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Cobra 2000 VR4 does nothing...
- Thread starter 357
- Start date
You've got a few 6.3v and one 10v electrolytic caps in that circuit. Wouldn't hurt to change them out for 16v caps of the same capacitance value. Seems that when I've recapped radios in the past, it was usually the smaller voltage valued caps that always tested fail or near failure. It is an old radio - after all . . .
I have never checked VR4 other than if no carrier on ssb I never have turned VR4 to see if it even did any thing. that is a good question. if no ssb carrier I have never turned that VR just to see if it would throw a carrier.
maybe some of the older guy's has ran across this and wanted to see if it actually works when no carrier is being thrown.
good question
maybe some of the older guy's has ran across this and wanted to see if it actually works when no carrier is being thrown.
good question
Hmmm.
"Does nothing".....
First question in my mind is "how do you know?"
The carrier oscillator frequency trimmers can change the way VR4 behaves in a big way.
Have you set the 7.8015 on USB, 7.7985 on LSB and 7.800 on AM? If those are too far off, there won't be enough residual carrier for VR4 to make a difference you can easily measure.
What are you using to monitor the unsuppressed carrier output from the radio while transmitting in sideband?
I prefer to use a nearby receiver, with the clarifier turned away from the center of the channel. That way I get a heterodyne tone from the SSB transmitter's residual carrier output. We set VR4 for minimum S-meter reading on the monitor receiver. You will also 'hear' that null setting, since the noise you hear along with the weak carrier will be loudest when the carrier is nulled.
The driver and final bias settings will have a BIG influence on the radio's residual carrier output level in sideband transmit. If the bias is set too low on either the driver, final or both you may not see any response from adjusting VR4. Missing bias can completely remove your residual SSB carrier no matter where VR4 is set. Makes the SSB audio raspy, as well. Easiest clue is to turn down the mike gain so that the radio only shows a quarter of a Watt of SSB power. If the audio quality in your monitor receiver is raspy this way, and sounds cleaner when turned back up to normal power level, you have a bias trimmer or two that is set too low. Or a bias-circuit fault that prevents you from turning it up to a proper level.
There will be some interaction here. Too much SSB carrier can fool the meter's current reading when setting the bias trimmer, resulting in a bias setting that is too low for proper SSB audio. If a quarter Watt of residual carrier drives up the meter's current reading, you'll end up setting the bias-current trimpot too low for clean SSB audio.
I ALWAYS leave the test jumper for the driver UNHOOKED when setting the final bias. This shuts down the driver transistor and eliminates any carrier leakage that can falsely drive up the final transistor's current reading.
I would suspect a flaw in your setup before getting too excited about having some sort of mysterious failure in the radio's balanced modulator circuit.
The residual carrier leakage in SSB transmit from a Cobra 2000 should be VERY low when everything is aligned properly, but there should be enough of it to hear on a monitor radio a couple of feet away. Enough to set VR4 for a minimum S-meter reading, at least.
73
"Does nothing".....
First question in my mind is "how do you know?"
The carrier oscillator frequency trimmers can change the way VR4 behaves in a big way.
Have you set the 7.8015 on USB, 7.7985 on LSB and 7.800 on AM? If those are too far off, there won't be enough residual carrier for VR4 to make a difference you can easily measure.
What are you using to monitor the unsuppressed carrier output from the radio while transmitting in sideband?
I prefer to use a nearby receiver, with the clarifier turned away from the center of the channel. That way I get a heterodyne tone from the SSB transmitter's residual carrier output. We set VR4 for minimum S-meter reading on the monitor receiver. You will also 'hear' that null setting, since the noise you hear along with the weak carrier will be loudest when the carrier is nulled.
The driver and final bias settings will have a BIG influence on the radio's residual carrier output level in sideband transmit. If the bias is set too low on either the driver, final or both you may not see any response from adjusting VR4. Missing bias can completely remove your residual SSB carrier no matter where VR4 is set. Makes the SSB audio raspy, as well. Easiest clue is to turn down the mike gain so that the radio only shows a quarter of a Watt of SSB power. If the audio quality in your monitor receiver is raspy this way, and sounds cleaner when turned back up to normal power level, you have a bias trimmer or two that is set too low. Or a bias-circuit fault that prevents you from turning it up to a proper level.
There will be some interaction here. Too much SSB carrier can fool the meter's current reading when setting the bias trimmer, resulting in a bias setting that is too low for proper SSB audio. If a quarter Watt of residual carrier drives up the meter's current reading, you'll end up setting the bias-current trimpot too low for clean SSB audio.
I ALWAYS leave the test jumper for the driver UNHOOKED when setting the final bias. This shuts down the driver transistor and eliminates any carrier leakage that can falsely drive up the final transistor's current reading.
I would suspect a flaw in your setup before getting too excited about having some sort of mysterious failure in the radio's balanced modulator circuit.
The residual carrier leakage in SSB transmit from a Cobra 2000 should be VERY low when everything is aligned properly, but there should be enough of it to hear on a monitor radio a couple of feet away. Enough to set VR4 for a minimum S-meter reading, at least.
73
Thanks Nomad.
I did what you suggested and turned the clarifier to one side and I could hear the carrier balance and nulled it out.
problem solved, thank you.
I did what you suggested and turned the clarifier to one side and I could hear the carrier balance and nulled it out.
problem solved, thank you.
Rock on!Thanks Nomad.
I did what you suggested and turned the clarifier to one side and I could hear the carrier balance and nulled it out.
problem solved, thank you.
Sometimes the simplest solution is the best one.
73
I had an issue with a radio as you are. In that case, i had turned every possibility and was at a wits end to find the problem. Even went as far a replacing AN-612.
I finally discovered the problem. I first thought that it was the pot I took it out and it tested fine. But after re-installing it in the radio, I thought the problem lay elsewhere. Poking about the circuit with a nylon tool, I accidentally bumped the pot with the tool while TXing. Suddenly it worked. Seems that a cap was wedged against the pot on the board and distorted the pot in such a way that it became intermittent. Pressing on it made it work. So, I made some more room for it by slightly grinding off enough of the pot so as to let it not bind against any surrounding part.
Not saying that is the problem.
I finally discovered the problem. I first thought that it was the pot I took it out and it tested fine. But after re-installing it in the radio, I thought the problem lay elsewhere. Poking about the circuit with a nylon tool, I accidentally bumped the pot with the tool while TXing. Suddenly it worked. Seems that a cap was wedged against the pot on the board and distorted the pot in such a way that it became intermittent. Pressing on it made it work. So, I made some more room for it by slightly grinding off enough of the pot so as to let it not bind against any surrounding part.
Not saying that is the problem.
glad to see this got solved, and just wanted to throw some more info on the heap.
when setting the carrier null on SSB, i always use an external SWR meter in the FWD position with the sensitivity turned all the way up.
this way you can see it register on the needle, and can null it out, even if it's not enough current to nudge the wattmeter at all.
also, glad to see you around here Nomad!
LC
when setting the carrier null on SSB, i always use an external SWR meter in the FWD position with the sensitivity turned all the way up.
this way you can see it register on the needle, and can null it out, even if it's not enough current to nudge the wattmeter at all.
also, glad to see you around here Nomad!
LC
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@ BJ radionut:list'n 50.125 USB...working in shop...also 2m FM Simplex freq's 6.52...6.46 ... 6.55 etc..
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@ BJ radionut:I was look'n your way Evan...was working on some lighting in the garage...you were just "Thr" but w/ fade...you were gone...so did not fire the heater and call+1
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