Frequency counter problem

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NightRanger3151
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Location: Vandalia, Missouri

Frequency counter problem

Postby NightRanger3151 » Sunday 17th May 2009, 7:59

Radio Make and Model: Cobra 2000 GTL
PCB Number: Main PC-497AB
PLL Number: MB 8734
Test gear available:
Symptoms: Frequency Counter is stuck on 92.2000 on receive and 0.0000 on transmit.

What is working: Radio works fine, except for frequency counter.

What has been tried: I did the improvement on the technical bulletin #1232 but it didn't change a thing.

Did it suddenly happen ?: Got radio this way.
Anything else that may be of help?:

Image

Classic CB
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First Name: Andy
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Re: Frequency counter problem

Postby Classic CB » Tuesday 19th May 2009, 5:44

Hi: Does the Frequency change when you move the selector? If not then this would point to a PLL or Power supply fault.
Check the voltage on pin 9 of the MB8734 PLL chip, it should be 8v DC if not then this points to a power supply fault & you should check the MB3756 regulator etc.
Apparently this is a reasonably common fault when the PLL section is not running properly.

Hope that helps: Andy @ Classic CB :)

NightRanger3151
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Re: Frequency counter problem

Postby NightRanger3151 » Wednesday 20th May 2009, 11:51

Classic CB wrote:Hi: Does the Frequency change when you move the selector? If not then this would point to a PLL or Power supply fault.
Check the voltage on pin 9 of the MB8734 PLL chip, it should be 8v DC if not then this points to a power supply fault & you should check the MB3756 regulator etc.
Apparently this is a reasonably common fault when the PLL section is not running properly.

Hope that helps: Andy @ Classic CB :)


Thank you for your reply!

Frequency counter does not change when the channel selector is moved. I should add on SSB the counter on this radio reads zero on both transmit and receive.

Pin 9 voltage shows 8.16.

I've read in a few places where it is suggested to replace D528 (1/2 watt zener diode) with the 1 watt diode 1N4734. I've got that ordered and will put that in and see if it helps.

Since pin 9 checks ok, anyplace else I should look?

NightRanger3151
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Re: Frequency counter problem

Postby NightRanger3151 » Thursday 21st May 2009, 9:23

I put the 1N4734 diode in D528 and it made no difference at all.

dlambeth
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Re: Frequency counter problem

Postby dlambeth » Saturday 5th May 2012, 3:21

I had the same problem with my 2000 and was never able to get it working. I tried every fix and modification you can imagine, but none of the fixed it. I replaced all CAPS, did the Cobra Bulletin Fix (1232), replaced the Diode but no enchilida. May be some other component bad, but it's not worth it for me to pour through the entire circuit. I'll just wait and find a replacement on ebay or something.

Regards,

hackindave
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Re: Frequency counter problem

Postby hackindave » Wednesday 9th May 2012, 15:38

I've seen a few 2000's that act this way. If all goes back to the power supply. These are built with a power supply that is capable of running the radio, but when peaked, it is lacking. Then the power supply also lacks and due to low voltage, parts tend to fail. I bought a Madison that the clock was very dim in. Someone had cranked the power supply to full voltage to get an extra watt out if it. The clock (not counter) had 2 burnt resistors from getting too much voltage.
dlambeth, you might try checking the voltage supplied to the counter.

NightRanger3151
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Re: Frequency counter problem

Postby NightRanger3151 » Wednesday 9th May 2012, 16:48

I tried many things to get that frequency counter working. Replaced all resistors, did all the fix's I could find online etc. Ended up just buying one on eBay that worked and put it in. Still working today.

hackindave
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Re: Frequency counter problem

Postby hackindave » Tuesday 10th Jul 2012, 23:20

It copuld be a cap problem. They are a PITA to replace. They are thru plated with sleeves thru the board. So you have to be careful or you will pull the sleeves out and have a open connection. I HATE re-capping the freq counters, but as old as they are now, the counters and P/S need to be done

Monk
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Re: Frequency counter problem

Postby Monk » Tuesday 9th Jul 2013, 10:19

I have found the traces on the circuit board weren't
prepared right at the factory. I have had luck by
solder wicking all the solder off the board, scratching
cleaning the traces and re soldering.

Monk


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