Cobra 2000 GTL ideas...

[Cable Guy]

I replaced the NTE driver with a 2166. Re-biased for 25 ma and that brought the AM pep to 15 watts, SSB went up to ~18 watts. That's closer to where I want it and it looks good on the scope too. I did replace the limiter, that made SSB sound better too. Speaking of SSB, this radio had an expo kit, but it drifted too much for my taste on SSB. I took it out when I replaced the pll with a 8719, so I think I'm going to add the switches instead for the poor man's channel expansion. I'm still undecided on that.

 

[nomadradio]

switches instead for the poor man's channel expansion.

This is what we would recommend back in the day to a sideband operator. And for someone who only cared about AM, the Expo 100 kit was good enough and a bit simpler to find a channel.

73

 

[Cable Guy]

One more thing, a really little thing, I have noticed about this particular radio. When squelched Rx, any mode, any volume setting, I can faintly hear some clock/counter noise. The volume knob will make it a little louder, but you have to crank it up. Unhooking the power to the fc module removes it completely.
The noise is continuous and regular, reoccurring in precise intervals. I feel some noise is feeding back from the counter power into the rest of the radio.
As far as I can tell, it isn't being transmitted with tx, on the scope output carrier is perfectly flat with no mod, no indication of noise. I think I will probe the power and see if I can find where it leads.
I don't recall hearing it before the tsb was done, but it may have been and I just didn't notice.
Maybe I can get a sample to upload here.

Edited to add audio file and more info.
I also removed any corrosion from the grounding points on the fc during the tsb. Sounds kinda like a clocking frequency getting out. Maybe I missed something doing the recap and tsb on the fc.

 

[nomadradio]

I can faintly hear some clock/counter noise.

A very familiar symptom. The cure is to replace all 11 (or is it 12?) electrolytic caps in the clock/counter module. The rapid on/off switching that takes place in the digital logic chips in that module generate RF and audio noise. It's filtered by those capacitors. Until they wear out. We consider this module to be the canary in the coal mine, so to speak. Temperature tends to dictate lifespan for aluminum electrolytic capacitors. The highest temperature in a Cobra 2000 is inside the clock/counter module. Tends to make them fail first. Recapping that module will cure the oddball noises. And also provides a hint that the electrolytics in the rest of the radio will follow this trend in the next year or two. Or maybe sooner.

73

 

[Cable Guy]

I have already recapped everything. There isn't 1 old alum'lytic standing in the crowd. It's really not noticable, unless the volume is full tilt, and the counter does work, just a detail I noticed. I will check over the caps again.