Customer has a Tempest 2400 system in use at for stage productions, They connected it up to an existing Clear-Comparty-line system (RM-704 is where the Tempest base is patched in).Channel B sounds fine from the beltpack,Channel A has a very noticeable delay / echo when they switch to thatchannel.   Asked customer if they had nulled the tempest base station afterconnecting the base to the existing PL system?, The response was "What is nulling and no we didn't dothat". 
Showed customer how to nulling the PL circuits on the Tempest basestation on both PL channels, after nulling the two channels customer reportedthat the echo was gone and it sounded great. 
When connecting any equipment to a PL system , if that new prodcut has a nulling circuit then you should null the newly connected product to the existing PL system to reduce echo, distortion and feedback
To Null the PL circuits on a Tempest basestation you can use the auto-nulling feature on the Tempest -ensure the PL circuit connected to the Tempest basestation is powered by the external PL system
Auto-Null Procedure-Turn OFF all TALK buttons on both the wired and wireless systems. Since Tempest monitors a self generated tone to adjust the null characteristics, any sounds entering through either the wired or wireless intercom systems will interfere with the nulling process. -When the Auto-Null or Manual Null menu screens are accessed, Tempest sends a Mic Kill signal to all wireless BeltStations, but the wired communication stations will need to be turned OFF individually, or a hardwired Mic Kill signal will need to be initiated. -On the Tempest BaseStation press MENU to open the Main Menu. Rotate the Volume Control to scroll through the menu options to the “Wired Intercom Settings” and select “Auto-Null.” Select “Null Now” to start the Auto-Null process. -Normal functions will be interrupted for about 10 seconds during the Auto-Null process. Progress of the null process can be observed on the BaseStation LCD display.-The BaseStation display will indicate “Auto-Null Complete” once the Auto-Null process is completed. Press MENU to escape to the Operational screen.-Activate Auto-Null for new connections or whenever the 2-Wire system changes significantly, such as when additional wired belt packs are added or removed, or when cable lengths are changed significantly
During Auto-Null, wired intercom users will hear the nulling tones and communication will be interrupted. It is recommended that this feature should be used only during setup and not during live events