BEOVISION 3400

The fault on this set is intermittent loss of brightness. It
occurs only occasionally, and is quite unpredictable. After
an hour or two it corrects itself. I’ve replaced the 12V zener
diode which stabilises the 12V supply to the signal stages,
also the LIF tantalum capacitor in the vision detector can,
and have tried cleaning the pins of the 12HG7 luminance
output pentode.
It would be helpful to know which of the c.r.t. electrode
voltages changes when the fault is present. If the first anode
voltages fall, the 840V supply rectifier 8D9 (BY184) and
the associated network 8R22/8C32 are suspect. If the grid
voltages are varying, check the clamp voltage source components 3R49/3R50/3D4/3D5, then check back to the
tracking circuit transistor 3TR3 and its associated components. If the cathode voltages are varying, which is more
likely, the 12HG7 valve itself is the prime suspect. The d.c.-
coupled luminance channel transistors 1TR15-18 could be
responsible, though failure of the rivets at each end of
brightness control track is more likely. Another possibility
is variations in the -210V line, due to the rectifier (8D6,
BA148) or its series resistor 8R16 (2.2k SZ, 1W). These are
fed from the line output transformer.