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COBRA 2000 GTL AM carrier issue

Unit 75

Unit 75

"TRAINMAN"
Jul 29, 2014
526
1,018
153
Central Alabama
OK... Has anybody ever experienced something like this??? I key the mic in AM mode. The carrier starts at 2w and rises to about 5w and then goes back down to just over 4w where it remains stable. I key it right back up and it does it all over again. SSB is fine for output (about 15w PEP) All regulators are good. Electrolytics around the regulators and driver/final have all been replaced. Voltage on driver and final collectors are stable. I checked the AM carrier pot and it is good with no voltage drops during tx. Power supply is also stable. I took a video of it but I could not upload it as the site errors to the .MOV extension.
 

If you have a power mic try it with the mic gain turned down. You might try it into a dummy load as well.
 
using a stock hand held into a dummy load. The dynamike is turned all the way off...no modulation...just a dead carrier. I also changed the bias regulator again as well as the 1306 driver....still the same.
 
Just thinking out loud......
I would think it would be a little harder to judge whether SSB is having this issue or not because no or little output unless you DO modulate.

Do you have any kind of tone generator, maybe even a double tone that you could feed into the mic so that you have stable mic audio going.... and THEN key up and watch the meter/scope? Just a thought.....
 
Oh it has power output. There is nothing bad about a 4.25w carrier. I did inject a tone. It shows more wattage with the 1000Hz tone injected but the output is doing the same thing by moving up and dipping back down a bit and stabilizing. I can whistle into the mic and the audio swings to about 8w. With no modulation, the carrier starts around 2w and climbs to 5w then it drops down to a little over 4w. After that it stabilizes and the carrier stays a little over 4w without dipping. I can hold a 4.25w carrier for a few minutes and it is solid. It is not an audio problem. It swings from that 4.25w up when I whistle into the mic. Audio is great on AM and SSB. Radio talks fine on SSB with good clean audio and good power. The problem is only in AM mode when you initially key.
Understand that this issue is within the first 4 or 5 seconds when you key up. If you stayed keyed...it's fine and you can talk and it sounds good.
 
Last edited:
You probably need to change all those original electrolytics everywhere or the radio will remain intermittent for signals coming from oscillators etc. A check of all the crystals, the 3 legged crystal filter and vfo for rusted legs (glue problem) is probably not a bad idea. Cobra 2000 is a great radio with good fat watts when it works.
 
I have had a couple of the older radios doing that, it was the metal pots corroded, one time de oxit fixed it, other time had to replace the pot.
 
nomadradio...radio has a new bias regulator.... new bias trim pots for bias adjustment. driver bias set at 30mA and final bias set at 60mA. bias is set correctly and is stable.

sp5it... voltage measurement at TP7/TP8 is correct and stable
 
Do you have rf probe?
Measure RF voltage on collectors TR36, 37, 38, 39 and base of TR39 for fluctuations when power is not stable on the power meter.
Mike
 
Just my thought. Would you happen to have a carrier on SSB? Can you listen to this radio on another radio when keying it up to see if tou have a squeal?
 

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