Daniels Electronics MT-3 Instruction Manual page 11

Ptt output
Pin B24 on the backplane connector is an open
drain output (Q9), which is pulled low anytime
the transmitter is keyed and the synthesizer is
locked. An N-channel MOSFET Q9 is capable of
sinking currents up to two amps and is activated
by Q8, which is activated by the Qualified PTT
signal (JS2-6) of the synthesizer module. The
Qualified PTT signal also controls the LED
ENA line for diode D1 on the front panel board
and enable line for the MT-3 Amplifier Module
(JP1-1).
Ptt Voltage Switching
The PTT voltage switching circuitry is comprised
of transistors Q1 through Q7 and the associated
resistors. The base of Q1 is driven by the output
of U2d, which is the combined PTT signal from
all of the PTT inputs. When the transmitter
is keyed, Q1 is turned off and subsequently
transistors Q3, Q4 and Q6 are turned on.
Transistors Q3, Q4 and Q6 provide three
separate functions:
Q3 Provides the active low signal for the
synthesizer module PTT input.
Q4 Turns on Q5, which turns on the
+9.5 VDC Switched supply.
Q6 Turns on Q7, which turns on the
+9.5 VDC PTT Switched supply.
The +9.5 VDC Switched supply (Q5) can also be
activated by installing jumper J6 or by externally
grounding the TX Standby Line (pins B12 and
Z12). The +9.5 VDC PTT Switched supply and
the +9.5 VDC Switched supply both provide
+9.5 VDC but, depending on how jumpers J6,
J7 and J18 are configured, the transmitter's
standby mode will change.
front Panel Board
The Front Panel Board is attached to the MT-3
FM Transmitter Mainboard and is used to mount
the front panel switch, diode and microphone
connector. The main purpose of the board is to
eliminate a wiring harness for the front panel
components. Jumper J1, located on the rear of
the circuit board, is used to select whether or
not the MIC PTT line activates the transmitter's
TOT. Jumper J2 is used to select whether or not
RX Audio or 13.8 Volt is supplied to Pin 4 of the
microphone.
J1 X position MIC PTT NTO – no time-out-
timer (factory setting)
Y position MIC PTT WTO with time-out-
timer
J2 X position RX Audio enabled to MIC-4 pin
(factory setting)
Y position +13.8 Volts
supplied to MIC-4 pin
MT-3 FM Transmitter Mainboard Instruction Manual
7
Theory of Operation
IM20-MT3TXMN