[BC] FM25K

[David Senft]

We have 3 FM25K's running as backups. They were new as mains until about 10
years ago. Agree with all below; -add to this the note about replacing all
coax. The length between the IPA is critical, -it is cut for a proper match.
We used a Bext 500 Watt IPA replacement with Bext exciters. Good,
serviceable equipment but somewhat slow to "boot-up". The old IPA and the
hybrid combiner can be removed if you cap-off the air duct (2" cap). We
never used the Tx's with the exciter in the box from day-one as service
access is easier when it was in a separate rack. Check the main amplifier
filament leads for deterioration as the "shield" is part of the filament RF
bypass. Often the braid is shot. Replace the (plug-in) control relays on the
lower-left of the cabinet with hermetically-sealed ones for trouble-free
operation. The logic control board hasn't been a problem as such. Don't
forget to short together the AFC interlock lines running to the old MS-15 if
it is replaced as the controller requires a "NC" contact. Check the action
of the HV shorting solonoids and bleeder resistors for safety reasons, -I
almost lost my life on one once. A note if using the original IPA, -check
the operating voltages on the modules often. If the series-regulator
transistor shorts, the modules run at full "rail-voltage". While the Tx will
appear to be fine, the minute you shut it off and back on again, the RF
transistors will blow. In almost all the cases where the modules blew, the
regulator was shorted. A quick test is to roll the Tx power down, if the
LED's on a module don't fade, it's hot.      
D. 

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