscilloscope Almost Done

T

T.C. Mann

Jan 1, 1970
0
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.
 
T

Tim Williams

Jan 1, 1970
0
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
 
T

T.C. Mann

Jan 1, 1970
0
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 potthere,
form -10 to + 10 V or so, to test.
This will cut the TV signal and create a nice brightnes control.

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.
 
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