When I first started in rental tv service engineering I was called to a customer with an RGD deep 17. The complaint was a bar of interference across the screen about 1.5 ins wide, which came and went. This was the start of a series of calls where no fault was seen including a return to workshop where no fault showed.
The customer by this time was getting irked and I was once again dispatched to visit to have one final look before changing the set. The day was a dull dismal one and the customer lived in an old cottage. Whilst I was starting to again see if I could get a manifestation of the fault, the customer came in and said “Bit dark today boy light help. Suddenly there it was a bar right across the screen like a wobbulator was being fed in. On changing the bulb no further problems occurred . These were the days when channel 2 BBC was the only one available, and it appeared that the filament of the bulb was oscillating at or near 50mhz.