Telex MCE325 manual Sidetone Null Test and Adjustment, Two-Channel, Four-Wire Mode

Table 4.2 USMB frequency response and level check.

10 kHz	-55 dBu (1.4 mV)	-5.7 ±3 dBu (0.28 – 0.57 VAC)
	–45 dBu (4.35 mV)	2.2	±2 dBu (0.79 – 1.26 VAC)
	–25 dBu (43.5 mV)	2.5	±2dBu (0.83 – 1.32 VAC)

Table 4.3 Mic to line response; 4-channel, 2-wire mode.

INPUT	INPUT
FREQUENCY	LEVEL	OUTPUT LEVEL
1 kHz	-45 dBu	-0.8 ±2 dBu (0.56 – 0.89 VAC)
100 Hz	-45 dBu	-0.8 ±2 dBu (0.56 – 0.89 VAC)
100 kHz	-45 dBu	0.8 ±2 dBu (0.56 – 0.89 VAC)

Two-Channel, Four-Wire Mode

1Remove the 200 ohm terminations from intercom channels 1 through 4.

2Program the MCE325 to be in two-channel, four-wire mode. (See Table 2.3, configuration 6 for switch settings. See Figure 1.11 for names of buttons in standard two-channel mode.)

3Connect 200 ohm termination resistors across the four-wire A and four-wire B channels:

Four-Wire A: J8 pins 2 and 3

Four-Wire B: J10 pins 2 and 3

NOTE Normally, the four-wire outputs would be terminated with 600 ohms for four-wire operation, but 200 ohms are used for testing purposes.

4Set the MIC ON button to on. Set all other front panel buttons to off.

5Connect an AC voltmeter across the Four-Wire A output.

6The signal generator should still be connected to the mic input. Set it to 1 kHz, -45 dBu.

7Activate CH 1 TALK.

8The output level should read –0.8 dBu ±2 dBu (0.56 – 0.89 VAC)

9Connect the AC voltmeter across the Four-Wire B output.

10Turn off CH 1 TALK and activate CH 2 TALK.

11The output level should read –0.8 dBu ±2 dBu (0.56 – 0.89 VAC)

12Remove the 200 ohm termination resistors.

Sidetone Null Test and Adjustment

NOTE This procedure verifies sidetone null operation and optimizes nulling for an ideal 200 ohm line termination. Since actual intercom line impedance may vary from the ideal, the MCE325 may have to be readjusted slightly during actual use.

1Reprogram the MCE325 to be in four-channel, two-wire mode (Table 2.3, configuration 2).

2Reconnect 200 ohm termination resistors across each of the four intercom channels.

3Set the MIC ON button to on. Set all other front panel buttons to off.

4The signal generator should still be connected to the mic input. Set it to 1 kHz, -45 dBu.

5Activate the CH 1 TALK button.

6Monitor the AC signal level at U5 pin 7.

7Adjust R57 (Sidetone 1 on front panel) for maximum null (minimum signal level) at U5 pin 7. The signal should be 0.025 VAC or less.

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