Hello all, this is my first post here and I’m interested in learning about the more technical side of CBs and need some advice on troubleshooting a Cobra 29NHST. I’m hoping the symptoms I describe will help someone point me in the right direction. Sorry for the long post; I’m just trying to provide a good description. The radio is a ’97 model (china) I bought new years ago and worked fine until some juice or soda spilled in the faceplate. Lesson learned: don’t mount a cb under cupholders.
I had an upcoming road trip so I took it to the local CB repair guy to take a look at it. He got it working. He said the board had a lot of junk on it (juice/soda). He cleaned it, removed some spotty rust and fixed a few poor or broken grounds. He said he replaced the audio driver final, 9/19/Nor switch and peaked and tuned it. (He said having him fix the backlighting wasn’t worth the cost. I agreed with him.) It tested 6.5W deadkey, 27W swing, 100%++ modulation. He then hooked it to the meter to show me, but the unit wouldn’t RX or TX, even though he just had it working. He was a bit perplexed. He suspected the rotary channel selector so he opened it up and used contact cleaner in the selector. It worked, he showed me how it tested on the meters again, sounded good, so I took it home. This guy worked on my brother’s Cobra 25 as well and the results were very good.
Before leaving for my trip, I tried the radio again and found it would not RX or TX again. So I got a new 29LTDclassic radio on sale for the trip. After the trip, I couldn’t get in contact with the repair guy. I decided to open up the radio since I wanted to fix the backlighting (this is the NH model with incandescent bulbs, not the NW model with integral board), and then also spend some time attempting to troubleshoot the problem. I applied more contact cleaner to the rotary channel selector. It worked for a few minutes, but then no RX or TX. Figuring I had nothing to lose, I removed the rear plate of the channel selector and gently spread the aqua colored ‘slices’ of the channel selector apart to try and get more cleaner in. As before, RX and TX worked sporadically.
My setup is a Wilson 5000 mounted on the front left corner of the truck bed; 2002 F350. I ran a dedicated ground wire from the mount to the truck frame. The new LTD radio works fine with this. SWR on the radio meter reads about 1.2. Swapping in the 29NH radio, it reads 3.1 on its SWR meter (ouch). Is this because of the peak & tune work done? I need to get another diagnostic SWR meter to see what’s really going on as my original grew legs and walked away.
Here is the main symptom of how the radio is operating: When turned on, the radio will immediately RX and TX when put to channels 32 through 40. The other channels will not RX/TX immediately. I can get the other channels to receive, but I must start at 32, then drop to 31 and it will come in after about a minute of waiting. About 15 seconds before the channel comes in, I hear a very faint rapid clipping sound through the speaker which slows to about the speed/sound of an idling helicopter and gets louder right before the channel comes in. Once in, I’m ok if I stay on the channel and RX and TX work. I can move down to 30 and wait another minute for it to come in. I can use this method to bring all the channels in, one at a time in sequence, if I’m patient enough. If I switch channels, I’ll need to wait to bring previous channel back in, but not as long. I did most troubleshooting a few months ago and it seemed the cold made it take much longer to bring a channel in. It’s as if something needs to ‘warm up’ in order to receive and transmit.
Do you think this is a channel selector issue, or something deeper? Is the peak and tune causing issues? I’ve been perusing the 29 schematic diagram, factory service manual, and service bulletins, learning what I can. At this point I’m curious what other people might diagnose. Right now I only have a multimeter, so my ability to diagnose is a bit limited.
Thanks for reading the long post and any advice you give.
I had an upcoming road trip so I took it to the local CB repair guy to take a look at it. He got it working. He said the board had a lot of junk on it (juice/soda). He cleaned it, removed some spotty rust and fixed a few poor or broken grounds. He said he replaced the audio driver final, 9/19/Nor switch and peaked and tuned it. (He said having him fix the backlighting wasn’t worth the cost. I agreed with him.) It tested 6.5W deadkey, 27W swing, 100%++ modulation. He then hooked it to the meter to show me, but the unit wouldn’t RX or TX, even though he just had it working. He was a bit perplexed. He suspected the rotary channel selector so he opened it up and used contact cleaner in the selector. It worked, he showed me how it tested on the meters again, sounded good, so I took it home. This guy worked on my brother’s Cobra 25 as well and the results were very good.
Before leaving for my trip, I tried the radio again and found it would not RX or TX again. So I got a new 29LTDclassic radio on sale for the trip. After the trip, I couldn’t get in contact with the repair guy. I decided to open up the radio since I wanted to fix the backlighting (this is the NH model with incandescent bulbs, not the NW model with integral board), and then also spend some time attempting to troubleshoot the problem. I applied more contact cleaner to the rotary channel selector. It worked for a few minutes, but then no RX or TX. Figuring I had nothing to lose, I removed the rear plate of the channel selector and gently spread the aqua colored ‘slices’ of the channel selector apart to try and get more cleaner in. As before, RX and TX worked sporadically.
My setup is a Wilson 5000 mounted on the front left corner of the truck bed; 2002 F350. I ran a dedicated ground wire from the mount to the truck frame. The new LTD radio works fine with this. SWR on the radio meter reads about 1.2. Swapping in the 29NH radio, it reads 3.1 on its SWR meter (ouch). Is this because of the peak & tune work done? I need to get another diagnostic SWR meter to see what’s really going on as my original grew legs and walked away.
Here is the main symptom of how the radio is operating: When turned on, the radio will immediately RX and TX when put to channels 32 through 40. The other channels will not RX/TX immediately. I can get the other channels to receive, but I must start at 32, then drop to 31 and it will come in after about a minute of waiting. About 15 seconds before the channel comes in, I hear a very faint rapid clipping sound through the speaker which slows to about the speed/sound of an idling helicopter and gets louder right before the channel comes in. Once in, I’m ok if I stay on the channel and RX and TX work. I can move down to 30 and wait another minute for it to come in. I can use this method to bring all the channels in, one at a time in sequence, if I’m patient enough. If I switch channels, I’ll need to wait to bring previous channel back in, but not as long. I did most troubleshooting a few months ago and it seemed the cold made it take much longer to bring a channel in. It’s as if something needs to ‘warm up’ in order to receive and transmit.
Do you think this is a channel selector issue, or something deeper? Is the peak and tune causing issues? I’ve been perusing the 29 schematic diagram, factory service manual, and service bulletins, learning what I can. At this point I’m curious what other people might diagnose. Right now I only have a multimeter, so my ability to diagnose is a bit limited.
Thanks for reading the long post and any advice you give.