When a 2 channel belt pack (like a RS-702 beltpack) goes into oscillation (howling) make sure that BOTH channels are terminated.
Clear Com 2 channel belt packs have an amp for each channel. Both channels are powered by the same leads, +30 V on pin 2 and ground on pin 1. Therefore both amps are up and working whenever the belt pack is plugged into a live intercom circuit. If you only require 1 channel of the belt pack the other needs to be terminated. There are 2 ways to do this. The easiest is to short pins 3 and 4 of the 6 pin XLR together or to take either pin 3 or 4 (the unused of the 2 channels) and terminate it to ground through a 5,000 ohm resistor.