Cobra 142GTL with 4-wire D104 TUG8 Ground Loop

[midnight]

What's the best spot to glom a ground on a Cobra 142 GTL? Can I just unscrew a cover screw and add a pigtail there to ground? I'm having intermittent ground loop problems with the D104 (squealing) when I get closer to the radio. It will do it one night and not the next, with no changes to the mic or radio at all. When I hold the mic and touch the radio simultaneously, the squeal stops, so I assume that giving the radio a true outside ground connection is the solution, along with inline accessories on a common ground.

[cbrown]

Check the shield connection in both the base of the microphone and the 4 pin connector to make sure they aren't broken.

Does the squealing start and stop without you moving the microphone?

[Tombstone (R.I.P.)]

I agree with cbrown. Perhaps a shielding problem in the mic cord, but if the shield connection is broken off somewhere I don't tink that the radio would key up and wouldn't have modulation. I had a D-104 come in with the same problem. Someone had put a different cord on it with different wiring colors. They had to compensate the different colors when they wired the plug. This meant that the white wire didn't have the shield around it, so no shielding on the audio wire. They had the mic wired with the audio wire being another color, but unshielded. They just hit and missed 'till the mic worked. I doubt if that's the problem though. There's a possibility of a polarity problem, Try reversing the plug on the power cord in the wall socket or power strip, however you're plugged in. Maybe your running overmodulated, try not to go over 100%. The amp in the mic might be set too high, just suggestions, a common ground will probably cure the problem, good luck!

Tombstone

[cbrown]

Excellent point, Tombstone. I didn't even think about that, and I had the same thing happen when someone brought in a Turner +3 to me to repair. Someone at some time replaced the stock microphone cord with an aftermarket cord, and they just match the colors, not knowing the white wire was unshielded.

I switch the audio wire to the proper shielded wire, and then pasted the new wiring diagram inside the base.

[Tombstone (R.I.P.)]

Thanks, cbrown. I was making all of the elimination suggestions for removing feed back that I can think of. I just hope he gets rid of the squeal and lets us know about it. Probably something simple? I'm sure that someone else may have the clue?

Tombstone

[Trapper John]

Hi Midnight,

The Cobra 142GTL has a 5 pin mic connector and requires a receive control circuit in the mic.

The T-UG8 stand (4 wire) doesn't have one.

You will need to use a T-UG9 stand (6 wire) which does have the required receive control circuit.

Wiring for the D-104/T-UG9 stand to the Cobra 142GTL:

Pin 1: White (Audio Hot)
Pin 2: Shield (Audio Ground)
Pin 3: Black (Receive Control)
Pin 4: Blue (PTT -)
Pin 5: Red (PTT+)
Yellow - Not Used

During receive the blue and black wires are connected to each other by the 3PDT switch in the neck of the T-UG9 stand (the T-UG8 stand has a 2PDT switch).

When you transmit, the black wire is disconnected and the blue makes contact with the red (through the switch) activating the transmitter and disabling the receiver.

I hope this helps.

73,
Trapper

[Tombstone (R.I.P.)]

The D-104 TUG8 is listed in the Master repair manuals as compatible with that radio. I'm not questioning your information, Trapper. The "L" Pad modification (if the TUG8 board has that capability. A resistor on the audio wire may also help him to get rid of the squealing. A few radios must have this to reduce the mic's output, this eliminates the squeal.

Tombstone

[Sandbagger]

Aug 15, 2010 23:15:50 GMT -5 Tombstone (R.I.P.) said:

The D-104 TUG8 is listed in the Master repair manuals as compatible with that radio. I'm not questioning your information, Trapper. The "L" Pad modification (if the TUG8 board has that capability. A resistor on the audio wire may also help him to get rid of the squealing. A few radios must have this to reduce the mic's output, this eliminates the squeal.

Tombstone

You are correct Tombstone. While the Cobra 142 has a 5 pin mic jack, pins 4 and 2 are internally connected together and both are ground. A standard 4 wire mic set for "electronic" switching will work properly. In fact, it's been my experience that the older 4 wire mics use direct shield to stand grounding where the newer 6 wire mics isolate the audio grounds to prevent shorting the mic if it's accidently touched to the radio chassis of some radios like some Johnsons. But by isolating those grounds, those newer mics are more prone to RF feedback than the older mics are.

I have a 1974 vintage 4 wire TUG8 D104 and a late 80's vintage 6 wire Silver Eagle. There have been radios I've had where the Silver Eagle would squeal, and the older 4 wire mic did not. I eventually modified the 6 wire mic to restore all of the solid chassis grounds inside the mic, and that made the squeal problem go away.

Squeal problems can be tough. But in many cases, the mic squeal is only a symptom of a bigger issue. If you are running an ungrounded "stick" style antenna, it's very likely that the feedline shield is radiating RF which can create a "hot" RF condition inside the shack. Grounding the radio, the antenna, and building a blocking choke (or replacing the antenna with one with radials) can go a long way toward reducing or eliminating RF feedback.

[Tombstone (R.I.P.)]

Yep, yep, Sandbagger. A low as possible SWR and not going over 80 or 90% of modulation will help eliminate squealing and feeback too and plenty of grounding where possible. I've had to put the resistor on the audio wire on quite a few six wire mikes. I had to do the "L" pad thing a few times too. Almost every Navaho I've had has had to have the audio wire resistored also. Over the years I've had way more trouble with the mics that have the bottom key up bar rather than the ones with the switches in the stands. Mainly the push bar in the stand and trouble with the switches in the bottom. I use a Silver Eagle on my Cobra 2000 and have had to replace the switches twice since it was new. I've never had to change switches in the stands.. Astatic should have left out the bar in the base and kept building them with the switches in the stand. I'm out of suggestions unless the squeal problem is due to hacking the modulation limiter in the radio.

Tombstone

[Trapper John]

Refering to the 142GTL owners manual:

Page 18, Alternate Microphones and Installation

For best results, the user should select a low-impedandce dynamic type microphone or a transistorized microphone. Transistorized type microphones have a low output impedance characteristic. The microphone must be provided with a five-lead cable. The audio conductor and its shielded lead comprise two of the leads. The third lead is for receive control, the forth is for grounding and the fifth is for transmit control.

73,
Trapper

[Tombstone (R.I.P.)]

I just thought of the one possibility that we haven't discussed. I've had this happen more than once with a TUP9. If the battery gets low enough, (7 volts and below), the mics would squeal on key up. I don't know if that holds true for the TUG8's or not, but worth checking.

Tombstone

[Sandbagger]

Aug 16, 2010 16:29:15 GMT -5 Tombstone (R.I.P.) said:

I just thought of the one possibility that we haven't discussed. I've had this happen more than once with a TUP9. If the battery gets low enough, (7 volts and below), the mics would squeal on key up. I don't know if that holds true for the TUG8's or not, but worth checking.

Tombstone

You're right, I've had that happen to me as well.

[Sandbagger]

Aug 16, 2010 16:23:15 GMT -5 Trapper John said:

Refering to the 142GTL owners manual:

Page 18, Alternate Microphones and Installation

For best results, the user should select a low-impedandce dynamic type microphone or a transistorized microphone. Transistorized type microphones have a low output impedance characteristic. The microphone must be provided with a five-lead cable. The audio conductor and its shielded lead comprise two of the leads. The third lead is for receive control, the forth is for grounding and the fifth is for transmit control.

73,
Trapper

I know that's what it says, but the reality is that the audio ground and the switching common (ground) are electrically connected to the same point on the chassis.

Trust me, I must have wired hundreds of 4 wire mikes to the 5 pin Cobras (and Presidents) over the years and I've never had an issue.

[cbrown]

Aug 16, 2010 19:18:25 GMT -5 Sandbagger said:

I know that's what it says, but the reality is that the audio ground and the switching common (ground) are electrically connected to the same point on the chassis.

Sandbagger is correct. I've run a jumper from pin 2 to pin 4, but that's basically for looks and isn't required. On a 4 wire microphone you can just skip using pin 4 (switch common).

[Tombstone (R.I.P.)]

I wonder if Midnight has made any progress? It's been six days since advice was asked for and thirteen posts of experienced people trying to help. I would like to know if the problem has been cured and if any of our suggestions helped. Just curious to hear about it.

Tombstone

[midnight]

Sorry my first reply to Cbrown did not post; then I got busy with work but had time to work with the problem last night (tried to post then too but it did not post and I lost a long message). I checked out the wiring but all seems well; the replacement cord is from a little-used '70s-era Astatic Trucker II mic (nice, manly cord!) and checks good for continuity even while stretching it. So before I got into the resistor thing, I decided to finish up my common ground project and ran a ground wire into the shack and built a bus for it, tying in all equipment in use. After guessing at a spot to ground the Cobra, I hooked it up and was able to increase Mic Gain quite a bit over the previous without any squealing and got good reports. I can get close to the radio without squealing now, so the RF in my shack may be taken care of.

Thanks to everyone for weighing in BUT I'm still wondering if my chosen ground point is the best spot on this radio.

Kind Regards,
Mid

[midnight]

Forgot to mention that the Solarcon "stick" antenna has always been grounded to 8' (our water table is at 8' here) and now the common ground for the equipment is on the same ground rod, thanks!

[cbrown]

Aug 24, 2010 7:06:00 GMT -5 midnight said:

Forgot to mention that the Solarcon "stick" antenna has always been grounded to 8' (our water table is at 8' here) and now the common ground for the equipment is on the same ground rod, thanks!

The Solarcon is probably putting RF currents on the outside of the coaxial cable. Since you seem to have cured it, I wouldn't worry about it, but if the problem pops up again I'd look into adding an air choke right at the coaxial input of the antenna. People also call it a 1:1 balun.

[Tombstone (R.I.P.)]

Congratulations. Midnight! Removing squeals from a mic/radio, that can be very frustrating. Your grounded bus bar is nice to have too. I'd leave everything alone now that you've elimated the squeal. cbrown is right about the Antrons wreaking havoc. Glad you found the problem.

Tombstone

[midnight]

I wonder what it is about the Solarcon/Antron antennas putting RF on the outside of the cable? My Browning 68 R/T transmitter with a UG8 used to put a little open-ground-type sting/buzz on my lip when I'd talk, and when I turned on a little two-tuber that Fixr worked on for me, it would squeal and then have to be turned down (the mic gain on the D104) to get rid of the squeal. It's a little off-topic, but the Browning transmitter is currently down (won't key up, and the receive kind of decays with no power output on keyup) and when I have it fixed, it will be very telling to see what happens when the bird is grounded to that bus bar. (BTW, Greg gave that bird the 100K overhaul in late 2007, and it worked flawlessly on a daily basis for a couple of years, so I assume (read: hope) it's not a big problem, just maybe a roached resistor or something.)

Cbrown, thanks for the advice on the air choke. Can you elaborate on how it attaches to the setup (inline or like a ferrite core on the outside of the coax?) and how it works? Is it like a toroid core setup?

Thanks everyone again very much for the terrific response; I will be paying close attention to the TUG8 resistor cure/L pad fix in future on radios here that are grounded but squeal anyhow. And also, sorry for the long wait to respond. This computer is in my shack and for some reason does not always like to complete posts after I've typed them.

[midnight]

Oh, yeah: How much longer do I have to be a mud duck? <Big Grin>

[Sandbagger]

Aug 24, 2010 20:25:40 GMT -5 midnight said:

I wonder what it is about the Solarcon/Antron antennas putting RF on the outside of the cable? My Browning 68 R/T transmitter with a UG8 used to put a little open-ground-type sting/buzz on my lip when I'd talk, and when I turned on a little two-tuber that Fixr worked on for me, it would squeal and then have to be turned down (the mic gain on the D104) to get rid of the squeal. It's a little off-topic, but the Browning transmitter is currently down (won't key up, and the receive kind of decays with no power output on keyup) and when I have it fixed, it will be very telling to see what happens when the bird is grounded to that bus bar. (BTW, Greg gave that bird the 100K overhaul in late 2007, and it worked flawlessly on a daily basis for a couple of years, so I assume (read: hope) it's not a big problem, just maybe a roached resistor or something.)

Cbrown, thanks for the advice on the air choke. Can you elaborate on how it attaches to the setup (inline or like a ferrite core on the outside of the coax?) and how it works? Is it like a toroid core setup?

Thanks everyone again very much for the terrific response; I will be paying close attention to the TUG8 resistor cure/L pad fix in future on radios here that are grounded but squeal anyhow. And also, sorry for the long wait to respond. This computer is in my shack and for some reason does not always like to complete posts after I've typed them.

Here's a link to a site which goes into some detail on constructing an air wound balun/choke: www.hamuniverse.com/balun.html.

Basically, the fiberglass "stick" type antennas with no radials, are basically end fed 1/2 (or 5/8 in the case of the Imax) wave monopole antennas with an impedence transformer at the feedpoint. But without radials, proper decoupling of RF currents cannot occur and they tend to use the shield of the coax cable as a counterpoise. These RF currents travel down the shield (and radiate as well, which is a primary source of TVI issues) and into your shack. The balun acts as an RF choke to block these RF currents from traveling down the feedline. The balun needs to be placed as close to the antenna feedpoint as possible for best performance.

Of course, a better solution would be to repalce the "stick" antenna with an "old fashioned" metal ground plane with radials, such as a Maco 5/8th.

[cbrown]

Sandbagger summed it up rather well.

[midnight]

Outstanding explanation, thanks Sandbagger. When I go to the trouble of accessing the feedpoint of the Solarcon, it will be to unscrew it and replace it with whatever 5/8- or 3/4-wave Ground Plane I've gotten my hands on (wonder how the Sirio model works with those many counterpoises?). Really, had I known that I was going to get back into radio to this degree, I would have gone that route in the first place. Good stuff and very edifying; thanks for taking the time to explain.

[Sandbagger]

Aug 30, 2010 20:58:39 GMT -5 midnight said:

Outstanding explanation, thanks Sandbagger. When I go to the trouble of accessing the feedpoint of the Solarcon, it will be to unscrew it and replace it with whatever 5/8- or 3/4-wave Ground Plane I've gotten my hands on (wonder how the Sirio model works with those many counterpoises?). Really, had I known that I was going to get back into radio to this degree, I would have gone that route in the first place. Good stuff and very edifying; thanks for taking the time to explain.

That's why this forum is here, so we can share experiences, pain, and knowledge. I know there is nothing more frustrating that when something doesn't work as it should, but you have no idea which direction to go to solve it. Hopefully all the advice here helps.

[Tombstone (R.I.P.)]

A friend of mine that is in a low elevation went with the Sirio 2016. His Super Mag antenna shorted somehow. He said that the Sirio is built well and easy to assemble. His signal came up slightly and the receive is a little better but nothing spectacular. It's mounted on about 65 feet of tower. Whatever you're going to do I would still put a 1-1 balun as close to the antenna's feed point as possible as per Sandbagger's suggestion.

Tombstone

[cbrown]

Adding the blaun doesn't affect performance, I added one to my Interceptor I-10K when I installed it. Just a preventative measure in case of stray RF traveling back down the coax shield.

[Sandbagger]

Aug 31, 2010 8:42:46 GMT -5 cbrown said:

Adding the blaun doesn't affect performance, I added one to my Interceptor I-10K when I installed it. Just a preventative measure in case of stray RF traveling back down the coax shield.

The I-10K is a very well engineered antenna. With those 4 nice sized decoupling radials, you shouldn't need the balun.

[cbrown]

You're right Sandbagger; I shouldn't need it, and I don't. It's just old habits are hard to break. I always install an RF choke loop on all my antenna installations.

Sort of an "ounce of prevention is worth a pound of cure" thinking.

[Sandbagger]

Sept 1, 2010 8:35:04 GMT -5 cbrown said:

You're right Sandbagger; I shouldn't need it, and I don't. It's just old habits are hard to break. I always install an RF choke loop on all my antenna installations.

Sort of an "ounce of prevention is worth a pound of cure" thinking.

I heard that!