5
Circuit Analysis
Receive Audio Path
Receive audio from the phone line enters through transformer T1. The control for Receive line input, R89, adjusts
the signal level passed to the remaining receive audio circuitry. This signal is coupled through audio gate U9-B to
the compression amplifier formed by U2-A, B and C. The compression amplifier operates by sampling the output
of U2-C through C5 and R32 and then rectifying it with D1 and D2. The rectified DC voltage, which varies as the
receive audio level varies, controls the impedance of Q2. The varying impedance of Q2 controls the signal level input
to U2-B. C6 and R8 control the compressor attack and decay time. U2D and it's associated circuitry form a high
pass audio filter which reduces 60 Hz components by as much as 40 db.
The compressor output is passed to the notch filter through R19. The notch filter is made up of U3, sections A,
B, C and D, and their associated components. It is tuned to the notch frequency by adjusting R39 and R38. When
properly tuned, the 2175 Hz component of the audio signal is reduced by as much as 50 db. The output of the notch
is passed to the front panel volume control through audio gate U9-A. Padded output audio is gated through U6-C
to the receive audio amplifiers U4 and U5. Low level audio for the handset earpiece is provided by U7-D and adjusted
by R96. It is then gated through U6-B, buffered by U7-C and sent to the handset.
Transmit Audio Path
Mic level audio is fed to preamp U7-A or U7-B, depending on which mic is used. Preamplified transmit audio is
routed through audio gate U6-A or U6-D, depending on which mic is used, to the mic level adjustment R75. R75 sets
the signal level passed to the compression amplifier formed by U2. The compression amplifier operates by sampling
the output of U2-C through R32 and C5 and then rectifying it with D1 and D2. The rectified DC voltage, which varies
as the transmit audio level varies, controls the impedance of Q2. The varying impedance of Q2 controls the signal
level input to U2-B. C6 and R8 control the compressor attack and decay time. U2-D and it's associated circuitry form
a high pass audio filter which reduces 60 Hz components by as much as 40 dB.
The compressor output is passed to the notch filter through R19. The notch filter is made up of U3, sections A,
B, C and D, and their associated components. It is tuned to the notch frequency by adjusting R39 and R38. When
properly tuned, the 2175 Hz component of the audio signal is reduced by as much as 50 db. The output of the notch
is gated through U9-D and fed to the phone line driver circuit of U10-A and U10-C. U10-A and U10-C form a push-
pull amplifier to drive transformer T1.
Transmit Mode
When the PTT line is grounded, pin 24 of U17 senses the ground and provides timed logic high outputs from pins
10 and 11 enabling audio gates U11-B and U11-C. U12 simultaneously passes a 16 bit digital word to the monolithic
sine wave generator U12 determining the frequency to be output to control tone amplifier U13. The control tone
amplification is adjusted by R80. Amplified control tones from U13-B are fed to the phone line driver circuit of U10-
A and U10-C. U10-A and U10-C form a push-pull amplifier to drive transformer T1. Simultaneously, pins 12, 13 and
14 of U17 are manipulated to control transmit and receive audio gating through control lines 1, 2 and 3.
Parts List
TR Series main PCB #700-DRBB-200 Rev. B
Reference
Description
CPI Part #
CAPACITORS
C5, 6
1uF Elec Rad
208-4052-105
C7,14,27,37,39,40,44,48,53,60
.1uF Mono Cer Dip
208-0092-104
C8
470pF Cer Disc
208-0002-471
C9
.47uF Elec Rad
208-4052-474
