Check out this web link sir: http://www.w8ji.com/ameritron_811h_572_amplifier_trouble_shooting.htm
Issue with the band switch? You said 75 worked but others won't.
Have your Ham Radio friend call Ameritron & ask for Rob Hood.He builds those amps & is a great guy.He won't talk to you & help you if you don't have a call sign since they of course want licensed Hams using them where they are Legal.He can give pointers & has no issue with helping solve problems with their amplifiers.Mike Eunis is another one if Rob is unavailable.
SIX-SHOOTER