Decca 30 Chassis: Line Lock
The picture is OK for the first few minutes, during which the channel can be changed without ill effect. Once the set has warmed up however, channel change, adjustment of tuning, severe ignition interference or even switching a radiogram on/off, results in line lock being lost.
There’s a sure fire drill for this symptom, which is due to line oscillator drift. Replace the PCF802 line oscillator valve, the two flywheel sync discriminator diodes D402/3 ( Types 1N914 or 1N4148 are suitable) and the three electrolytics in the line oscillator circuit (C419, C423 and C425). Then short TP400 and TP401 and adjust the line oscillator coil L401 for vertical floating lines. Finally remove the short.