Cobra 2000 audio problem with Turner +3 desk mics

I wonder if the squealing mic has a 5 pin connector on a 4 wire cable. When using adapters from 4 pin to 5 pin, a sequel is produced from connecting the ptt ground with the mic audio ground. Worth a look see.

dss56 said:

The handheld stock mic works well and no squeal or noise in the audio when turned all the way up. Both AM and ssb ok.

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Does rather point to a separate problem in each mike.

You didn't reverse the wires on the socket for pin 2 with pin 4, did you? Probably not, since the hand mike works okay.

Using a 4-wire mike presents a math problem. Five doesn't equal 4.

You can connect the mike's ground to just pin 2 and get squeal issues on receive, connect to pin 4 only and get feedback squeal on transmit, or jumper pins 2 and 4 together, and get feedback in both transmit and receive. Some folks have reported that changing C18 to a 1000uf part helps. Could be that C18 is over 40 years old and just not doing the whole job any more.

Check the plug on the Sidekick for continuity between pin 2 and pin 4. Should be an open circuit. If there is any continuity, that's a problem that needs to get fixed.

73

dss56 said:

So i got in a Cobra 2000.gtl it had a roger beep and a train noise toy. I removed them and had to rewire the wires back to the mic jack on the radio.
I have a Turner +3 and a Turner super sidekick desk mics.
both older 5 pin connectors.
Tr24 was removed in radio but I replaced it back removed the noise toys and got radio transmit/receive ok.

The handheld stock mic works well and no squeal or noise in the audio when turned all the way up. Both AM and ssb ok.
When i hookup either turner mics I have to adjust the mic audio on radio to about 1/4 way up and the same on the mic in the AM mode.
If i go past those points a noise in the audio happens and you can see the external power meter move from 5watts to about 9watts.
There is either a buzz or noise in the audio. I tried to add .01 cap from shield to audio on the mic and no change.
If i keep level so there is no noise then you have to talk real loud so you can get some audio out.

I checked all the audio stages and only had to add back the tr25 transistor.
is there some mod to the mics that someone has done to stop the squeal.
Both mics are the black +3s not the gray one, 1 a 4 wire and 1 a 5 wire. Both transmit and receive fine just the noise on the audio when turned up past 1/4 way up.
Thinking of getting a regular +3 gray deck mic and see what happens.

thanks for any replies /help

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I do not see where you are claiming that either one of these microphones worked on this radio, just prior to removing the noise toys. Maybe the problem is your setup will squeal with any power Mike because of poor RF ground?

One loose coax connector can cause this entire problem.

dss56 said:

I tried to add .01 cap from shield to audio on the mic and no change.

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Just my 10 cents, I'm working on theory here, not practice.

Having the cap at the mic won't stop RF picked up on the cord from traveling into the radio. And since the radio already has 1nF caps at the connector, another cap at the mic probably won't help.

That cap would not be shunting the RF to ground anyhow, it is merely shunting it around the mic element and back toward the mic amp. You cannot shunt RF to ground when it is originating on the ground wire, you need to instead block it from travelling back on the other wires. Making a low impedance path for RF to go around the mic element is NOT what you want to do here!

What I am seeing in the 2000GTL schematic is that pin 4 serves as chassis ground for RF via C406 and simultaneously board ground for low impedance RX audio via ferrite bead L405. The audio ground at pin 2 has a more serious inductor forming a low pass filter to pass only mic audio and no RF.

Chances are the inductor in the audio ground path has too much reactance to be passing the low impedance RX audio through it, hence the separate ground paths for RX switching and the mic audio.

By using pin 4 for audio ground, the RF picked up by the mic cable shield and the mic audio are sharing a common ground path, and although it is a short section of wire that is common between the two signal paths, it's enough for the modulated RF to superimpose itself on the mic audio and cause a feedback loop.

The little ferrite bead at pin 1 of the connector has the right idea, but it isn't enough to block the strong RF signals picked up on the shield from getting in. Thats the reason for the low pass filter in the mic ground, to break that loop to RF. Perhaps adding another bead at pin 1 would do it, but I would go with an LC or RC filter in the audio line inside the mic to more firmly block the RF. They did it inside the radio, so why not do it in the mic?

Remember, the goal is to break the RF path in the audio circuit, not to help the RF get through it by bypassing it from ground to the audio wire.