(2) When keying the intercom from a crew member's or driver's C-2298
(Figure 7), a ground is applied through the handset to the 22-30 volts DC at
J803 pin C of each crew member's or driver's C-2298. Current flows through
the circuit, which causes the K501 and K502 to energize, keying the AM-1780.
(Trace this circuit on your system diagram.)
The CR521 will block the keying circuit to relay K503.
Intercom keying circuit (crew member and driver).