I have a Cobra 148GTL. TX light changes and carrier is present but there is no modulation. Checked microphone appears to be working. Replaced Capacitor C176 with a 16v 47uF identical capacitor (in the audio amp stage). Problem affects both SSB and AM. Receive works great. Carrier activates other radio just can't here voice.
The problem is most likely the audio ic . open the radio and see if the diode 44 is blown or see if you see anything that looks like it got hot around the plug area. if not contact me and i will buy it. is it an old or new 148 gtl.Read more
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It might not seem obvious but on many, if not all, current CB radios have separate wires coming from the microphone for RX and TX. It seems like the mic wire for TRANSMIT may be open. It is a common failure.
If the relay contacts are closing. You may have blown the output transistor, however a broken wire in the microphone can also create the symptoms you describe. .
you can check by using a #47 light bulb soldered to a PL259 connector as a dummy load. If it lights, the carrier is present, but you may not have voice modulation which will show as differences in light intensity as you speak. If no modulation, the microphone is the first thing to look at.
The relay contacts may also be very pitted or corroded and not achieving continuity.
That's usually a defective amc stage that produces RX audio and TX modulation. It's an integrated circuit that will require a tech fixing it. You will have a transmitted carrier with no modulation, and no sound in your speaker. However, make sure you have a good mike plugged in first, and substitute a known good mike to verify it's not the mike. With the WX band working, the above may not apply.
See if you have a carrier...signal going out, but no modulation. Get your Tech to check the mike wiring/circuitry inside the radio, also.
i am having this trouble as well. The ics was my thought too, but I think it is in the modulation circuit somewhere. another article i read said to check this.
"Look for the collector of TR-23, you should read about 4.5 V with a dead key. If you find that this voltage is not present, check on the opposite side of R-120 for 6.5V. Most likely this voltage will not be present; if this is the case, C18 has shorted and needs to be replaced. This is a 330uf, 10V electrolytic capacitor beside L31. To prevent "future" problems, replace with a 16V unit."
As of yet, i havent gotten that far. note this is from the cobra 2000, but they are very much alike. you might cross check the resistor capacitor numbers though.
btw, isnt the mb3756 the pll ic?