• You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.

Constant loud squeal when I turn on my Cobra 2000GTL radio

SmilinDoug

New Member
Aug 27, 2022
18
4
3
62
Hi everyone. I just registered. I'm hoping someone can help me out. Whenever I turn on my Cobra 2000GTL I get a constant loud squeal. Turning the volume down doesn't change a thing. I've tried every switch and knob on it and still get the squeal. I've swapped out microphones, speakers, and antenna's. Checked the SWR, nothing. I've checked on AM, LSB and USB, the squeal is still there, and on every channel.

My Dad passed away last year and I got his shop and everything in it. He sold and repaired CB's all through the 60's, 70's, 80's and 90's. He's got other radio's that work fine, and I want to fix this one. It was built in 1991 as far as I can tell.

Thank you for any help you guys can give me.
 

nomadradio

Analog Retentive
Apr 3, 2005
5,557
8,175
573
Louisville, KY
www.nomadradio.com
Of course you aren't supposed to key the mike unless a proper antenna or a dummy load is connected to the antenna socket.

But that's the first question that comes to mind, what happens when you press the push-to-talk button on the mike?

The oldest Cobra 2000 radios are 43 years old. And the newest would be 28 or 29 years old. This is not a typical fault for this model. Just the same, a whole category of component inside that radio called "aluminum electrolytic" capacitors will deteriorate and fail from age alone. The heat and stress of operating the radio will speed up the failure of those parts. If you hear the phrase "re-cap" an old radio, it means replacing all of them due to age alone. It's quite a list for this model.

Just the same, if I had to guess, I would blame the biggest, fattest filter found on the main radio circuit board as the culprit. This would be C172, located near the TA7222P audio amp chip. This is quick and easy to verify if you have a spare 1000uf capacitor. Just poke the ends of the new cap's leads up against the foil pads where the old cap is installed. If the squeal goes away, C172 is the cause. And if it doesn't make any difference, you have a different problem.

Even so, most of those electrolytic capacitors serve to filter the separate circuits in the radio so they don't interfere with each other. This sort of fault just smacks of a failure in that department.

I want to fix this one.

Helps to have tools and someone on hand who can read a schematic diagram.

73
 

SmilinDoug

New Member
Aug 27, 2022
18
4
3
62
Of course you aren't supposed to key the mike unless a proper antenna or a dummy load is connected to the antenna socket.

But that's the first question that comes to mind, what happens when you press the push-to-talk button on the mike?

The oldest Cobra 2000 radios are 43 years old. And the newest would be 28 or 29 years old. This is not a typical fault for this model. Just the same, a whole category of component inside that radio called "aluminum electrolytic" capacitors will deteriorate and fail from age alone. The heat and stress of operating the radio will speed up the failure of those parts. If you hear the phrase "re-cap" an old radio, it means replacing all of them due to age alone. It's quite a list for this model.

Just the same, if I had to guess, I would blame the biggest, fattest filter found on the main radio circuit board as the culprit. This would be C172, located near the TA7222P audio amp chip. This is quick and easy to verify if you have a spare 1000uf capacitor. Just poke the ends of the new cap's leads up against the foil pads where the old cap is installed. If the squeal goes away, C172 is the cause. And if it doesn't make any difference, you have a different problem.

Even so, most of those electrolytic capacitors serve to filter the separate circuits in the radio so they don't interfere with each other. This sort of fault just smacks of a failure in that department.



Helps to have tools and someone on hand who can read a schematic diagram.

73
 

SmilinDoug

New Member
Aug 27, 2022
18
4
3
62
Thank you! I did not know that about those capacitors. I didn't say anything in my post but I replaced the TA7222P audio power amplifier already. I figured that would be the most logical part, and it was only about $3. I'll look around in my Dad's shop and see if I can find a 1,000 micro farad capacitor. Thank you again.
 

SmilinDoug

New Member
Aug 27, 2022
18
4
3
62
So I couldn't find a 1,000 micro farad capacitor in my Dad's shop, so I ordered one. I had to order a pack of 5, but they ended up being about $3 a piece. I touched the leads on the other cap in the radio, and there was no change. Is there a board layout in the forum some where? I'd need it for the top and bottom of the board. Would one of the transistors cause this problem?
 

1iwilly

Well-Known Member
Dec 7, 2008
553
318
73
60
so this squeal is there all the time without squeezing the mike
spitballing here if it is on transmit i wonder if the radio has talked back a resister where the speaker jack is like a 100 ohm
 

SmilinDoug

New Member
Aug 27, 2022
18
4
3
62
so this squeal is there all the time without squeezing the mike
spitballing here if it is on transmit i wonder if the radio has talked back a resister where the speaker jack is like a 100 ohm
Right when I turn on the radio it's got that squeal, all the time. Without keying the mike.
 

1iwilly

Well-Known Member
Dec 7, 2008
553
318
73
60
Right when I turn on the radio it's got that squeal, all the time. Without keying the mike.
ok try this as a test= put a jumper from pin 2 to pin 3 that's just for receive
bottom right corner =pin-1 20220905_212821.jpg
Mic Wiring
Stock

1- Audio
2- Shield
3- Receive
4- Sw Com
5- Transmit
 
  • Like
Reactions: SmilinDoug

SmilinDoug

New Member
Aug 27, 2022
18
4
3
62
ok try this as a test= put a jumper from pin 2 to pin 3 that's just for receive
bottom right corner =pin-1 View attachment 60484
Mic Wiring
Stock

1- Audio
2- Shield
3- Receive
4- Sw Com
5- Transmit
I'll try that hopefully tomorrow. I live a few miles from my Dad's house. And hopefully my wife doesn't have any projects to do here at our house. Thank you for the help! I'll let you know what happens.
 
  • Like
Reactions: 1iwilly

SmilinDoug

New Member
Aug 27, 2022
18
4
3
62
please at least try a different mic.
LC
I already have. I thought I wrote that in the first post.

There might be two bad microphones. I doubt it but I'll give it a shot trying the jumper. Besides I'm learning more troubleshooting procedures and I do like learning things.
 
Last edited:

nomadradio

Analog Retentive
Apr 3, 2005
5,557
8,175
573
Louisville, KY
www.nomadradio.com
In the 43 years since we first started repairing that model, I think I've seen at most two radios that did this. Maybe only one. As such I have almost no experience with that fault. Got lotsa experience with other stuff we see go wrong with it, but not this exact symptom.

Touching the ends of a new capacitor's wire leads to the circuit board foils beneath C172 is hard to beat in the cheap and quick departments. And if a bad C172 is the problem, the result is immediate. The noise will instantly disappear as long as the new cap's leads are touching. A smaller cap, 470uf or one larger than 1000 will serve to perform this test. This is a filtering component, so its capacitance rating is not critical. Just has to be "enough" to prevent feedback issues.

73
 
  • Like
Reactions: SmilinDoug

dxChat
Help Users
  • No one is chatting at the moment.
  • @ ButtFuzz:
    GR&T Super MC-11/A Crystal Synthesized and is in SAMS #22 I have no further information on this radio.
  • @ Robert Dinsmore:
    I have schematics and Manuals I need someone to work on it
  • @ Robert Dinsmore:
    I have a MC-7 as well W D-104!
  • @ Robert Dinsmore:
    does anybody have any Ideas for working on a General MC-11A