Cobra 139xlr 40 cn SSB no audio, carrier ok

Sounds like you may have shorted out the audio amp ic4, or maybe t1 transformer. Can you hear yourself talk through the pa ?

 

I see you posted this on radioreference.com

Well I have a couple questions for you.

Do you have instructions for the install?

Which echo board is this, there are a few.

I doubt very much if you blew anything up in the radio, it sounds like it wasn't wired right.

 

I’m not a big fan of Echo boards, but it looks like the Delta 2 is fairly simple to install. The red wire goes to 12 V of the power jack, the black wire goes to the chassis ground. Now these boards have two different grounds. They have a PCB ground, and a chassis ground. The two ground wires on mic in and out should have went to pin one on the mic jack. I believe the 3300 microfarad capacitor is the filter for the power supply. I’m not sure but maybe hooking the 3 ground wires together damaged the delta board?, and in turn the delta board did something to the audio amp??? Or maybe you put the ground leads to the positive side of C304? I believe pin 4 is the input to your audio amp. With the radio powered on, volume up, rf gain up, squelch down, and no antenna hooked up. You should be able to touch pin 4 with your finger and hear a buzzing sound if the audio amp and circuit is working.

 

After re-reading your post I just realize you said you used earbuds, and you heard a hum. Don’t use earbuds, that’s a good way to hurt your ears. Use a good working external speaker. I assume the hum you heard was when The radio was in PA mode? Also start with the mic gain turned way down, and as you’re speaking in the microphone slowly turn the mic gain up to see if you hear yourself talking through the external or internal speaker.

Also when you checked the audio chip pins with your finger for a humming or buzzing noise, you may need to use an external speaker just in case the internal one went bad. You may also need to check for cold/broken solder joints, you can use a pencil and with the eraser end, push on the pins to see if you get any sound or not. If audio transformer T1 got shorted out, it would be real hot to the touch or the electrical looking tape around the transformer will be like shrink wrap, and be real tight against the wires. You may want to check, I believe it’s pins 9 and 10 for 10 to 12 Volts also.

I know this may seem like over kill to some, but since we don’t know what exactly happened, and it’s pretty tough to troubleshoot over a forum. So it’s best to check out the basics.

 

Yes, use your finger to touch the pins on the IC audio chip itself (the soldered side of the board). I couldn’t find a schematic on that audio chip, but I believe it’s actually pin number 1, that should have the supply voltage that could could be as low as 5 Volts. PIN number 10 should be the audio output pin. If you have an oscilloscope, it would be really easy to check the output pin to see if you have any modulation coming out the audio chip.

I haven’t worked on a cobra 139, but if it’s like the other cobras,
the number 1 pin should be the pin closest to the front of the radio.