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Cobra 148 Repair: Squealing on transmit | Radioaficion Ham Radio

Cobra 148 Repair: Squealing on transmit


June 3, 2003          Cobra TECHNICAL SERVICE

Cobra CB Radio MODEL(s):   148GTL, 148NWST

DESCRIPTION: Squealing on transmit.  Possible causes:
•  High SWR
•  Inadequate ground
•  Microphone sensitivity of radio is too high.  Specification:
−  From 1 to 2mV for 50% modulation with Soundtracker off
−  From 0.5 to 1mV for 50% modulation with Soundtracker on

ACTION: One of the following suggestions should solve the problem:

1. Check the SWR of the antenna, make sure it is below 3.

2. Connect a ground wire to a screw on the side of the radio, to ground the

chassis of the radio.

3. Remove the top cover plate and tighten all the screws around the edge of the

circuit board.

4. If the radio still squeals, the microphone sensitivity of the radio is too high.

Change R126 to a 12KΩ resistor.  

If R126 was changed, check maximum modulation.

NOTES: Procedure for checking microphone sensitivity.

Checking Microphone Sensitivity

1. Connect a calibrated audio source to the microphone input.

2. Connect a modulation meter to the antenna connector.

3. Turn Soundtracker off.

4. Adjust the audio source for 50% modulation.

5. The level of the audio source should be between 1mV to 2mV.

If the audio level is below 1mV, increase R126.

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