Mic causing hum when connected

[Lkaskel]

Hi Everyone,
So, I'm in the prcess of getting some mics back to working condition. I have a Turner +3 that I recapped (just the electrolytics), cleaned the contacts and installed a new 4 pin connector. When I plug it into the radio (Cobra 139XLR) I get a mild hum from the radio. I have another working +3 as well as a +2 that does not cause this. In fact, this is the only mic that causes the hum. The radio will receive and transmit with this mic. I did check the shield to be sure it was intact and it looks good.

Any thoughts?

 

[KneeBiter]

Hi Everyone,
So, I'm in the prcess of getting some mics back to working condition. I have a Turner +3 that I recapped (just the electrolytics), cleaned the contacts and installed a new 4 pin connector. When I plug it into the radio (Cobra 139XLR) I get a mild hum from the radio. I have another working +3 as well as a +2 that does not cause this. In fact, this is the only mic that causes the hum. The radio will receive and transmit with this mic. I did check the shield to be sure it was intact and it looks good.

Any thoughts?

Do you have a hum on tx side as well? Dead key and listen on another radio.

 

[nomadradio]

Last time I ran into this it was caused by using an unshielded wire for the audio. Turned out someone replaced the mike cord, copying the wire colors exactly. Just one problem. The shielded audio wire was red on the old cord, but white on the new one. Rewired both ends of the cord to feed the audio down the shielded wire and the hum was magically gone.

Maybe this problem won't be that simple, but it's worth a look.

73

 

[Handy Andy]

Mic wiring...4-pin - similar and uses Uniden pinouts.

Pin 1 Shield (all grounds go here sort of thing)
Pin 2 Audio
Pin 3 TX - Shorts to Pin 1 to turn on TX
Pin 4 - RX - Speaker "-" shorts to Pin 4 to enable speaker.

IT doesn't do that but Pin 1 and 4 short all the time at the socket. (Think PC122 - Galaxy - only needs 3 pins to work, but uses 4.

Now there is a "thru the mirror" error in the schematic if you get this from CB Tricks - I'm not going to change it, because it can be viewed in a perspective of mirror imaging...

Thru the looking glass, behind the radios face - you can see the Mic Jack is oriented with Pin 1 to your lower left at the index notch...where the wires are soldered side - male pins sticking out but away from you.

Same can be thought of thru the view perspective of the female end on the PLUG side as well.


The pin for pin is correct, just verify the wires for the function is correct.

 

[Lkaskel]

All good suggestions so thanks!!! I did verify the wiring of the plugs but I did find that the shield was corroded green and that coorsion went down into the cord. I am wondering if the shield is partiall broken. I have ordered some new mic cord and when it arrives I'll hook it up and let you know.

Thanks as always!!!

 

[Handy Andy]

Remember the cord is stranded braid. So that means the braid strands don't go all the way thru to be soldered - so the shield conductivity fails wherever those strands that got corroded - stop and they don't always electrically reconnect onto another strand of wire.

So there's that reactive element with the corrosion too...

 

[Lkaskel]

Last time I ran into this it was caused by using an unshielded wire for the audio. Turned out someone replaced the mike cord, copying the wire colors exactly. Just one problem. The shielded audio wire was red on the old cord, but white on the new one. Rewired both ends of the cord to feed the audio down the shielded wire and the hum was magically gone.

Maybe this problem won't be that simple, but it's worth a look.

73

This is VERY interesting!!! I was looking at the mic tonight getting ready to install a new mic cord. I noticed that the shield was wrapped around the red wire not the white. Everything about this mic tells me that this is from the factory and not that this has been replaced. Tomorrow I am going to swap the red to be audio on the plug and as well as on the contacts. We shall see.

Thanks!!!

 

[Lkaskel]

This is VERY interesting!!! I was looking at the mic tonight getting ready to install a new mic cord. I noticed that the shield was wrapped around the red wire not the white. Everything about this mic tells me that this is from the factory and not that this has been replaced. Tomorrow I am going to swap the red to be audio on the plug and as well as on the contacts. We shall see.

Thanks!!!

Ok, so this issue is fixed. I rewired it so the red wire was audio and no more hum. After working on it I think that someone may have replaced this cord in the past. They did a great looking job so there is that. One other thing. This mic is actually labeled as a Midland mic but is just a relabled +3. It's off white and the cable is a very light gray so it looks perfect together. I do have new mic cable that was suggested by KneeBiter https://geistnote.com/gotham-audio-gac-3-5-8mm-o-black-or-brown-sold-per-foot-not-terminated/. This cable is not coiled but it is VERY nice cable and very reasonable cost. I guess I will use it at some point.

Thanks again!!

 

[AudioShockwav]

Ok, so this issue is fixed. I rewired it so the red wire was audio and no more hum. After working on it I think that someone may have replaced this cord in the past. They did a great looking job so there is that. One other thing. This mic is actually labeled as a Midland mic but is just a relabled +3. It's off white and the cable is a very light gray so it looks perfect together. I do have new mic cable that was suggested by KneeBiter https://geistnote.com/gotham-audio-gac-3-5-8mm-o-black-or-brown-sold-per-foot-not-terminated/. This cable is not coiled but it is VERY nice cable and very reasonable cost. I guess I will use it at some point.

Thanks again!!

Some of the " precision series" Midlands had the re- labeled Turner +3 mics.
They also included built in slide mounts on the sides of the radio, real nice rigs.

73
Jeff