G.E.C. BT2155

After working perfectly for a few minutes after
being switched on, the screen goes dark so that
only the highlights of the picture can be seen. A few seconds after this an audible ” plop ” is
heard and the picture returns to normal. The
sound remains normal throughout this series of
events.
I have replaced the valves in the tuner and have
checked all the other valves in the receiver i.f.
stages by substitution. Also I have changed C121
(0.11.LF) in the cathode lead of the c.r.t. and tested
the 120kû resistor in the same lead. -P. W. Ellis
(Alconbury, Huntingdonshire).
You should check the tube base voltages. The
pin 11 voltage should be about 120 -150V normally:
note whether this varies.
The voltage on pin 2 should be about 100V at
the control of the brilliance; again check for
variation. The . pin 10 voltage is via a 2.2Mû
resistor and may appear low, but if the voltage
necessary to check for variation, and if the voltage
is noticed to fall, check the 0.5µF (C120) decoupling capacitor.