X-Y CRT INDICATOR
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
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
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.
for the deflection circuit 350 vDC, being +175v and -175v to ground,
these voltages being established by the two 33k divider chain.
tube half's anode resistor is connected to the +175v rail.
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.