AKG C414B-ULS with low output

[PhantomBox]

Calling out to all mic gurus:

I have an AKG C414B-ULS with low output.
Please correct me if I'm wrong: these condenser mics have FETs that need to be calibrated to a certain voltage that polarizes the capsule. If this voltage is too high by just a few volts, it looses low frequency response, and if it's to low, the output is reduced drastically.
There are 1 or 2 FETs that regulate this voltage with a fixed resistor between gate (G) and source (S).
This resistor(s) might have drifted from its original value.

I have the following questions:
Which FETs do I have to check and what is the voltage that should be present?
Does anyone know how to take the PCBs off the frame in order to get to the underside for measuring? I don't want to force anything without wrecking the mic.

Thanks in advance.

 

[sgrossklass]

Not a mic guru, but here are the exploded view and schematic (including voltage test points) that I found on the interwebs. Had to zip 'em since they are b/w jobs that are too big. If you convert them to greyscale, you can easily resample them to 50%, makes them easier to view.

 

[basreflex]

try to find the +62V point on the schematic and check that with a modern digital multimeter. most likely on the gain switch. the schematic has given other points to check: prufspnanung=testvoltage. moisture/dirtleakge could reduce the bias voltage for the capsules and cause reduced sensitivity. replacing preventively all electrolytic capacitors is always a good idea.