Keeley modded TS9DX Tone control not working

[FirstTimer007]

Hi,
I have only built fuzzes and tranny drives etc., but have been asked to repair this pedal.
Don't know where to start...only have part of an email sent to pedal owner from Keeley saying to check

'the gray wire that should be on the left side of the board above wires 12 and 11.  Make sure that this wire has a solid connection'.
I have checked that, and it tests OK, I don't know what now to check. I have no schems or layout etc for the stock TS9DX pedal, and nothing on the Keeley mods.
These mods were done by Keeley, including true bypass and include a 2 position switch added to the front.
Scratching my head now... do I use an audio probe here?
Thinking one of the 'lytics has gone bad, or the 4558?
Thanks in advance
Sean