b. Possible bad equipment. If during step 1d and 2, the AM-1780 relays
click (when the key switch on the handset is pressed and voice sidetone is
heard when speaking into the microphone) the intercom is receiving operating
voltage. This indicates the AM-1780 power lamp is burned out. However, if
the AM-1780 relays do not click and voice sidetone is not heard, there is no
intercom input power.
Trace the intercom DC input power circuit and identify the
possible bad items of equipment (Figure 28).