Hi, I've got an HP/Symmetricom 58502A 10MHz frequency reference distribution amplifier. It has two 10MHz signal inputs and it auto-switches between inputs if one of them goes down. The 58502A had been working fine for the couple of years that I've had it, but recently it failed in a strange way where it thinks it has input signals on both A and B inputs even when no signals are present, and when when signals _are_ present, they don't make it to the outputs, even if I tell the controller to hard-select one of the inputs via SCPI commands over the RS-232 port. The hardware status lights display consistently with the erroneous interpretation of input signal state. The User's Guide which contains only a simple block diagram that shows a 2-to-1 switch on the inputs that feeds status to the microcontroller, and funnels the active input to the 1-to-12 output amplification and isolation block. I'm ready to crack it open and start poking around now, but I was hoping a time-nut might have experience or suggestions with these units and/or a service manual if such a beast exists. Cheers, John Stone johns@ks.uiuc.edu -- NIH Center for Macromolecular Modeling and Bioinformatics Beckman Institute for Advanced Science and Technology University of Illinois, 405 N. Mathews Ave, Urbana, IL 61801 http://www.ks.uiuc.edu/~johns/ Phone: 217-244-3349 http://www.ks.uiuc.edu/Research/vmd/