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Message |
    
Mayor513
Intermediate Member
Username: Mayor513
Post Number: 180
Registered: 3-2002
| | Posted on Saturday, August 11, 2007 - 8:15 pm: | 
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--Hi, all. What is causing the frequency counter in my Cobra 2000 to jump around? For instance, I can be on a channel on receive and it will show the proper frequency, but as soon as I transmit, it shows numbers like, 30.659, 30.678, 30.772, etc., etc. What can I do to fix this? Thanks. |
Radioreddz
Member
Username: Radioreddz
Post Number: 57
Registered: 9-2006
| | Posted on Sunday, August 12, 2007 - 2:07 am: |
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wow i have the same problem. reads fine on AM receive but when i transmit it will show 30 sumthin as well. but when i swith over to SSB it will read 351835 and stay the same when i transmit. when on SSB reading 351835 thats ch 38 when i switch to ch. 39 it will read 351935 the center two numbers change right up trhu the band. |
Kid_vicious
Senior Member
Username: Kid_vicious
Post Number: 2609
Registered: 9-2004
| | Posted on Monday, August 13, 2007 - 12:38 am: |
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well guys, sorry to say it, but this is a symptom of age, and its only going to get worse.
teh freq. counter in these radios is about the hottest place in the radio and its the first place the electrolytic caps will dry out.
there are also two FETs that can go bad, but usually its the caps that cause the jumping.
the solution is to remove the entire assembly, and replace all the elec. caps in it. there are about 11.
however!!!
this PC board is double sided and plated through.
so if you've never done this type of soldering before, and dont own the desoldering iron required, its best left to a pro.
(the bulb wont work, neither will braid)
it can be fixed, but its not easy.
best of luck,
matt
anyone wanting a "clean signal", just look to the left and build one of these!!!
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2600
Advanced Member
Username: 2600
Post Number: 594
Registered: 7-2002
| | Posted on Monday, August 13, 2007 - 1:40 am: |
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Does it read correctly on SSB receive, or does it go nuts on sideband?
Mostly I recommend replacing all 9 or 10 (I think) electrolytic filter capacitors on the counter/clock pc board. Once that first one goes bad it gives some part of the display a case of 'jitters', usually, or causes digit segments that should be dark to flicker weakly. Replacing them all is probably cheaper than the labor to pin down "just one" of them that has become and open circuit. Besides, once the first cap goes bad, the next is seldom far behind.
But if it won't read correctly on SSB receive, this points to more trouble than filter caps alone.
A 2000 that won't read on SSB receive has a problem with its carrier-frequency input.
The 2000's display is pretty unique for the late 70's. It reads two frequencies, counting one then the other. Takes both numbers, and does the math to calculate the operating frequency.
One of the two inputs is from the PLL/channel selector. This will be in the range of 34-35 MHz. The other input is from the "carrier" frequency generated by the 7.8 MHz crystal. That input is only active for SSB (T and R) and for AM transmit only.
On AM receive, the 7.8 MHz crystal is turned off, and the counter's internal computer inserts the number "7.8000" to calculate the channel frequency to display. Trouble when you transmit on AM, and on SSB points to the second (7.8 MHz) input circuit. Several possibilities there.
73 |
Kid_vicious
Senior Member
Username: Kid_vicious
Post Number: 2612
Registered: 9-2004
| | Posted on Monday, August 13, 2007 - 10:25 pm: |
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i never knew that about the counter generating its own "7.8000" number on AM.
very cool!
i learn something new everyday.
matt
anyone wanting a "clean signal", just look to the left and build one of these!!!
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Radioreddz
Member
Username: Radioreddz
Post Number: 59
Registered: 9-2006
| | Posted on Tuesday, August 14, 2007 - 11:30 am: |
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i guese as always i have the worst problem. on SSB receive it still reads the 351835 on ch. 38. if i switch to channel 39 it will read 351935 and thats on transmit and receive. on AM when on receive it reads normal but on transmit goes bonkers. |
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