The PLF-250 Power Line Filter was developed by Technisonic to provide an extra ordinary level of power line conditioning to counter excessive 28 Volt DC ripple resident in some airframes. Investigation has determined that if ripple on the aircraft 28 Volt DC bus becomes excessive, the UHF function will be inhibited and CTCSS tones will become modulated (distorted) by the noise. This filter will correct problems relating to the operation of the Technisonic TFM-403, TFM-500 and TFM-550 in the transmit mode. Typical difficulties include no transmit in hi power mode, or unable to open a repeater using a CTCSS transmit tone. If you encounter either of these difficulties, confirm the condition by operating the radio(s) with the aircraft generators switched off. If the radio(s) operate normally on battery power only, the use of the Technisonic PLF-250 filter is recommended. The Technisonic part number for the PLF-250 is 021214-1.