Cobra 2000 Restoration........    

A revered solid state classic finds its way into my hands.   

 

 

I've written a couple of other restoration articles, when I rescued a pair of Tram D201's  from a sure fate at the scrappers, as well as bringing a Browning Golden Eagle MKIII back to full glory. I've also restored my original Lafayette Comstat 25, and have two more Trams waiting to be worked on. These radios all have one thing in common, they were all tube-type radios, and at least 36 years old.  In my article, "Classic Radios", I touched on the highly subjective topic of what makes a radio a classic. It was generally agreed upon, that tube radios were defacto classics, but there were also many solid state rigs which also made the grade as well.  Mostly these were 23 channel rigs, made before the January 1977 date when 40 channels became legal.  But also included in the broader "classic" definition are radios which are over 25 years old.  25 years was picked since automobiles are considered antique after 25 years, so that number made sense for radios as well.  Six years ago, when the local classic radio aficionados agreed on this, radios made in 1981 and older would qualify. Today, we're up to 1987 as the cutoff. It's tough to imagine the lackluster radios from the late 80's as "classics", but the years are flying by and 25 years old is what it is.  While the 80's were pretty much on the downside of the CB boom, and both the quality and selection of domestic radios was shrinking in proportion, one of the most popular radios to come from the 1980's though, was the Cobra 2000. This radio was first manufactured in 1978, and was produced into the early 90's. When you consider that a typical radio model only lasts for 3 or 4 years, the longevity of this model is a testament to its desirability and popularity.  Arguably, the Cobra 2000 is to the 80's, what the Tram D201, or Browning was to the 70's. It certainly qualifies as a "Premium Rig".

Along with my tube classics, I also have a good selection of solid state classics in my collection as well. Typically though, they have required far less restorative work to bring them back to their former glory. A little cleaning, an alignment, maybe a couple of replaced parts and away they go.  So it's been tough to call any of my solid state radio projects a "restoration", in the same vein as the Tram D201 or Browning Eagle projects.  So where do you draw the line where a "freshen up" or simple repair operation ends and a bona-fide restoration begins? I'm sure that everyone has their own idea, but as far as I'm concerned, the Cobra 2000 I have recently received will definitely qualify as a full restoration.

First off, everything is relative. If you want a sample of a classic radio that's in clean, 100% working condition, especially one that's a popular and still sought after like the Cobra 2000, be prepared to pay a king's bounty for it. The full retail price (and who pays retail?) of the 2000, when it was still in production, was around $400.  Now, over 15 years after production ended, clean examples of the 2000 are still fetching $400 (or more!) on places like E-Bay. So knowing that I like to find deals on the cheap, it doesn't take much imagination to conclude that my 2000 was nowhere near clean, 100% working order.  No, my 2000 was a victim of a Frankenstein tech, as well as suffering the effects of its age.

When I inspected my latest project, I made a mental list of what was wrong and, more importantly, what was RIGHT. The big thing for me was that the frequency counter was still working. These radios are known for the counters failing, and since the major parts are no longer available, it makes repair prospects a little dicey. The receiver appeared to work as well, but seemed weak. The transmitter was not putting out any measurable power, but it could be heard on another bench radio. "Bad final" was what was told to me by the seller. In addition, the radio had an add-on roger beep board as well as 2 extra crystals and the usual PLL cuts and jumps to give the radio a host of extra channels. The bad news here was that 2 holes were drilled in the front panel to accommodate the two PLL switches.  The front panel headphones jack was replaced with a variable dead key pot. The tone control had been replaced by a 3 position switch for the extra crystals.  The SWR/modulation meter was sticking, and the meter lamps were burned out.  And to put icing on the cake, the radio was used by a heavy smoker and it reeked of stale smoke, and there was a film of yellow tar over the whole surface of the cabinet, the faceplate, and the knobs.  Yea, I think this officially qualifies this dog as a restoration candidate.

I started off my project by attempting to clean up the outside and to minimize the smell of stale smoke. The knobs cleaned up rather well, as did the faceplate decals. I have heard that there are places that sell new decals, so I may end up going that route when the project is completed. I also blew out a ton of dust and crud from the inside of the radio. All pots were given a shot of tuner cleaner and rocked back and forth. When the bottom cover was removed, I noticed that the headphone jack and the tone control was wrapped up in electrical tape and left hanging. The roger beep board was wrapped in a plastic bag and tie-wrapped to the main wiring harness.  Shaking my head, I strongly resisted the urge to whip out the cutters and start removing all this junk.  I promised myself that I'd wait until I got the radio to work a bit better before I started "de-junking" it. I then fired the radio up to see what electrical work I needed to do. The channels were all there, and I noticed that the clarifier had been modified for extended range and was sliding down nearly 2 channels. So now I had to sit back for a moment and ponder the usefulness of this mod. Any straight up PLL rig moves in 10 Khz steps and can be made to fill in the gap (RC) channels with a simple 3 position switch (and in fact that mod had been done to this 2000).  So with all 10 Khz gaps filled, why would anyone need to slide the clarifier any more than +/- 5 Khz, much less 15+? With that much clarifier range, tuning in on SSB will be more difficult, and there always the potential for instability issues.  But to each his own I guess. As stated before, the transmit power was not there so I set about to replace the final. I had a spare 2SC-2312 from a junk radio, so I dropped that in. But the transmitter still was not making any power. Hmmmm....... The driver then? That didn't make sense as I could hear modulation, and the driver and final are the only places where modulation is applied, so if the driver is bad, there should be no modulation. On a whim, I decided to give the radio an alignment. Well lo and behold, the power started coming up. Every coil and transformer in the transmitter was way off, which makes me shake my head yet again. SSB power was running at about 18 watts PEP and the dead key power was at 4 watts. But there was now another problem. There was a constant tone on AM which modulated at nearly 100%. Varying the mic gain control had no effect. Even all the way down, the tone was still there. At first I thought it had something to do with the add-on roger beep board, so I now had a good excuse to rip the darned thing out. But after removing the roger beep and reconnecting the wires to the mic jack, the tone was still there. Curiously, the tone was not present on SSB though. Thus began the long and arduous process of tracking this issue down.  During the process of troubleshooting, I decided that it was time to remove the remaining modifications and return the rig to as close to factory stock as possible.  The tone control was returned to its rightful spot, along with the headphone jack. The "aux hole" jack, however, had been cut and was not in the rig.  I will have to scrounge a suitable replacement.  After much time was spent trying to isolate the section responsible, the problem turned out to be C18, a 330uF 10V cap. This vintage Uniden-made radio (like others) was built with a number of 10V capacitors in them.  For whatever reason, the 10V caps do not seem to age well, and tend to fail far more often than other types in these radios.  I've run across similar problems in other Uniden-made rigs, and between my own experiences and those of others, we've nicknamed this particular ailment the "10 Volt Blues".

After correcting the "constant tone" issue, the radio seemed to function well enough to take out for a test drive during that Wednesday night's Classic Radio Roundup on Channel 13. Before doing that however, I replaced TR24 (the main modulation limiter), as it had been previously removed (no surprise there).  But even with TR24 replaced, neither the AMC or the ALC seemed to work.  Nothing else in the circuit seemed out of place, so I tabled that issue and proceeded to run the rig on the roundup.  Immediately, I noticed that the receiver was definitely not up to snuff, so I gave it a quick and dirty alignment, and managed to get the sensitivity up to a usable level. But once again, I noticed that someone had added an additional crystal filter in the I.F. (presumably to improve adjacent channel rejection).  Crystal filters have loss associated with them, so there's a trade-off between a tighter IF and best overall sensitivity.  So I will most likely remove them as the need for a sharper I.F. is not required at my location, where my nearest CB neighbor is 3 miles away. The local C.R.R. crew overall liked the sound of the radio, and decided that my Turner +3 desk mic provided the overall best fidelity. I'll most likely do some fidelity improving mods to the radio once I correct all of its ills, so another musical mic contest will be in order at a future date.

During the next opportunity I had to work on the radio, I gave the receiver a complete alignment with my test equipment. I also set about to undo the extended clarifier range and set it up for a far less touchy coarse range of +/- 5 Khz, and a fine range of +/- 500hz. I removed a choke, which had been placed in series with D35 to give extended tuning range. I added a resistor to ground from the ground side of the coarse clarifier pot to give a total of 10 Khz of tuning. I set the extreme ends of the range with L22, 23 and 59. Centering the center slot frequency was accomplished by placing a resistor from the clarifier pot wiper to ground.  Setting up the fine clarifier pot was accomplished in a similar manner. So one more item gets the "DONE" stamp. The next item on the list was the sticking SWR/MOD meter. This is also a common problem with these radios. The good news is that replacement meters can be found, but they typically cost close to $30, which is a bit salty for a simple meter.  Fortunately, there is a way to "fix" a stuck meter. Usually, you can loosen the bearing screw on the movement a slight amount which forces the needle to use a different place on the jewel as its bearing.  So after removing the meter from the radio, and giving a careful 1/4 turn of the screw, the needle freed up and the meter appears to be working again.  Granted, the "fix" may not last all that long, but I've done this before on countless radio meters, and they usually work fairly well. I also replaced the meter lamps. I had thoughts on replacing them with white LED's.  But the color frequency of the light is not the same as the warm glow of a filament lamp, so I opted for original type bulbs.

A few days later and it was time to tackle the inoperative modulation limiters again. I had to chuckle at all the effort I was putting into restoring the AMC operation. There was a time when I would have clipped them out and felt good about it. But now here I was trying to get them working again. My desire for clean audio nowadays pretty much demands it. I started off by using the scope probe to trace the feedback signal back through the AMC circuit. But without a working radio to compare it to, I couldn't tell if what I was seeing was correct or not. After tracing and checking voltages, I noticed that a resistor had been cut (and 10 years ago, when my eyesight was much better, I would've spotted it sooner). I wondered why someone felt the need to clip the resistor when the main AMC transistor was already removed. But for whatever reason, they did it. The good news is that once the resistor was replaced, the AMC circuit became functional. I also had to replace the AMC pot, as it had a broken connection and could not be properly set. SSB ALC was set for 15 watts and AM modulation was was set for just over 100%.

Now that the modulation limiter situation has been rectified, it was time to revisit the receiver. It seemed to be "ok", but not overly sensitive. I had mentioned before that a previous owner had added a pair of crystals to act like an additional filter to the AM I.F. Knowing that this mod most likely added a bit of additional loss to the I.F., so I removed those and realigned the radio. This time, sensitivity improved until I was now measuring 12db S+N/N at .3 uV. This is more in-line with expectations.

A subsequent test drive showed improved performance, but I also noticed that the transmitter carrier level fluctuated. So I suspect another dirty potentiometer will need to be replaced.

Well another week has gone by and the Cobra 2000 is pretty much done except for that nagging carrier fluctuation issue. It's not a dirty pot as I first suspected, but rather some level of intermittent in and around the area of the AM power regulator/modulator. I can make the power jump by moving/flexing parts in the area. Well, after an exhausting session of poking, flexing, tapping, and measuring, I finally tracked down the culprit. It would seem that one end of thermal protection diode D49 on the final transistor had a cold solder joint on one end, and it would intermittently break contact causing the final transistor bias to take off, and that caused the power to drop. I guess I'm lucky it didn't pop the final. But in any case it's all fixed now.

And here is the finished product in all of its glory:

 

I'll publish a full review of the radio at a later date after I've played with it for a while, and taken care of any issues that might pop up.

 

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