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The trouble with this set is that the h.t. rectifier diode and
the associated 3.952 surge limiter resistor keep going open- circuit – it’s happened three times in four months. Twice this happened when the set was switched on: the 2A mains fuse
also blew on these occasions. On the other occasion the set
had been on for a quarter of an hour.
There may be different causes for the fault. A likely cause
for its occurrence at switch on is that the line oscillator is
slow to start up, the line output stage then drawing
excessive current. The coupling capacitor C427 in the line
oscillator stage is suspect in this event. We assume that a
BY127 is being used as a replacement. If so, random
occurrence of the fault could be due to its protection
capacitor C603 being defective, or a fault in the h.t.
reservoir capacitor C602. It’s possible that your mains
supply is full of spiky transients, in which case the GPO
or the Electricity Board may be able to assist.

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