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The trouble with this set is that R725 and R735 have
overheated to such an extent that the values cannot be read,
while fuse F4 has blown.
It’s difficult to relate these faults, unless the thick film unit
has failed with the result that the trip circuit is not
operating. R725 (1801(52) simply forward biases the beam
limiter diode W722. F4 protects the 24V supply to the field
timebase and, via R735 (3.9Q), the signals panel. Either of
the two decoupling electrolytics C171 and C196 on the
signals panel could be leaking. If C196 is a tantalum type it
should be changed to an aluminium type. The voltage at F4
can rise excessively if there’s a fault in the field timebase: in
this event, connect a 12052 5W resistor from F4 to chassis
and check the field timebase circuit.

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