TELEFUNKEN 711 CHASSIS

The initial problem was no sound or raster. The 4A mains
fuse was blown, and D425, which is in series with the
regulator thyristor, was short-circuit. These were replaced,
giving sound and a full raster. After about ten seconds
however the raster collapses. This becomes a pulsating
operation, with the raster on for two seconds then off for
two seconds, the sound remaining on all the time. I’ve
checked the transistors in the power supply circuit, also the
reservoir electrolytic, and tried a new tripler in case a fault
here was overloading the set in some way.
Diode D425 was omitted in later production, and
Telefunken recommend its replacement with a wire link to
overcome the first problem mentioned. The pulsating effect
is due to the protection circuit coming into operation. If you
check the h.t. (U1) line you will find it varying in sympathy
with the fluctuations. The idea is that the h.t. line is
automatically shut down when the line output stage draws
excessive current: this is monitored by rectifying the flyback
pulses at tag 4 of the line output transformer to provide a
reference voltage for the monostable circuit (1552/553)
which is non -operational under normal conditions. If the
picture is normal before the pulsating starts, the fault is
probably in the protection circuit. This can be proved by lifting
one end of R557 to see whether a normal, stable picture is
obtained. If all now seems satisfactory, check the transistors
in the protection circuit (T551/552/553) for leakage
and replace as necessary. Finally, check that the h.t. voltage
is correctly set at 190V by monitoring test point M421 and
adjusting the voltage by means of the preset R429.