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The original fault was intermittent field collapse. The set
was not used for several months, and now the fault is intermittent top foldover, which becomes apparent when the set’s been on for an hour or so. All capacitors in the field
oscillator and charging circuits have been checked by
substitution, and the resistors are within tolerance.
Top foldover (slow flyback) suggests that the field output
stage is not operating correctly. We suggest you check by
substitution the 16029/16039 field output transistors, the
mid -point voltage adjustment preset 5RV4 and the
bootstrap capacitor 5C35 (250µF), also the AC128 driver
transistor. The field scan balance control 6RV2 on the scan
control panel is suspect for intermittent field collapse: also
check the connections to 6L20 for dry -joints.

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