Hello, I am attempting to install a second channel in one of these analyzers and have successfully performed this mod before, twice. Now apparently there is something that needs to be done if I am dealing with a unit with serial number previous to 3641A (or something close) according to the online manuals I am supposed to contact Agilent for some reason. Symptons are... after installing any second channel A20 board the unit will not power up. I have turned the option on internally and changed the single/dual channel jumper on the A35 board and still nothing. I have even installed another power supply from a working unit which has two channels??? Still nothing. Any help would be greatly appreciated.