RANK A823 CHASSIS
This set works correctly for the first two hours after
switching on, then simultaneously looses brightness and
colour, with picture ballooning. The fault seems to be
temperature dependent, with a long cool down period before
normal viewing is restored. With the fault present the e.h.t.
drops to about 10kV, with a noticeably reduced spark at
the tripler input. The first anode supply is also reduced, but
the h.t. remains unchanged. I suspect the tripler.
The tripler could be responsible, but only if the h.t. and
flyback voltages are equally distributed between the two line
output transistors during the fault. We say this because this
fault is commonly due to one half of the line output stage
becoming short-circuit, as a result of failure of one of the
BU105 line output transistors or one of the 0.0047µF
flyback tuning capacitors 6C5/6. If 6R6 and 6C4 are
present, they should be removed when carrying out the
repair. Be sure to rebalance the stage with 6L4/5 as
described in the manual.