tektronix: Cure for dot-pattern on trace

When I'd finished the restoration of this magnificent instrument, I noticed what appeared to be a dot-pattern on the trace. This was a mystery, as I didn't recall seeing this when the instrument was first powered.  

Extensive investigations yielded nothing: there was no modulation on either the EHT, the grid or the X-plates.  However, recalling that the colllector voltage sweep is derived directly from the mains supply (via transformers and an internal  variac), I tried addng a 0.47uF X-rated capacitor  across the output of the variac (shunt wiper to ground). This largely cured the problem, and confirmed that the noise was mains-derived. Subsequent correspondence with a gentleman in the USA who had exactly the same issue (blog at lazyelectrons.wordpress.com) confirmed the diagnosis; he traced the problem to a modern 9.5W LED lightbulb.  

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