EFJohnson 761X, 764X service manual Receiver Adjustments Vhf Models, Vhf Adjustments Cont’D

ADJUSTMENT PROCEDURE

VHF ADJUSTMENTS (CONT’D)

2.Select the low power channel in the middle of the band (Test Ch. 5 - 160.050 MHz). On the computer screen, scroll to “MOD N” if setting narrow band deviation or “MOD W” if setting wideband deviation.

3.Monitor the transmit deviation with a communica- tions monitor set as follows: HPF = Off, LPF = 20 kHz, De-Emphasis = Off, Detector = (P–P)/2.

4.Key the transmitter using the test cable switch and

set the following maximum deviation by pressing the adjust keys (←/→, PgUp/PgDn, or spacebar/

backspace). Unkey the transmitter.

Wideband (30 kHz) Models - 4.2 kHz Narrow Band (12.5 kHz) Models - 2.1 kHz

5.If the transceiver operates on both narrow and wide band channels, select Test Ch. 7 and also adjust the deviation on that channel (see note in Table 6-1).

6.4.4 DTCS WAVEFORM ADJUST

1.Select a channel in the middle of the band pro- grammed for DTCS (Test Ch. 8 - 160.050 MHz). On the computer screen, scroll to “DTCS N” if setting a narrow band channel or “DTCS W” if setting a wideband channel.

2.Key the transmitter and view the demodulated sig- nal on the CRT of a communications monitor.

3.Press the adjust keys on the computer so that the waveform appears as follows:

Set For

Flat

Waveform

4.If the transceiver operates on both narrow and wide band channels, select Test Ch. 8 and also set the waveform on that channel (see note in Table 6-1).

6.5 RECEIVER ADJUSTMENTS (VHF MODELS)

6.5.1 BANDPASS FILTER ADJUST

1.Select the channel on the low end of the band (Test Ch. 2 - 146.050 MHz).

2.Connect an RF signal generator to the antenna jack. Set the output for the channel frequency at a level of 3.2 µV (–97 dBm), modulated with 1 kHz at the following deviation:

Wideband (30 kHz) Models - 3.5 kHz Narrowband (12.5 kHz) Models - 1.75 kHz

3.Adjust the filters automatically or manually as follows:

Automatic Adjustment Method 1

(Adjusts all filters)

a.Select “BPF T1” on the screen and adjust for “0”. Repeat for T2 – T4. Reselect “BPF T1”.

b.Press the F9 key and all filters are automatically adjusted for peak levels.

Automatic Adjustment Method 2 (Adjusts only one filter at a time)

a. Select “BPF T1” and press F8 to automatically adjust it for a peak level.

b. Repeat for the other three filters.

Manual Adjustment

a.Connect a SINAD meter with a 4-ohm load to the external speaker jack (see Figure 6-4).

b.Select “BPF T1” and press the adjust keys ( ←/→, PgUp/PgDn, or spacebar/backspace) to obtain minimum distortion.

c.Repeat for the other three filters.

6.5.2 SQUELCH ADJUST

NOTE: The squelch level can also be set from the front panel as described in Section 3.3.6.

1.Select the channel on the low end of the operating band (Test Ch. 2 - 146.050 MHz).

2.Connect a SINAD meter with a 4-ohm load to the external speaker jack.

November 1998
Part No. 001-7600-001	6-6