ZETRON 640 DAPT XTRA, Installation And Repair Manual, Page: 101 / 210

Adjustment Procedures
025-9416 89
Hybrid (BAL R and BAL C) Adjustment
The BAL R and BAL C potentiometers isolate the trunk card audio output (TO TEL) from the
telco line input audio (FROM TEL). The two audio paths are separated into transmit and receive.
This is accomplished by adjusting the resistive and capacitive impedance of the active hybrid
balance circuit. Some adjustment is usually required to compensate for the particular telephone
line impedance to the telco central office. The audible result of proper hybrid adjustment is
elimination of sidetone (microphone audio) heard in the earpiece of the line.
If the phone lines are swapped around or the telco changes the lines, this procedure should be
performed again. The telco CO occasionally “rotates” lines, swapping a different hybrid line
termination circuit for the previously existing circuit. This is common during routine mainten-
ance of the trunk. Since the telephone company may perform such updates unannounced, it is
good practice to readjust the hybrids every 2-4 months.
Note: If the system application uses voice prompts or alphanumeric modem paging, the
hybrid circuit balance is particularly important to eliminate audio distortion and
sidetone (echo) problems.
Follow the steps below to perform the BAL R and BAL C adjustments:
q
1. Remove the top cover of the Model 640 chassis (if not already done).
q 2. Remove the plate covering the trunk adjustment pots.
q 3. Connect the voltmeter to the appropriate pins that extend off the sides of the potentio-
meters. Refer to Figure 51.
q 4. Set the meter range to 2 V
RMS
AC or less.
q 5. Dial (using any touch-tone phone) into the trunk by way of the local telco CO.
q
6. Enter the System Prompt Access PhoneNumber (0200 default in the Model 640 data-
base). The paging terminal responds with 1 long and 4 short beeps.
q 7. After the beeps, enter the SecurityCode (0200 default in the Model 640 database).
The paging terminal responds with 3 short beeps.
Note: If either of the above numbers does not work, enter the Model 640 database to verify
the access numbers. From the main menu, select 1 - System Access Phone Numbers.
Then select 3 - System Prompt Access to view the current settings.
q 8. After the beeps, enter 999. The paging terminal responds with a single beep, followed
by a long pause (approximately 5 seconds). Then a continuous 1000 Hz tone plays.
q
9. Set the AGC KILL switch to the kill position (toward the rear of the chassis). Do
NOT kill the gain control until ready to make actual adjustments, as DTMF decoding
usually does not work with this setting.
q
10. While the 1000 Hz tone is playing, adjust the BAL C pot for a minimum reading on
the voltmeter.
q 11. Likewise, adjust the BAL R pot for a minimum reading on the voltmeter.