THORN 2000 CHASSIS

This set has given several years trouble free viewing. It’s
now developed the following fault however. Every few
minutes the colours unlock, forming not so much Hanover
blinds as Venetian blinds. The trouble can be temporarily
cured by pushing lightly on the push-button tuner for the
particular station. The tuner has been repaired, so I assume
it’s a decoder fault.
Decoder problems with the 2000 chassis are quite rare.
From your description, it seems that the reference oscillator
is drifting off lock. Suspect components are the electrolytics
C3 and C7 (both 4,0) in the burst gate/amplifier circuit,
the 0A91 diodes (W1-4) in the burst detector circuit, the
BC107 d.c. amplifier transistor VT3, and the crystal. Since
these components are inexpensive, we’d replace the lot then
adjust R13 (detector balance) and R15 (oscillator
frequency) as described in the manual.