There is a 2-3 dB difference in mic station level between older revision and newer revision 508V/H units.  The boards in the unit themselves are calibrated at the factory to have the exact same gain, but when a microphone is plugged in it may output a higher or lower level depending on the revision of 508 it is plugged into.  This is because on the older mic stations, the base unit supplied about +/-15V to the microphone (i.e., to the pre-amp in the microphone), whereas on newer mic stations, the base unit supplies about +/-12V to the microphone.  This causes the microphone to put out a slightly lower signal. 

If a facility has all of the same generation of mic station, there is no problem as zone levels were set to accommodate whichever level is coming out of the mic stations.  However, if a facility has a mixture of mic station revisions, the difference may be noticed.

To compensate, one can carefully adjust the mic station base unit gain to compensate for the difference in levels, i.e., either turn down the older units or turn up the newer units.  This is done via a potentiometer on the 508V or 508H board.  This board is the one with the keypad buttons mounted to it.  Most boards have have only one pot on them.  If there are two, it is the one closest to where the beeper is mounted on the opposite side of the board.  On the 508H board is is on the bottom of the mic station as assembled. 

To increase the gain, turn this potentiometer Clockwise.  It has been found that to compensate for the difference in mic station generations, it is only necessary to turn the pot 1/8th of a turn.  Since the potentiometer has an X in it (two crossing slots for adjustment), this means changing the pot from showing an X to showing a + (or Cross) or vice versa.