A.K.A. The "Bus"
by Spitfire, 441
This is the long anticipated review of the Cobra 98, also known in the local area as "The Bus". For an explanation of how this rig got that rather dubious nickname, check this out and see if you don't see the resemblance. If not so anticipated by others, at least I could not wait to finally review this radio. I really wanted this radio to perform well as I like the looks. But I believe it is not fair to review a radio before it is fully restored and operating to as close to original specification as possible. Well, the time has finally come. So sit back and relax and Iíll relate how this radio started and the load of little, and not so little, problems I had to overcome to get this radio to finally become a very fine performer.
While I am on the subject of performance, the night before I wrote this was one of those nights where the ground wave 11-meter propagation was enhanced. I was out giving the "Bus" a drive. While magpie-ing on channel with some of the locals, a station in Upper Black Eddy Pennsylvania shouted to me and complemented me on my audio quality. Upper Black Eddy is about 35 to 40 miles from my location, and not a place we usually hear from. He said that my signal was not very big however my audio carried the day. You have to love these tube rigs when they're working right. They just have great audio.
Now back to the story. So just how did the "Bus" arrive at this destination? It took a circuitous route through a series of detours. The restoration of this radio went somewhat concurrent with the CAM 88ís, as they are very similar in circuitry. The only truly major difference is that the Cobra 98 has a built in SWR bridge. This radio was another Ebay purchase. It was bought after having a couple of the CAM 88ís already on the classic radio shelf. Although this radio arrived in very good clean cosmetic shape, electrically it was a mess. I started out by testing all the tubes and replacing all the electrolytic capacitors. I did not have to replace the big filter can. It seemed to be ok, which is a good thing as my supply of filter cans has now been exhausted. So after that bit of surgery and replacing the weak or bad tubes I gave it an alignment. Things were looking pretty good so far. The receiver came up very nice and pretty much on par with the 88ís. On the transmit side of things though, it didnít look so good. The power was up to 3 watts carrier, but it had back swing with modulation. On the air reports were not very favorable with any appreciable mike gain. I had to drive it with barely any audio to keep from distortion. Bummer, what is the problem now? So after some on-air discussion of the radio's symptoms, it came down to a problem with neutralizing the final. I am using a new Russian 6BQ5 and had good results with those tubes on the 88ís and also used the Russian tube on my Lafayette HB 400 audio and output, so I figured it should work ok here as well.
So back to the test bench to find the problem. I started probing around with my newly acquired and increasingly invaluable piece of test gear, a digital capacitance meter which I, again on Ebay, purchased. So with probes in hand, I started checking for capacitors and resistors falling out of value tolerance. I zeroed in on the mica variable capacitors for tuning and neutralizing the final and driver. As I tuned them through their range, they didnít have the range called for in the schematic. So again back to see if the parts 88 (remember that? It's still offering up parts.) had any good mica variable caps. Well I found one out of two good. But I still needed another. I got lucky there, as I had one in the spare parts bin. That one came courtesy of my invaluable friend at Zerns. So after installing those replacement caps in the "Bus", and tuning it up with the valuable assistance of the spectrum analyzer, things were looking real good there. Clean signal, and seemingly good loud audio. No back swing, but no forward swing either. Now it was time for a radio check. On-air reports were better, but I still couldnít drive it with a lot of audio without clipping distortion. While it was an improvement from before, I was not out of the woods just yet. Back on the test bench, where I started looking more in the audio section. The problem turned out to be bad modulation clipper diodes. It would seem that this radio has been a culmination of all the problems of previous radios plus some, all conspiring to test my troubleshooting and repair capabilities. While inside the radio this time, I decided to replace one of the synthesizer crystals which was 5kc off frequency. After a donated crystal from the parts '88 again, the "Bus" is now on frequency on all channels +/- 300 cycles. So back on the air again to see if that got it. Can I get a radio check? This time the "Bus" was able to take all the audio a Night Special amplified D-104 could pump into it. It did this with loud crystal clear audio and no distortion. Output power had forward swing with modulation now. Finally I had gotten the "Bus" up to where I could be happy to take it out for a drive. Receive was good and transmit was, as some said, the best sounding radio I have. Of course, as mean old Mr. Murphy, of Murphyís Law fame would have it, it was not to last long. After a few days of running the "Bus", I started to notice some extra static in my receiver. I discounted this to RFI from the wife's bedroom TV. Well a day or so after the static on receive first arrived, I turned on the rig to take a drive, and it surprised me by letting out a big cloud of the proverbial magic smoke. So now the "Bus" takes yet another trip back to the test bench. Things were not all doom and gloom though. The transmitter was still there, but receive was gone. Opened the rig and found a burnt 10k resistor charred all black. It was the high voltage feed to the tube in the 2nd IF. I replaced it, and went on the search for a cause. It also stressed some wires and components in the high voltage supply. The root cause turned out to be a .01 disc ceramic along with a 10pf mica capacitor which was breaking down under high voltage. More surgery there and time to test-drive the "Bus" again. In retrospect, I should have paid greater attention to the arrival of the static.
The features on the rig are pretty basic. A Volume, Squelch, Fine Tune, "Dyna Boost", and PA/CB switch. In addition to the standard S/RF meter, a built in SWR bridge is included. The Fine Tune is nice as it is a continuously variable capacitor instead of a switched +/-. The fine tune is good for about 2.5kc of shift +/-. The modulation light presents a good indication of output modulation. The "Sí meter does a fair job of displaying relative signal level, and it is fairly linear. The power output meter is pretty much on. The meter is of a good size to be easily read. I also like the way the meter is backlit instead of lit from the front.
Now for the bench numbers:
Sensitivity: .22uV for 10db S+N/N
Adjacent channel selectivity: 45db @ 10Khz.
IF: 1st 37.5 Mhz, 2nd 475khz.
AM Power: 3.5 watts with forward swing.
AM Modulation: 100+%
2nd harmonic suppression: -60db+
Other spurious emissions: -55dbc.
This is where the Bus has taken us now. I was using it last night and having a total blast. It has screaming audio and good sensitive and quiet receive. And of course, it has that warm tube audio sound on both ends. I did take the liberty of fashioning a set of custom "wheels" or knobs for the "Bus", with the help of some aluminum stock and a metal lathe. With much thanks to Firestarter for the use of his Frankensteinís Laboratory. It has given a very nice enhancement and feel to an enjoyable radio.