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575 trace issue resolved

John
 

Problem posted originally under #149304

What looked like dot-pattern on the horizontal trace(s), on closer inspection due to superimposed X-plate HF signal which gives velocity modulation of the spot.

This is a 234V model and we live in a relatively rural area with plenty of PV installations. Anyway, after much hunting around and decoupling of various supplies, it did turn out to be intereferance on the incoming mains. It is more-or-less cured by adding a 0.68uF class X capacitor across the primary of T702. Only when T701 is in the +/- straight-through position is the problem still visible, albeit diminished.
John