Cobra 2000 counter

[GarageNET]

I've got a cobra 2000 here, and the counter is displaying "92.2004" on am and "99.9990" on USB/LSB. Replaced FET's 501 and 502, and d528 (common problems according to the web..?..). Seems as if the signal coming from the board is not correct, or not present, because I can pull the connector off the side of the counter (with the 2 coax's) and nothing changes on AM (SSB goes to all 0's). Any ideas? The radio works fine otherwise.



73

Vince


</p>

 

[NorthStar34]

Good Chance the module needs replaced or rebuilt.


</p>

 

[Tweek]

I know this is an old thread, but just in case...I have one I picked up that's doing the same thing. I'm in the process of resurrecting the rest of the rig (lots of other problems), but noticed my counter doing the same thing. Have any of you had a similar problem and located the problem (short of cramming another counter in)?

 

[kopcicle]

https://www.worldwidedx.com/threads/cobra-2000-gtl-reads-92-0000.37105/

 

[loosecannon]

since i am in the thread linked, i should probably key up and say that over the years i have found that just replacing the electrolytics in the counter module has cured this problem most of the time.

i actually can't remember that last time i did that tech bulletin, and i think i only ever did it to my own radio.

replace the electrolytics and see how that works.
remember that this board is double sided and plated through so be careful with your desoldering.
LC

 

[nomadradio]

Learned the hard way to follow the counter's two input signals with a 'scope. That tool is the swiss-army knife for fixing counter faults.

The two JFET input preamp transistors are vulnerable to damage if the counter's input plug (the one with two thin gray coaxes) is pulled or put back while the power cord is plugged into the wall. Simply pushing the power button won't protect anything from surge stress. You have to pull the AC plug out of the wall before pulling or pushing any connectors on the clock/counter module.

There is a slug-tuned input coil for each input in the module. They don't go bad that often, but it's on the list of things to check, following the input signals downstream. A compulsive slug-twister can adjust them far enough out of whack to cause trouble. Only way to peak them is to watch the signal coming out of each one.

There are a handful of small chips that handle the two input signals, before they reach the fat 5032 counter chip. Not a frequent failure, but on the list of possible suspects.

Changing all the electrolytics is the only sane way to approach the problem at first. But there is no guarantee this will get it working again.

But a very good chance that they will cause trouble if you fix some other fault and leave the old ones in there.

73

 

[Tweek]

Thanks for the link and all the great tips, this is the first of these I've ever tackled. I'll report back with what I find, in case it helps anyone else. This rig is loaded with problems, lots of hackery Looks great outside, but that's about where it stops.

 

[nomadradio]

displaying "92.2004" on am and "99.9990" on USB/LSB.

Missed this detail the first read. The "92.200 means that the 35 MHz VCO input to FET501/L501 is not getting through.

The "99.9990" reading means that the 7.8 MHz input to FET502/L502 is working. Everything in the path of the 7.8 MHz signal must be working to get that reading.

You changed FET501, so this should narrow it down to L501 feeding into IC512, and then into two sections of IC507. Everything downstream from (and including) IC505 has to be working to get that "99.999" reading.

Following the 35 MHz signal with a scope, one step at a time down the circuit is the only way I know to pin down where the VCO signal is getting lost. Seeing that L501 shows a proper peak when the slug is turned would be a big deal. Pin 1 of IC507 is where I would probe for that. Seems to me I blew out the chip at IC512 once when the tip of the 'scope probe bridged two adjacent pins together. IC507 should not be as fragile if the tip of the probe slips.

One last thought. Did you check the FET you removed from FET501 to see if it was bad?

73