Feedback in Mic Insert on AM (Cobra 148 GTL)

[Spanish]

Let's dive in.
So i rewired another hand mic as my original mic cord (4 wire) was way too short. Found another longer cord (4 wire), wired it up and now i have this annoying feedback in the insert. (Dynamic at 12 o clock).
Original shorter lead dynamic all the way open.
Also, just to clarify it is wired correctly (New cord).
Pin 1 > Audio
Pin 2 > Screen
Pin 3 > RX
Pin 4 > NC
Pin 5 > TX

Any help would be appreciated.
73
Spanish

 

[BC Coyote]

I would guess the new longer cable is poorly shielded and you are getting RF in the audio wire within the cable as a result. Solution: Use a better quality mic cable with proper shielding. I've head this exact issue many times by the way!

 

[doffo]

This is related on what I was having when I used an adapter to use 4 pin cobra mics to my old Cobra 140 GTL 5 pin socket. Only this time, it made modulation too harsh to the point of distortion.
As Nomad explained, its because the ground of the mic element is separate of that from the common ground of the radio. So you will have to use all 5 pins to make that happen.

You will need to source a cable that has a total of 5 wires. (If it has a braided shield wire and 4 other wires, then that works just fine. Use the shield wire for the Negative side of the Mic element.)

****As mentioned by BC Coyote, use a good cable.

 

[Spanish]

I would guess the new longer cable is poorly shielded and you are getting RF in the audio wire within the cable as a result. Solution: Use a better quality mic cable with proper shielding. I've head this exact issue many times by the way!

Thanks, will ask a buddy of mine to plug in his mic and check

 

[Spanish]

Thanks a lot guys. Makes sense as my Leson has the 5 wires and have no issues.
Will keep u all updated. On the subject about mics. Anyone ever tried doing a mod on the Leson Circuit? My one doesn't seem so sensitive. Read about where they change the 2sc1815's to 2sc945

 

[nomadradio]

Oh no, Mr Bill!

The perennial arithmetic puzzle. Four doesn't equal five.

The Cobra SSB radios had five pins to provide a separate ground pin each for audio and for transmit/receive switching. This served to keep noise voltages out of the mike audio, since pin 2 carries only the mike's audio current.

This leaves you three choices wiring a four-wire cord to a five-pin plug. Connect your one and only ground to pin 2 and get squeal problems in transmit. Connect only to pin 4 and get squeal problems from the speaker in sideband receive. Or bridge pin2 to pin 4 and get some of each.

I sell a technical solution to this on fleabay. It replaces the mike socket with a 4-pin. Contains a relay that handles the separate pin-4 ground circuit without connecting it to the pin 2 side.

https://www.ebay.com/itm/1169779606...pid=5336136228&customid=&toolid=10001&mkevt=1

The only reason this toy exists was for customers who wanted multi-tone roger beeps in this kind of radio. Those boards don't have a relay on them. The result of this is a deafening beep from the receive speaker when you unkey. This gadget cures that problem when using a 5-tone or double beep. It also lets the radio receive without a mike attached. Not the reason we make it, just a bonus.

You could install a SPDT relay inside the radio to handle pins 3,4 and 5 separately from the mike. Odds are you'll hear a loud "POP!" when you unkey. Discovered how to stop this. Delay the speaker being turned back on. Your thumb naturally does this on a stock mike. But a relay inside the radio is too fast. Our gadget provides that delay. Took us a while to explain the reason for the POP!, but delaying the speaker was the fix.

Our gadget is the gold-plated solution, but feel free to experiment.

73