Atkinson Dynamics AD-27-6 Series C

The -6 indicates that it uses a 4-wire audio system, with separate paths for 'Send' and 'Receive'.  This allows units to be individually (and privately) addressed from a MASTER control location.

Series 3 ( or C) use a printed circuit board in place of earlier point-to-point wiring.  Is it better ?  My experience is that it is susceptible to vibration damage in earlier units due to inadequate support of the output transistor heat sink assembly.