TELEFUNKEN 709 CHASSIS

The set will work perfectly all evening provided the channel
is not changed. If another channel is selected after the set
has been on for some time, it’s impossible to obtain a picture. The situation is the same on all channels. The set
works normally when first switched on from cold.
The tuning voltage stabiliser diode GR971 (ZTK33) has
a tendency to cause slight tuning drift when warm, since it
then stabilises at a slightly different voltage. This effect is
often sufficient to throw the set off tune when a different
channel is selected. Another area that gives trouble on this
chassis is the gated a.g.c. circuit. This is on the main signal
board, alongside the i.f. unit. Operation of the circuit can
become very critical when, as the set warms up, either of the
transistors T171 (BC237A) or T172 (also BC237A) starts
to break down, reducing the voltage at pin 3 of the i.f. strip
so that overloading occurs. Under these conditions it’s extremely difficult to tune the receiver to any station at all.