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Hi Mark,

This is what I heard, PMT tubes are supposed to have extended life.

Symptoms are:  
Channel 2 went down while acquiring live images.  A user might have 
accidentally pumped up the Argon laser power way above its baseline for some 
time.
Nothing could be detected even when switching lines from 488 to 543nm. A 
faint signal in 488 was seen when the detector gain, amplifier and transmission 
% were set up to the max.
All signals could be detected normally when light was diverted to channel 1.
Nothing wrong with the lasers, no burnt filters. We tested the voltage on the 
corresponding motherboard and all measurements are according the spec 
sheet. Nothing wrong with the power supply.
Any idea what else it could be?  Zeiss wants to change the whole module, but 
I am trying to find a cheaper alternative.

Thank you for any input.

Regards,
Ben