GRUNDIG 717GB

The fault at first appeared to be a defective tube, the symptoms being uncontrollable brightness with the controls ineffective and a faint blue glow in the tube neck. When the
service switch is moved to red raster or the line position
however operation appears to be normal. I’ve since found
that by turning the brightness control and the preset
brightness control range preset 8382 to minimum a dark
screen is obtained: then gently advancing R382 produces a
picture, though very dark. The picture then goes bright
again. The only other trouble I’ve had with this set was with
the VDR R704 in the degaussing circuit.
It’s most unlikely that the c.r.t. is the cause of this
trouble: much more likely are low c.r.t. cathode voltages.
The brightness and brightness range controls operate on the
luminance output stage black -level clamp in this chassis.
We suggest you replace R382 and set up the video as
described on page 14 of the manual. If this fails, there is
probably a defective transistor in the luminance circuitry:
suspect the output transistor Tr375 and its d.c.-coupled
driver Tr365, and the black -level clamp transistor Tr385.