I have read the FAQ (and much else) without finding a clue to the precise problem I have.
The set is a Thomson 28WT25US around 6 years old. It has a 28 inch 16:9 tube and ICC17 chassis. From about its second year there was an intermittent fault: the set would go into protection mode (error 27: = deflection failure more than three times). But it always used to recover within a few minutes. Recently however it failed to recover. I changed the LOPT/flyback transformer. That has not solved the problem however.
If I adjust the focus voltage to give a sharp picture the set fails quickly (within a few minutes and sometimes almost immediately) with error 27 as before.
But if I leave the focus voltage low then the set continues to operate for hours.
I have tried reducing the G2/A1's but that made no difference. So I don't think it can be arcing from G2 or a breakdown in the capacitor between G2 and ground (CB01 on the board at the back of the CRT). Right, please?
I have read that these sets can have problems with the capacitors around the LOPT (especially CL21 and CL22). But I am puzzled as to how that would explain the way the set is stable with the focus voltage low - unless (as is very possible) I am misunderstanding the way the focus voltage is tapped from within the LOPT.
I have looked for a capacitor between focus and elsewhere which might be breaking down but could not see one. So I am at a bit of a loss.
In short, any ideas please?
Please note I am a novice at TV repair and have only an ordinary meter - no access to scope or EHT meter these days.