Cobra 140 GTL microphone woes...

[doffo]

Hello fellow CBers!

Last night got my two Cobra 140 GTL radios off the shelf, and it was shortly after I noticed my mic's audio being way too loud.... This was using one of those 5 pin to 4 pin adapters. I tested various 4 pin microphones and all did the same exactly thing. Normal talking and it was too loud to the point of distortion.

I found my 5 pin Uniden microphone from a Grant XL radio, and that microphone sounds great! No distortion and no clipping. Audio sounds normal. I did notice the 142 GTL has a Dynamike adjustment whereas its missing from the mobile version. I followed the wires and noticed the area it went right into on the board. On the mobile version, it just goes straight off the microphone jack to the board. It got me thinking, is the way the 4 pin to 5 min microphone adapter wired up the issue? The two pins for Common and Shield are tied together. Surely doing that wont upset the radio and make it too sensitive and over modulate?

Only thing I can think of trying would be is to put a potentiometer in the spot and try to see if adjusting it will back off on the sensitivity from the mic.

Anyone else had this issue? Thanks for reading. Have a great day everyone.

-Doffo 898

 

[brandon7861]

So I had just deleted my post thinking I gave bad advice, but I am not sure now. Maybe it was good advice.

I suggested checking to see which radio pin the adapter uses for ground and made the recommendation to use pin 2 only as a ground for both the PTT and mic so that the mic audio still utilized the RF choke that is between pin 2 and ground, but I second-guessed myself because in RX, the audio must also pass through that choke.

After a moment of thought, I don't think that choke will be an issue at audio frequencies and should be heavy enough to pass a few watts of audio, so I think that suggestion is still valid.

 

[nomadradio]

I sell an overpriced solution to the 4-pin mike/5-pin radio dilemma.

https://www.ebay.com/itm/1165841599...pid=5336136228&customid=&toolid=10001&mkevt=1

The original 5-pin socket comes out, and the five wires get soldered to this board, along with a power wire for the relay it uses. The four-pin mike really only needs pins 1, 2 and 3. The radio will receive with the mike unplugged now.

It serves to keep the original pin 2 ground circuit isolated from the pin 4 ground circuit.

Of course, people respond "ground is ground, why two pins for it?"

Pin 2 carries only the tiny audio current from the mike cartridge. Pin 4 carries the brute-force switching voltage for transmit, and the high audio current of the speaker ground for receive. Those currents create a voltage drop. That voltage won't be much, but the mike input is sensitive to literal microVolts of audio input. Keeping the two ground circuits apart prevents them from interacting. Noise and feedback are what that will usually cause.

For years we simply added a relay inside the radio when someone had a roger beep that lacked its own relay. The five-tone roger beep is one of these. If you don't do that, your roger beep will come blasting out the speaker. The relay keeps the speaker muted until the beep finishes and drops the transmit side. The "just add a relay" solution created a side effect. A loud "POP!" in the speaker when you unkey. Figured out that adding a small delay to our board's relay would mute the pop, the same way the switch inside the hand mike will do.

It's the gold-plated solution, but if you fancy a double roger beep or the five-tone it will simplify the install.

The external adapters are a band-aid. They either use just pin 2, or just pin 4, or tie both together for the ground connection. One method causes feedback squeals on transmit. The other one makes the receiver audio howl in sideband receive. If you turn down the volume control that usually stops it. The third option tends to get you both symptoms.

When the 5-pin radios first appeared around 1979 this caused great consternation from folks who insisted on using their old 4-wire mikes. I would explain that they need "new" mike with a six-wire cord. Mostly we just tied pins 2 and 4 together and soldered the four-wire mike's ground braid to it.

But it's a dice roll. You can wire a 5-pin mike plug with that jumper from 2 to 4. No telling what's in the adapter. Could have been made for a totally different 5-pin radio. The Uniden HR-2510 comes to mind.

Might work okay. You never can tell.

73

 

[doffo]

Appreciate both of you chiming in.

That explains why it sounds overmodulated/horribly loud on the Cobra 140 GTL to the point of distortion. The adapter seems to be ok with the two pins shorted out on a Cobra 148/Uniden Grant XL, but at least you can back off on the mic gain on those radios. Thanks for the link Nomad. I did add it to my wish list.

 

[Lawrence lin]

You're probably right about the adapter — mixing Cobra and Uniden pinouts without the right wiring can cause exactly that over-modulation.<br><br>Quick test: try your Cobra mic with the adapter. If it does the same thing, adapter's your culprit.<br><br>The Dynamike on the 142 exists for this reason — mobile version doesn't have it, so whatever the mic puts out goes straight through. A small pot (500ohm) in series with the mic line could work as a fix.<br><br>Let us know what you find!