[Tig] PEC limiting MK3

[woz]

I have a situation with a mk3 telecine of one of the PECs (blue) limiting
prematurely. This showing as extreme changes of  shading etc.at the limit,
(slew?) when, with the same light levels, the other PEC's are perfectly
linear.
It (the blue channel) is also behaving too slowly in its response to change
when light levels to the PEC are altered.
If the light to the PEC is reduced it behaves in an acceptable and linear
way, so in the interim I have put an additional filter in the dichroic box.
I (naturally) changed the PEC for a brand new one and swapped the pec
control board, but to no avail (surprisingly because it looked  to me almost
certain to be the PEC!)
I have not yet changed the head amp, which I will shortly substitute with
another.
Power supply voltages etc seem normal.
Anyone else had a similar experience?

Regards
Woz