Until now it has been fine, I went to shoot so.e skip on 38 lower and reports came in it sounded horrible and barely legible. I change battery in mic same problem..went to AM and no issue at all. What could this possibly be?
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Cobra 2000 gtl ssb issue
- Thread starter ShadowDelaware
- Start date
"Until now it's been fine."
My 1985 daily driver would start right up every day for decades until this morning. Now it knocks, misfires and stalls when I put it into gear.
The newest Cobra 2000 is over 30 years old. The oldest one around 45 years old. Could be an age issue.
Try tightening the circuit board's mounting screws around the edges of the big circuit board. Not a lot of wasted effort if it turns out not to help.
A 'scope would quickly reveal if a problem with the Automatic Level Control, or "ALC" circuit is allowing the sideband signal to overdrive the final. This flattens the voice peaks in a way you can see. The hams call this "flattopping", when voice peaks get cut off and flattened. Could even be oxidation on the ALC trimmer pot VR11. A drop or two of contact cleaner and turning it end to end a dozen times often cleans up a noisy trimpot. But now you'll have to set VR11. Best way is to put an average (NOT peak) reading wattmeter in line, turn up the mike and modulate a long "helloooooo". Turn the VR11 for max reading, then slowly turn it back the other direction. At some point you'll see the wattmeter "dive" down to half the max reading or less. This is the sweet spot. If you have a 'scope, just set VR11 so none of the voice peaks get flattened on top.
There is more than one way for a sideband transmit signal to get garbled, but those are the most common that I see.
And if electrolytic capacitors in the ALC circuit have failed, this can garble the sideband transmit, as well. C109 and C18 are on the hit list for this problem.
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My 1985 daily driver would start right up every day for decades until this morning. Now it knocks, misfires and stalls when I put it into gear.
The newest Cobra 2000 is over 30 years old. The oldest one around 45 years old. Could be an age issue.
Try tightening the circuit board's mounting screws around the edges of the big circuit board. Not a lot of wasted effort if it turns out not to help.
A 'scope would quickly reveal if a problem with the Automatic Level Control, or "ALC" circuit is allowing the sideband signal to overdrive the final. This flattens the voice peaks in a way you can see. The hams call this "flattopping", when voice peaks get cut off and flattened. Could even be oxidation on the ALC trimmer pot VR11. A drop or two of contact cleaner and turning it end to end a dozen times often cleans up a noisy trimpot. But now you'll have to set VR11. Best way is to put an average (NOT peak) reading wattmeter in line, turn up the mike and modulate a long "helloooooo". Turn the VR11 for max reading, then slowly turn it back the other direction. At some point you'll see the wattmeter "dive" down to half the max reading or less. This is the sweet spot. If you have a 'scope, just set VR11 so none of the voice peaks get flattened on top.
There is more than one way for a sideband transmit signal to get garbled, but those are the most common that I see.
And if electrolytic capacitors in the ALC circuit have failed, this can garble the sideband transmit, as well. C109 and C18 are on the hit list for this problem.
73
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Are you running it off the internal power supply ? My bet would be on a power supply problem that is not giving sideband transmit enough amperage. Nothing garbles SSB transmit faster than starving a radio for power..... AM draws far less amperage, hence it sounds OK. I'd start with the big electrolytic caps in the power supply........
I agree with both Nomad Coyote. Pretty much guaranteed to be an age-related issue. In your case, since AM is working well, you have a lot less circuitry to consider. Since your post didn't say anything about the receive being a problem, this limits the possibilities to the power supply and SSB-specific transmit circuits. Loose circuit board screws and other grounding concerns I lump under the power supply heading.
So my first thought was a power issue so I switched to an external supply and the problem persist. I will have the time today to take it down and open it to look further.
If the Cobra 2000 has a 12v plug on the back try powering it that way once.
I would look at the bias circuits for the driver and final. Bias problems can really mess up SSB transmit.
It would also be helpful to know what exactly the transmit does sound like. You say it's garbled, but that's pretty vague. Is it crunchy sounding, or is it more of a bubbling and burbling on the peaks, or... ?
Different sounds = Different possible problems.
Do you have a second radio you can listen to and see how the audio actually sounds ?
And just to put it out there....... More than once in my time on 11 meters I've been told by someone that my audio was awful / crunchy / distorted etc etc when there was nothing wrong with it at all and the problem was with the persons receiver !
One more good reason to have a second radio you can monitor yourself on !
Bubling and burbling would be a good definition.
I want to dive into BUT I had it in shop for repairs (from 9-28-23 to 5-20-24) and now it has a warranty void sticker on it that will peel if I open it. Image shows what was done. I do not want to void warranty on the work done.
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Wouldn't hurt to ask them to key it up and listen on another sideband radio before sending it home next time.
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It shouldn't need a trip to the shop less than 2 months after leaving the shop...that is for sure.
Unless some 1981 electrolytic cap randomly selected this week to fail.
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Luckily I have backup radios. I am taking it back to them to check out before I tear the warranty sticker. Damn sticker stopped me in my tracks.