Murphy has struck again, replaced the 990k input resistor on Ch A, all was working nicely, perfect square wave etc on both channels volts/div was reading spot on, nice steady trace, so put covers back on the night and switched off for dinner.
Came back after dinner, and thought that maybe I'd look at the offset on some ranges so I switched on and no trace, pushing beam find button had no effect either, using the vertical position controls then operate as screen illumination controls and absolutely no horizontal activity at all.
Back to basics reveals that the power rail -15V is down to 1V, pull of the A14 power bridge board and power on, check -15V and its back. Replace A14 and power, -15V is still up, check the 12v rails, both are fine then trace is back again and is looking fine .... Sweep seems to be working just fine again ARRGHHHH.
I'm pulling handfuls of hair out wondering just WTF is going on here, do I have a Tant playing up on the -15v rail??