Circuit diagram:
Description
If you are the proud owner of an old oscilloscope tube, you may be
interested in using it once more for its original purpose. All you need
are the right voltages on the right pins: in practice you may need to
peer closely inside to find out which pins on the base correspond to the
acceleration and deflection electrodes, in particular if there is no
part number to be seen on the tube. The tube we had for experimental
purposes was a 7 cm model of unknown provenance.
So the first step is to establish which pins correspond to the
heater, cathode, grids, deflection plates, and anode. With this done we
can make our simple oscilloscope as follows: connect the Y input via a
suitable capacitor to one of the Y deflection plates; for X deflection
we use a neon lamp oscillator to generate a timebase; and with a focus
regulator circuit we have a complete oscilloscope.
Operation of the horizontal deflection oscillator is visible as the
gentle flickering of the neon lamp. Whenever the voltage across the
parallel-connected capacitor reaches the strike voltage of the lamp, it
is discharged with a brief pulse of current. It is hard to imagine a
simpler way to generate a sawtooth waveform. The supply voltage of 300 V
is adequate for simple experiments, even if the tube is rated for
operation at 1000 V or even more.
Now, if a signal is applied to the Y input, we should be able to see
the waveform on the screen. It must be admitted that the design’s
sensitivity, linearity, trace size, bandwidth and triggering facilities
leave a little to be desired. Nevertheless we have shown how little
circuitry is required to make a real working oscilloscope.
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