PYE 725 CHASSIS

The set has been correctly set up for 185V h.t., but the
picture starts to pulsate after fifteen minutes from cold. In
case the over -voltage circuit had altered slightly, the setting
up was readjusted and a meter was then left across the 185V
line. After fifteen minutes, the line started to rise to over
200V, when the set was switched off to avoid damage. The
three presets RV916, RV917 and RV879 had no effect on
the rising voltage. What’s the cause of this rising voltage?
The usual cause of this trouble is that the thyristor mains
rectifier D888 is going into the diode mode when warm.
Replacement should cure the trouble – use a 2N4444 or
OT112