My first Cobra 2000 work...

[Hawkeye351]

Finishing up this Cobra 2000 GTL that was bought brand new back in the early 90s by a late friend of mine, rest his soul. He bought this radio brand new from cobra back in the early 90s and brought it plus an expo kit straight over to my shop before he even opened the box. I was the one to unbox this radio.

It's the first Cobra 2000 I ever worked on back in the day. The owner passed away and his wife put it up in storage. It's been in storage for about 15yrs.

The son finally decided to pull it out and hook it up. Bad idea after sitting in storage for 15yrs. Things just weren't right on it so he unhooked it and put it away until he heard I was back into working on radios.

I'm the only one that's ever been in this 2000 since it was brand new. Brings back memories for sure.

After looking over my old soldering work, I decided to redo a lot of my past work.

She's screaming now, lol...
New caps all the way thru, except the 50v caps.
Frequency counter was showing 92.000 all the time. Frequency counter module fixed.
Channel display had segments gone out, replaced the whole segment board.
Removed the old super slide choke from the varactor diode. Placed the diode alone back in place.
Removed the old way of unlocking the clarifier, did it the right way this time.
Replaced S/RF meter with a new one.
Fixed the stuck needle for the modulation/SWR meter.
Checked and replaced all lights with originals, not LED's.
Cleaned all controls and switches.
Realigned entire radio.
Reset bias.
Tuned output stage.
Set dead key to 2.5w.

Before:
AM - variable key set to 5.5w, swing to 10w peak, swing backward to 4w average.
SSB - 14w peak.

After:
AM - variable key set to 5w, swings up to 14w peak, swings forward up to 5.5w in average.
SSB - 18w peak.

This one truly has been a pleasure working on.

 

[guitar_199]

Just for fun.........

the "change in swing direction"......

what do you think you did that attributed to that? TO clarify... what do you think took it from "backward swinging" to "forward swinging"?

(I am still trying to work out a better understanding of "drivers/finals" and "modulation/swing"!!!)

Thanks,
Bob

 

[Hawkeye351]

Bias plays a big role in forward or backward average swing. Too much current draw in other words.

Over modulation.

Beefing up the output without beefing up the regulation side of things.

Removing limiters. The limiters are there for a reason. My theory is, those limiters in place will prevent your negative peaks from going above 100%. You can do certain other things to increase the positive peaks above 100% without removing the limiters, while the limiters prevent the negative peaks from going over 100%. You can accomplish 180% positive peaks while maintaining the negative peaks around 100%, but you need the right equipment and know how.

Proper alignment.

All these play a role in forward or backward swing.

My case with this 2000?
The factory passthrough regulator (1419 or the 1135) are flimsy and weak. Upgrading just that regulator with a 6487G will improve audio and stability a lot, not necessarily watts, but audio will definitely be improved. Plus driver and final bias being slightly higher than they should have been. And another is, the swapping out of the AMC Timing capacitor (C109, 10uf 16v) to a 4.7uf 25v (although, you can leave the factory cap in place and just strap a 22k ohm resistor across the caps legs on the solder side, I like this way the best). This prevents the AMC circuit from being so aggressive. And lastly, the VCO Buffers were low (this is where your audio sensitivity for transmit and receive comes into play).

These guys can explain it better than I can. You're on the right site for guidance on subjects such as this.