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How to covert from Mic Needed to RX to Not Needed

Wire Weasel

Wire Weasel

Senior Moment
Dec 13, 2008
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Starting new topic over from a diff. thread in other forum.

Many radios like some Cobras, Unidens, Midlands, Cybernets etc…. require having a mic plugged in for the Receiver to be on. Others like all Galaxy's & more do not have to have a mic plugged in.

Anyone know know how to convert these radios so that you do NOT have to have a mic plugged in for the Receiver to play over the speaker?

The immediate rig in question is a China Cobra 29LTD Classic

Thanks !!!
 
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Find ground on the mic jack and jumper it to the rx pin. This will lock the radio into rx mode. Using a dummy mic plug is best as you can replace it with a mic if you want to talk on it later. You should be able to find a schematic online for either the mic or radio to find which pin is which. Easy peasy.
 
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I gotcha bud. It involves installing a relay, but if you don't mind that, I can help you. I'll draw up a diagram and I'll post it here.


~Cheers~
 
Last edited:
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Here's a picture of the wiring and relay.

You'll want to remove the TX and RX wires that are soldered to the mic jack, as they will be resoldered to the relay as shown. For the common/ground, you can solder a wire on the PCB, or solder an extra wire from the ground pin on the mic jack. Personally, I go right off the mic jack. Solder a wire from the TX pin on the mic jack to the relay coil as shown, and solder a wire from the other coil pin to a 13.8VDC source, either from the power switch or from the power jack in the back of the radio.

This will allow you to have RX without a mic plugged in. This also works great when converting 5 pin radios to a 4 pin mic jack. Also works to directly key an amplifier if you so desire, using the other set of contacts on the relay, if the relay you use has a second set of contacts.

If you have questions, let me know. Hope this helps. :)

1d6d4132-4751-4365-a00b-a6a4eed4c656_zps826cipnh.jpg


~Cheers~
 
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Thanks CK & ET13 !!

Okay, I did the simple jumper as CK suggested. Indeed it closed the RX circuit so no mic is needed for RX - but, in the case of this China 29LTD …. it also created a Talkback condition …. which is unusable with the D104 as all it did was squeal. While I had the bottom cover off I tried a 10uf cap across the speaker and that had no effect on the POP I'm trying to rid myself of in the first place.

EX13 …. in case you didn't see the other Thread in the CB Equip. forum - the problem is I'm using a known working D104 TUG8 on a newer China Cobra 29LTD Classic (and I also experienced this problem on another Uniden-type mobile (Moxon). Radio talks fine but makes a loud POP when you unkey the mic. The pop does NOT go out over the air …. it's just annoying in the shack. 3 diff. working TUG8's & a Turner +3 all do the same thing. None of these mics do this on many other radios here such as several Galaxy's. What the Cobra & Moxon have in common is this circuit design whereby you have to have a mic plugged in to RX; and likewise the Galaxy's etc… do not. So I was guessing there was a commonality there.

So have you run across this Unkey POP situation before and any ideas on a fix ?? Thanks so much for any help !!
 
I have heard of that situation, and I believe it's caused by the relays inside the mics not breaking contact at the same time. Most mics have 2 sets of contacts, one set switches your TX/RX, the other allows the mic audio to pass. If they don't break contact at the exact same time, the result is the loud "pop" sound you get from the speaker in the radio.

The relay method I showed you should help eliminate some of that. It may not entirely disappear, but it should help. Also, another method to possibly help the popping sound is to put a .01uf bypass cap from the RX wire to ground. This will help dump some of the audio from the mic itself to ground.


~Cheers~
 
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Wire Weasel said:
Thanks CK & ET13 !!

Okay, I did the simple jumper as CK suggested. Indeed it closed the RX circuit so no mic is needed for RX - but, in the case of this China 29LTD …. it also created a Talkback condition …. which is unusable with the D104 as all it did was squeal. While I had the bottom cover off I tried a 10uf cap across the speaker and that had no effect on the POP I'm trying to rid myself of in the first place.
Click to expand...

You lost me. I thought you were wanting to use the radio to rx WITHOUT a mic connected. Connecting a mic with the jumper in place is an entirely different matter and will cause problems. Also a 10 uF cap ACROSS the speaker effectively shorts the output of the audio stage. Bad idea.
 
Hi CK … well I did. Many radios, such as Galaxy's … come designed such that the RX plays without a mic being connected. Others like this China Cobra any many Unidens & others require a mic to plugged in for the RX to play. This is the condition that I was seeking to convert. - From a Mic Needed to a NOT Mic Needed. … but yes …. I would then intend to connect mic & talk ;-)
 
Ok. This is why I mentioned about using a dummy mic plug. So that it would be removed in order to plug in a real mic and avoid any issues. Leaving a jumper in place soldered to the mic jack inside the radio and then plugging a mic jn would indeed creat a problem.
 
Wire -
This might be a simpler solution than using a relay
rx mod.png
If there is a positive voltage on TX pin 3 that exceeds 0.7 vdc, then the transistor will turn on (and go into saturation) and the speaker will be active w/o the mic being connected. When the mic is connected, in receive (red lines) the switch in the mic connects pins 2 (gnd) and 4 (RX) bypassing the transistor. When the radio is keyed (green lines) Pin 3 is grounded (turning off the transistor) and the mic switch puts radio into Xmit. You may need to add a resistor between pin 3 and the transistor base, but I don't think so. Any general purpose small signal transistor (like a 2N2222) will work just fine. 73s.

- 399
 
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no conversion should be necessary. a cobra 29 microphone works on a 10 meter radio just fine without any modification.pin number 1 to pin number 4- those two pins connect to give you receive in a Cobra 29. if you don't want that function simply cut the wire from pin number 4.
Wire Weasel said:
Starting new topic over from a diff. thread in other forum.

Many radios like some Cobras, Unidens, Midlands, Cybernets etc…. require having a mic plugged in for the Receiver to be on. Others like all Galaxy's & more do not have to have a mic plugged in.

Anyone know know how to convert these radios so that you do NOT have to have a mic plugged in for the Receiver to play over the speaker?

The immediate rig in question is a China Cobra 29LTD Classic

Thanks !!!
Click to expand...
 

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