Why are my 2 channel belt packs howling?

Why are my 2 channel belt packs howling?
Are you using one of the channels of the belt pack or both?
If you are using only 1 channel of the belt pack please see "Two Channel (2 Channel) Belt Pack Howling" - solution number 00000169

There are 2 primary causes for howling or feedback on wired belt packs. The most common cause is a lack of termination on the line. The next most common cause is a loop between the headset microphone input and the headset speaker output.
To check the terminations:
1. Check the rear of the Master Station and insure that each party line channel has one and only one termination switch set to the on position. If howling continues ensure that each termination is good by turning off the power to Master Station, remove the cabling from the affected channel and measure the resistance between pins 1 and 3 of the main station with the termination switch set to on. It should read 5,000 ohms. If it does not the Master station needs to be repaired.

2. With the system up and running and the belt pack on the system, try turning on the call light from the main station for each channel. If the belt pack's call light for the called channel remains off then the wiring attached to pin 3 or pin 4 of the XLR's is broken and that's causing one of the belt pack’s amplifiers to be unterminated. If another belt pack shows the same symptoms then the problem is in the wiring. If another belt pack is fine then the belt pack is not seeing the termination and needs to be repaired.
If there is no break in that wiring then please see if you can answer these questions:
3. Does the howling occur with the mic on but not with the mic off?
4. Does the howling go away if the volume is turned all the way down?
If either (or both) of those conditions are true, then it is likely that there is some kind of feedback loop in the headset or headset to belt pack wiring that is causing the trouble. (There is a 70dB difference in level between the microphone input and the headset speaker output and if a loop exists [even an inductive one] between those 2 points you can get howling). If either of those conditions are true then look at the wiring of the headset. A couple of possible issues are that the mic hot lead is not shielded from the headset speaker leads, the mic shield might be on the wrong pin of the XLR or perhaps not properly terminated on the ground pin.


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