1-17. POWER SUPPLY SIMPLIFIED BLOCK DIAGRAM
The power supply circuitry in the modem may be divided into two main categories: the main chassis power supply and
the special purpose supplies that are located in scope module A2 and loop battery module A5. The power supply
circuitry in the modem consists of circuit breaker CB1, a transient suppressor, a preregulator, a +20 volt regulator, a
scope high-voltage power supply, and two dc loop current supplies. The power supply circuits convert an unregulated
input of a nominal +27 volts dc into a transient-protected, regulated +22 and +20 volt dc supply for operation of the
modem circuitry. Unfused, unprotected +27 volts dc (nominal) is available to auxiliary equipment by an internal
connection in the modem.
Main Chassis Portion of Power Supply . Circuit breaker CB1 serves as an on-off switch and protects against long-
term overloads. The +27 volt (nominal) input is applied to the transient suppressor, which protects semiconductor
components against damage from short duration high-voltage spikes caused by switching transients from other units
connected to the common +27 volt source. The transient-protected +27 volts is applied to a preregulator. The
preregulated +22 volt output operates relays, powers the special purpose power supplies in the A2 and A5 modules, and
operates the +20 volt regulator, located in receive audio module A1. The +20 volt regulator in turn supplies power to the
five modules (A1 through A5) plus meter bias board A7 and the input isolation amplifiers, also located on board A7.
Special Purpose Power Supplies . The scope high-voltage power supply converts the preregulated +22 volts dc into
+1,000 volts dc for operation of the scope module crt. Dc loop 1 and 2 power supplies convert the preregulated +22 volts
dc into a +127 volt dc power supply to supply current for dc loops 1 and 2.
1-18. SIMPLIFIED METERING AND MONITORING CIRCUITS
The operating condition of the modem can be checked with front panel meter M1, by observing the crt display, or by
audio monitoring with an external monitor speaker or headphones.
Metering. Dc loop current for either loop, center frequency of the received signal centering (DISCRIMINATOR
position of S3), +20 volt regulator output, and the level of the receive input to the MD-522A/GRC may be read from
meter M1. The circuitry associated with the meter contains sampler resistors, bias resistors, and detector diodes
necessary for proper meter readings in the various positions of METER FUNCTION switch S3.
Scope Display. Mark-space signals from the frequency discriminators in tty channel 1 or 2 are applied to scope
module A2. Because of the phase relationships in the frequency discriminators, the major axes of nsk-mark and space
ellipses are at an angle of 30° and the major axes of fsk-mark and space ellipses are at an angle of 90° When the
center frequency of incoming tty tones is off frequency (because of a badly tuned receiver, misadjusted beat-frequency
oscillator (bfo), or off-frequency transmitted tty tones), the mark or space ellipse will reduce in size, or disappear entirely.
This indicates that the incoming signal is so far off frequency that one of the tty tones cannot pass through the frequency
discriminators. In diversity operation, where the stronger of two signals is selected, scope module A2 automatically
switches to display the stronger signal.
Monitoring Through Speaker or Headphones . The receive audio (tty tones and/or voice) input to the modem can
be monitored through headphones or speaker in all modes, except 85 Hz plus voice. In this mode, tty tones are filtered
out before amplification of the voice signal.