Trucki,
There are seldom serious fault on KM84, I have repaired/refurbished hundred of them over the years, it's mostly mechanical fault, screws as loosened and some time a shorts in a tantalum electrolyte.
But of course, there have been some exceptions, such as the failure of the capsule, bad FET transistor, interruptions in the transformer.
If you have about +10 volts on the FET transistor Drain, probably the amplifier works as it should, and it is said earlier, you can not measure the polarity voltage with a standard "DVM" meter, so if you have the opportunity to try another capsule, you should do this first of all.
However, if you suspect fault on polarity voltage, try changing the capacitors C6 and C10, but probably they are ok.
--Bo