Cobra 2000 gtl AGC mod or fix

[Chipper]

Hello i have a cobra 2000 that has a big issue with volume control and I was wondering if anyone knew a trick to get this radio to where I can set the volume and the ones closer to me won’t blow the speakers out but I can still hear the guys that’s at a bit of a distance away also. I’m constantly having the ones farther off comeback one more time while I crank my volume up some to hear them and rush to turn it back down so the ones closer don’t bust my ear drums. I’ve done the google thing and all I’ve found is a mod to C-23 adding a 10uf cap to bottom side of the board. Will that do it or are there some resistors I can change or is there something I can check to see if the original ones not working. I could really use some help on this one here and knowing where this circuit is on the board also will help me out too. Thanks again

 

[unit_399]

CHIP -
This is an ad published in SECRET CB years ago and is probably what you're looking for. I know Selman Enterprises is long gone, but maybe these things are still available somewhere. I had a couple but have no idea where they went. All I know is that they worked as exactly as advertised. I believe they were made by a company called DX Products. Good Luck.
- 399

 

[9C1Driver]

The Cobra 2000 has such a great receive I would think all it needs is a correct alignment. Even then that's kinda the nature of cb radio, closer stations will come in louder then distant stations.

 

[nomadradio]

One thing that prevents the radio's AGC from equalizing the audio between weak and strong stations is a loss of sensitivity.

Randomly tweaking tuning slugs inside the radio can reduce the receiver's sensitivity and cause this. Receiver adjustments are set for peak signal level with a weak signal feeding into the radio.

If the radio is old enough, and was exposed to humidity or condensation it may have one or more failed capacitors INSIDE one or more of the slug-tuned RF and IF coils. This will typically result in a tuning slug that appears to show a peak, but does so with the slug DEAD EVEN with the rim of the hole. If you see this, the apparent "peak" setting of the tuning slug is not a true peak. It's the slug reaching the end of its adjustment range. Turning the slug so that it now protrudes above the rim of the opening reduces the signal, as does turning the slug so it dips below that rim. This 'flush' position is the max inductance setting of the slug. Indicates the internal capacitor in that can has failed. A trimmer cap soldered across the pins underneath where the cap is connected inside can fix this sometimes without removing/replacing the coil.

Alignment of the receiver becomes a diagnostic tool when the radio gets up in years, or if it was in the hands of a tweaknician who disrupted the receiver's alignment.

73

 

[Chipper]

One thing that prevents the radio's AGC from equalizing the audio between weak and strong stations is a loss of sensitivity.

Randomly tweaking tuning slugs inside the radio can reduce the receiver's sensitivity and cause this. Receiver adjustments are set for peak signal level with a weak signal feeding into the radio.

If the radio is old enough, and was exposed to humidity or condensation it may have one or more failed capacitors INSIDE one or more of the slug-tuned RF and IF coils. This will typically result in a tuning slug that appears to show a peak, but does so with the slug DEAD EVEN with the rim of the hole. If you see this, the apparent "peak" setting of the tuning slug is not a true peak. It's the slug reaching the end of its adjustment range. Turning the slug so that it now protrudes above the rim of the opening reduces the signal, as does turning the slug so it dips below that rim. This 'flush' position is the max inductance setting of the slug. Indicates the internal capacitor in that can has failed. A trimmer cap soldered across the pins underneath where the cap is connected inside can fix this sometimes without removing/replacing the coil.

Alignment of the receiver becomes a diagnostic tool when the radio gets up in years, or if it was in the hands of a tweaknician who disrupted the receiver's alignment.

73

Ok I’ll pop the lid off and look at the cans but yea it’s been around a few technicians. I’ve removed a bunch of crap mods off this radio to be back stock as I can get it. Thanks for the help.

 

[sp5it]

Maybe AGC is just broken?

 

[Chipper]

Maybe AGC is just broken?

Do you know where and why I can check to test it?

 

[sp5it]

Remove D68 it should disable AGC completely. If no difference, AGC is not working.
I would check D12, 13,16 and 68 if not failed.
Mike

 

[Chipper]

Remove D68 it should disable AGC completely. If no difference, AGC is not working.
I would check D12,13,14, 16 and 68 if not failed.
Mike

Ok thank you very much for that. Is d-12 and d-13 a special diode or different from the others

 

[sp5it]

Switching diode. For a test 1n60 or similar germanium diode should work.

 

[Handy Andy]

(Fixed typos on this post)

This might help...
​

AGC amp "sees" a signal of carrier and envelope all as one, it's converted to an IF but unamplified from that section, from there it's sent into a full wave detector that uses a bridge (wein) of known reference - the varying audio is smoothed out and summed into the carrier to help keep you from getting blasted out. The Squelch control tap is for this so you can use Squelch and it works properly.

• When you asked about "loud signal blast" I went back to an old Cobra 148 that has a mod in it that uses the R43 AGC out line as a tap point - to go to D15's Banded end - using a 1N4148 and a simple 2.2K Resistor - looking this ACG section over here - reminded me that if nothing else, this mod applied more of the AGC difference signal that goes to the PIN diodes to trim - punch down - the RF signal (Strong adjacent) I think this was an OEM install - not a "Tech" mod.

Key parts are that bridge, if unbalanced, will not merge the carrier and audio detected in it, properly to "compress" the audio envelope so you don't get blasted by a wildly swinging radio.

When you don't have a carrier, extra signal is needed - that is where L14 in SSB modes comes into play.

@sp5it - is correct, that D68 sends power to the PIN diode section to help limit the input signal and prevent overload. Not guarantees it's foolproof, but limiting the input goes a long way. Timing is done thru those Electrolytic caps - too small it sounds monotones-ish, too large, you have an overload issue - can't keep up.

This is more of an end user event, because you can't prevent the locals from ruining your day - so some small tweaks to the Bridge caps can help - but it makes the radio deaf in some ways, or too sensitive if you use the wrong values. So the end user has to make the choice of what is acceptable to them - else it's best to get a good tune up and leave it alone.

 

[Chipper]

Thanks for the help again I had L7 and L9 slugs at the top of the can and put a 101 cap on L7 to bring it back in and a 68 cap on the L9 one now the low guys are real close now after the alignment was done. Thanks again y’all don’t realize how much you help me out on this page in keeping the locals around here doin what they love and keeping the hobby going here in West Virginia. Thank ya

 

[Chipper]

(Fixed typos on this post)

This might help...
View attachment 42565​

AGC amp "sees" a signal of carrier and envelope all as one, it's converted to an IF but unamplified from that section, from there it's sent into a full wave detector that uses a bridge (wein) of known reference - the varying audio is smoothed out and summed into the carrier to help keep you from getting blasted out. The Squelch control tap is for this so you can use Squelch and it works properly.

• When you asked about "loud signal blast" I went back to an old Cobra 148 that has a mod in it that uses the R43 AGC out line as a tap point - to go to D15's Banded end - using a 1N4148 and a simple 2.2K Resistor - looking this ACG section over here - reminded me that if nothing else, this mod applied more of the AGC difference signal that goes to the PIN diodes to trim - punch down - the RF signal (Strong adjacent) I think this was an OEM install - not a "Tech" mod.

Key parts are that bridge, if unbalanced, will not merge the carrier and audio detected in it, properly to "compress" the audio envelope so you don't get blasted by a wildly swinging radio.

When you don't have a carrier, extra signal is needed - that is where L14 in SSB modes comes into play.

@sp5it - is correct, that D68 sends power to the PIN diode section to help limit the input signal and prevent overload. Not guarantees it's foolproof, but limiting the input goes a long way. Timing is done thru those Electrolytic caps - too small it sounds monotones-ish, too large, you have an overload issue - can't keep up.

This is more of an end user event, because you can't prevent the locals from ruining your day - so some small tweaks to the Bridge caps can help - but it makes the radio deaf in some ways, or too sensitive if you use the wrong values. So the end user has to make the choice of what is acceptable to them - else it's best to get a good tune up and leave it alone.

Thanks for this I want to try this out and learn more of it