by Frank Hughes

This circuit utilises what can be best classified as "recycled componetry" ie. that which is not in current manufacture; notably the power transformer and cathode ray tube. The latter is an electrostatic focussing and deflection type and dates from the period of WW 2. For the X - Y CRT indicator shown:- the unit will display two signals (when present) simultaneously on the tube face, the resulting display being a composite of both signals. Its most common use is that of a tuning indicator for a RTTY FSK demodulator, the amplitude of the mark and space frequencies being applied to the X and Y inputs.

The circuitry is of elegant design because of the following circuit innovations, ie. one dual triode valve is used as both the X and Y deflection amplifiers and only one position control; potentiometer is used thus reducing component count. The focussing anodes and one of each of the deflection plates are operated at ground potential, thereby minimising any electrostatic field effects on the CRT trace due to stray currents, induced currents from transformer magnetic fields, hum loops etc.

The tube EHV supply is of simple design using readily available components, (the cathode is operated at high -ve potential to ground, consistent with good CRO design). The circuit can utilise virtually any type of CRT provided sufficient technical information is available to allow it to be adapted to the design ie pin connections, element voltages etc.


Because the CRT has a 4 volt heater the 6.3v heater winding voltage is "dropped" through a 2R2 5w resistor. EHV supply, voltage doubles the 750 v rms AC supply to 1500v DC and then feeds the CRT resistive divider chain. All resistors should be at least 1W rating. HV supply for the deflection circuit 350 vDC, being +175v and -175v to ground, these voltages being established by the two 33k divider chain. Each tube half's anode resistor is connected to the +175v rail. The cathode bias resistor is connected to the -175v rail. By adjusting same (position control) the voltage at the anode(s) of each valve can be made equal to ground potential. (0v). Because the anode of each valve is connected to a deflection electrode and its corresponding opposite is hard connected to ground; no potential exists across the two plates and therefore the (undeflected) CRT trace should appear as a spot in the centre of the screen (tube geometry and lineup permitting).

The schematic is so drawn to illustrate the "bridge" type nature of the deflection circuit. The position control moves the spot across the CRT face in a diagonal fashion. Because one set of deflection electrodes sensitivity is twice that of the other, the input signal to the more sensitive is attenuated by a 2:1 resistive divider.

If you have any questions related to this project then please refer them to the author Frank Hughes.

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