Under some conditions, use of a phone patch with a speech processor may cause a feedback condition. This condition results if the transceiver still has a very small amount of audio present at its output in the transmit mode. This can be caused because of less then 100% muting, or because of RF rectification in the receiver audio amplifier. Although this condition may be completely unnoticeable in a speaker, when connected to the phone patch, and with the added gain of the speech processor, a feed-back condition can result.

This feedback condition can be eliminated in one of two ways. In most cases using the phone patch without processing will eliminate the condition. The SP75 input switching can still be used to select the phone patch but the SP75 ON/OFF switch can be left in the OFF condition. 

If processing is desired for phone patch operation, the muting of the transceiver must be improved. This can be done without modification to the transceiver itself by adding an additional relay to break the audio output (speaker) line. The relay can be controlled from the linear amplifier contact of the transceiver as shown in the schematic below.