User's Manual

3-5
3.7 WIRING INSTRUCTIONS
Figure 3-2 shows all required connections and recommended wire sizes for the TFM-500
Transceiver. If problems with the correct operation of the UHF/FM Transmit function of a
TFM-500 are encountered on a specific airframe, a DC power line filter may be required.
Typical problems encountered are that UHF/FM will not transmit on high power or will not
open a repeater when using a CTCSS transmit tone. Investigation has determined that once
the ripple on the airframes DC (28V) power line becomes excessive the UHF transmit
function and tones will distort. If the airframes generators are turned off and the UHF/FM
transmit function works correctly from 28 Vdc battery power, the ripple on the DC power
line is excessive. The use of DC power line filter PLF-250, p/n 021214-1 is recommended
and is available from Technisonic.
3.7.1 Main Power +28VDC
The main power +28VDC (±15%) is connected to pins 7 and 14 of the transceiver. Both
pins should be connected.
3.7.2 Main Ground
Ground connections are made on pins 8 and 15. Both pins should be connected.
3.7.3 PTT (Ground Keying)
The PTT line is connected to pin 13 and should be floating when the transceiver is in
receive mode, and grounded during transmit mode. The PTT2 input is on pin 2 of the 9 pin
connecter.
3.7.4 Front Panel Back Lighting
Front panel back lighting connection should be made on pin 3 of the transceiver. The
opposite end of this lead should be connected to the panel lighting system of the aircraft.
Before connecting, verify the required panel lighting voltage (28 VDC or 5VAC) on the
transceiver configuration control label.
3.7.5 Audio Outputs (600 ohms and 4 0hms)
There are two audio outputs available (1 & 2). 600 ohm audio output 1 has audio from
both bands and the guard in single operator mode. When in dual operator mode, 600 ohm
output 1 has audio from the VHF band and guard only while the UHF audio will be on the
600 ohm output 2.
3.7.6 Audio Output Ground
Pin 10 is the ground for both the 4 ohm and 600 ohm audio output signals.
3.7.7 Mic Signal Input
The microphone input signal is to be provided on pin 6, utilizing shielded wire with the
shield grounded to pin 10. Microphone signal 2 is on pin 6 of the 9-pin D connector.
3.7.8 Memory Up/Memory Down
Remote scrolling through the 200 memory positions can be achieved by providing a ground
to pins 4 (up) and 5 (down) through a momentary contact cyclic switch. The memories will
scroll only on the band selected.