These sets generally don't have many faults in the picture processing but
the subsequent models had a lot of trouble with faulty tube guns in the
Philips tubes they use.

Its possible there is a short keeping the blue gun from cutting off and
hence throwing the balance to lose red & green.

Try tapping the neck of the tube whilst its running... use your judgement
about how strong- so as not to break the glass!!
This may clear the fault either temporarily / permanently.

Beyond that you'll just have to scope the drives to see if there is an
electronic fault.

regards
AW