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The problem is loss of brightness control: the picture is
displayed at full brilliance, with the brightness control
having little or no effect.
Check the PL802 luminance output valve and its anode
circuit: there should be 175V at pin 7 (anode). If all’s well
here, move to the brightness control circuit, which is linked
to the PL802’s control grid. The brightness control is slung
between positive and negative 1.t. supplies: there should be
between -2V and -5V at its wiper. The beam limiter
transistor TR535 is also associated with this circuit, and
there should be 2.7V at its emitter. If the fault lies here,
check the transistor and if necessary the associated
components. We assume that the c.r.t. first anode voltages
are correct: the voltage at the wipers of the presets should
be adjustable between 275V -710V.

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