ULTRA 6713

Once or twice every hour, or maybe longer, the raster goes
intermittently green for 1-10 seconds. This appears on top
of the normal colour picture. Sound and picture are normal
at all other times. Reducing the brilliance control setting still
leaves the green raster, as does reducing the colour control
setting. The fault also occurs on monochrome. Vibration, or
the length of time the set is on, have no effect on the fault
and I’ve tried looking for dry -joints in the green drive circuit
without finding anything here amiss. I suspect the colour
demodulator/matrixing i.c. but the trouble is so intermittent
that I can’t pin it down.
There are three leads from the c.r.t. base to the signal
panel. Interchange the red and green ones and if necessary
set up the grey scale. Watch in monochrome (colour turned
off). If the fault still comes up in green, monitor the green
first anode voltage. No change here or a drop in the voltage
indicates a faulty c.r.t. If the fault now comes up in red
however, suspect the green (now driving the red gun) output
transistor VT119, the green drive potentiometer R216 or, as
you suggest, the demodulator i.c. (IC3)”. The fault can be
further pinpointed if necessary by interchanging leads to
pins 1 and 2 (red and green outputs) of IC3 (watch in
monochrome). (Thorn 8500 chassis.)