GAI-Tronics IDA1000A DC Remote Adapter Installation and Service Manual - Manual (Page 21)

17 01/05
Theory of Operation
Power Supply
The power supply for the remote adapter operates in the linear serial-pass mode. Programmable Zener U4
serves as reference for the 5 V digital power supply. The reference voltage is 5.00 V dc, +/- 0.1 V dc
measured at emitter Q5. Op-amp U1a controls the series pass element Q5 to provide 5 V dc at the 5 V
source point and the power LED provides indication of this voltage level. The reference voltage from U4
is scaled and isolated through U1b to provide a 2.5 V source.
DC Control Current Detection
Audio and dc control line currents are input to the circuit board through pins 3 and 4 of modular
connector J1. AC current is shunted across the transformer secondary by C1, and dc current is developed
across R1. The dc current is translated to a voltage (referenced to 6 V) by amplifier U5 and fed to U6a, b,
c, and associated components.
If the translated voltage were produced by a line current greater than or equal to 4.4 mA (indicating a
transmit command), then U6-1 would output a high level voltage; if the translated voltage were produced
by a line current greater than or equal to 9 mA, then U6-7 would output a high level voltage. If the
translated voltage were produced by a line current of negative polarity and greater than or equal to 2 mA
(indicating a monitor command), then U6-14 would output a high level voltage.
A high signal on U6-1 forces a low at U7-1 (illuminating LED 2) and U8-1, resulting in a high at U7-4
(after a delay through R71 and C20, C52) which then opens the audio path from the wire lines to the
station, drives Q1(Q6) and thus forces transmission on F1 (in a multi-frequency station), and forces
Q3(Q8) to output an active low (high) on the push-to-talk line. The low at U7-1 disables the receive
audio path from the station when SW1 section 8 is down.
In a 2-frequency station, a high on U6-7 drives Q2(Q7) through NOR gates U9b and U9c and thus forces
transmission on F2. The circuitry consisting of latch U8b and U8c, U8a, U8d, U9 guarantees that the F1
line will be disabled in the event of an F2 command. Switch SW1–5 may be pushed down (with SW1-6
up) to cause the transmit frequency to revert to F1 subsequent to every transmit command; switch SW1-6
may be pushed down (with SW1-5 up) to cause the transmit frequency to revert to F2 subsequent to every
transmit command.
A high signal on U6-14 forces U7-10 low (illuminating LED 3) and U7-13 high, thus enabling transistor
Q4(Q9) to output an active low on the monitor line. If switch SW1-4 is up, the monitor function will
remain enable subsequent to a monitor command and until a transmit command is received. This function
is accomplished by the latch configuration of U7c and U7d.
A low on U9-4 (indicating detection of a line current greater than or equal to 9 mA) illuminates LED 4; a
low on U9-13 (indicating detection of a line current greater than or equal to 4.4 mA) illuminates LED 5.