ham.dmz.ro.sh_gm300.zip
look for a burned track
look for +12v supply to p.a
its more likely the driver transistor has gone
look for anything common to a 12v supply and not further back in the mixer
a quick test using a scope will reviel if there is drive locked frequency post mixer
to the driver transistor
the 12v usualy is common to both the pa and the driver
and often there is a series diode in the p.a somewhere to previent this type of damage
ive found this problem to be a burned choke usualy a white box or blob type
with two dots 2pin choke in series with the 12v supply to the driver and or pA.
and it when fried dumps any amplified rf to the 12v rail and not to the pa
so there is no drive and no output
you can pull the top cover off the white box type chokes and check the colour of the copper coil see if its black or gone pale gold colour then its fried
very often most transistors will survive reverse polarity
and youll get a burned track or chock or a regulator
again check the 12v rail at the pa and press TX to see if it dips
this will show that there is not enough avalible current and a regulator
usualy used to control power output is damaged
let me know here how you get on
i cant find the diagram or i would help some more