CTI Products TSAM Transmitter Steering & Audio Matrix - Manual (Page 70)

TSAM Installation and Maintenance Rev. 2.10 Chapter 8
CTI Products, Inc. Maintenance Theory
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8.2 Transmitter Board Electronics
The TSAM-T1 board contains the following subsystems:
TSAM-T1 Transmitter Board
• Power Supply
• Microcontroller Unit
• Programming Port Interface
• Serial I/O Bus (SPI)
• Logic I/O circuitry
• Console TX audio line receivers
• TX audio switching
• TX line drivers
8.2.1 Power Supply
The TSAM requires a nominal 20-24V AC or 24V DC power source. Power is fed from J101. Fuse F101
and varistor VR101 provide input over-voltage protection. Diode bridge D111 provides polarity
protection when the TSAM is operated from DC. The diode bridge, C235 and L101 provide AC
rectification and filtering for AC operation.
All other system voltages are derived from the filtered DC power source. Two switching converters
generate 5V DC and 12.7V DC. In addition to these voltages, several low current and reference voltage
regulators are used to supply board power. These include:
• -5V volt DC-DC converter
• 5V regulated reference
• 6.3V buffered audio ground references
- 5 V References and 6 V Analog Ground
The -5V supply and 5V reference supply, feed tone generators IC106 and IC107 and mutiplexers IC108
and IC109. The 6.3V audio ground references are supplied by virtual ground buffers IC137 on the
TSAM-T1 board and IC210 on the TSAM-R1 board.
+ 5 V and +12 DC-DC Converters
The 5V and 12.7V regulators are simple "Buck" type regulators. They both operate in a similar manner.
Only the 5V regulator is discussed in detail.
Filtered DC is fed to the 5V regulator (IC138). The IC and catch diode D112 convert the DC into a pulse
width modulated square wave. The square wave is then filtered by L102 and C238. The output is a lower
DC voltage that is proportional to the duty cycle of PWM square wave. This DC voltage is then fed back
to the regulator FB pin (IC138-4). If the feedback signal is lower than 5 volts, the regulator increases the
PWM square wave duty cycle till the voltage reaches 5 volts. If the voltage is above 5 Volts, the regulator
decreases the PWM square wave duty cycle till the output drops to 5 Volts. This change in duty cycle in
proportion to the feed back voltage provides the power supplies regulating action.
Operation of the 12.7V supply is similar. The 12.7 volt supply has additional filtering components L105
and C264. This further reduces switching noise in the output, as required by the TSAM analog audio
circuitry. It also has feedback resistors R162 and R163 that set the output voltage.