CPI TR20 Manuale di istruzioni - Pagina 5

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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

CAPACITORS

C5, 6

C7,14,27,37,39,40,44,48,53,60

Description

1uF Elec Rad

.1uF Mono Cer Dip

470pF Cer Disc

.47uF Elec Rad

CPI Part #

208-4052-105

208-0092-104

208-0002-471

208-4052-474