There have been a few instances of improper VOX operation when the MC-50 microphone is used. The sensitivity of the VOX-3 may be increased by removing R-4 and connecting R-3 directly to ground.
Also install a 220mfd 25Vdc electrolytic in parallel with C-6.
(These parts are all on the X54-1010-00 board.) If the VOX delay is insufficient, change C-10 (on the X54-0001-00 board) from 3.3mfd 50Vdc to a 10mfd 50Vdc capacitor. Shown below is a modified schematic showing these changes.