Section IV. DETAILED BLOCK DIAGRAM ANALYSIS OF TRANSMIT SIGNAL PATHS
This section presents a detailed description of the function of each stage in the signal path. The first half of the
transmit signal path is unaffected by the mode of operation selected by MODE SELECTOR switch S4. To understand
transmit signal path operation, it is necessary to learn the various frequencies generated in transmitter module A3 and
understand their functions. This is discussed in detail in the following paragraphs and summarized in paragraph 1-24.
1-20. SEND DC LOOP
The send loop is a dc circuit that contains tty send equipment in series with the send regulator power supply, internal-
external switch S1, and send oscillator power supply zener diode A5VR6.
Send tty equipment, such as keyboard, transmitter-distributor, or tape reader, causes current in the send dc loop (dc
loop No. 1) to flow or not flow, resulting in time synchronized mark or space pulses. The loop current can be supplied
either internally or externally.
· With A5S1 set for internal power supply current, regulated current supply 1 provides the loop current.
Regulated power supply current 1 consists of dc-to-ac inverter 1, bridge rectifier 1, and current regulator 1. Dc-
to-ac inverter 1, consisting of A5Q3, A5Q4, and A5T2, changes + 22 volts from preregulator A9 into an ac
voltage, which is rectified by diode bridge rectifier (A5A1CR7, CR8, CR9, CR10) to provide + 127 volts dc for
the operation of the send loop. Send loop current is regulated by current regulator 1. Front panel DC LOOP
NO. 1 switch S7 selects 20 or 60 mA regulation.
With A5S1 set for external current supply, current supply 1 is switched out of the send dc loop, and current
must be supplied from a source external to the modem. The external supply may be of either polarity, since
send steering quad A5A1CR11, CR12, CR13, CR14 routes the send dc loop current so that proper polarity is
maintained for the send sensing circuit. Note that the modem does not provide loop current regulation in the
1-21. SEND SENSING CIRCUIT
When current flows in dc loop No. 1, a voltage drop appears across send oscillator voltage supply zener diode A5VR6.
This voltage energizes send oscillator A5A2Q4. The output of A5A2Q4 is detected by envelope detector-doubler
A5A2CR15, CR16, providing positive pulse for a loop current and no pulse for no loop current. Thus, the send sensing
circuit provides a shaped output of proper level for operation of following circuitry. Also, the send sensing circuitry
isolates the dc loop from the rest of the modem, thus preventing interaction of grounds between equipment allocated to
the dc loop and the modem.