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Post by n4dxx on May 17, 2019 7:43:31 GMT -5
Hey all,I have a small issue with a cobra 2000..I just got this radio up and going after a few years of being stored away.I had to replace the frequency counter module..In AM mode Only when i key the mike the freq counter goes up in frequency around 2 HZ..If reading 27.2050 on rx when i key the mike it will go to 27.2052..And one other thing when i go from ch1 to ch40 On ch 1 it will read on rx 26.9650 and if i change to ch 40 it will read 27.4051..It has had the clarifier mod done but it is done correctly and i don't think it has anything to do with this issue as i have done the clarifier mod on many 2000's and never had an issue like this..Thank's and any help would be greatly appreciated.
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Post by tubefan on May 17, 2019 9:36:59 GMT -5
If you have a bench counter I would check to see if the radio is truly tracking correctly. If not then align. If it is then tackle the counter.
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Post by tubefan on May 17, 2019 9:39:09 GMT -5
If you do not have a bench counter a known working SSB radio would work. Listing to yourself on the other radio on SSB and tune yourself in . Then transmit on the other radio and listen on the 2000. If it's off then you know it is not tracking correctly.
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Post by n4dxx on May 17, 2019 11:23:26 GMT -5
Checked the coutner had a spare and it's doing the exact same thing..The radio is not tracking in AM only ssb seems fine..Will get out the service manual..
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Post by tubefan on May 17, 2019 11:33:32 GMT -5
There is a vr if I remember correctly that will tweak just the TX. Not home so do not have manual in front of me. Not too difficult to align.
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Post by n4dxx on May 17, 2019 12:32:58 GMT -5
Well that has been clarifier modded so the vr dosen't work..I was going to reverse the mod but someone completely removed the brown wire from the clarifier knob and i have no clue where it goes back
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Sandbagger
Administrator/The Boss
Posts: 6,247
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Post by Sandbagger on May 17, 2019 18:37:49 GMT -5
Hey all,I have a small issue with a cobra 2000..I just got this radio up and going after a few years of being stored away.I had to replace the frequency counter module..In AM mode Only when i key the mike the freq counter goes up in frequency around 2 HZ..If reading 27.2050 on rx when i key the mike it will go to 27.2052..And one other thing when i go from ch1 to ch40 On ch 1 it will read on rx 26.9650 and if i change to ch 40 it will read 27.4051..It has had the clarifier mod done but it is done correctly and i don't think it has anything to do with this issue as i have done the clarifier mod on many 2000's and never had an issue like this..Thank's and any help would be greatly appreciated. It's probably not the counter that has the problem, but the PLL circuit that needs a little attention. If the frequency error increases from channel 1 to 40, that indicates that the 10.24 Mhz reference crystal is running a little high in frequency. If it differs from AM and SSB, that could be either the mixer crystals or the carrier oscillator crystals. All of them can be easily aligned.
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Post by 2600 on May 19, 2019 0:08:59 GMT -5
If it does this on sideband, the previous suggestions are worth checking.
When you are receiving in AM, there is no signal to the counter's IF input. The computer that runs the counter substitutes "7.800" for that part of the display's internal arithmetic. When you key the mike, the 7.8 MHz carrier becomes active, and the frequency display uses that input for the display arithmetic.
Two adjustments matter here. The AM carrier trimmer coil L31 has to be set for exactly 7.800 MHz feeding into the balanced mixer chip.
And the trimmer cap in the counter must be set correctly as well.
I'll guess that both of them are slightly off.
Could be only one of them.
73
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