I have a trc 458 I got given to me. Previous owner said he accidentally hooked it up backwards. So now no receive ( just very strong signals) and it also has a 20 watt carrier on ssb. I also own a cobra 139 XLR and a Robyn 520d that also suffer the no receive problem.
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Trc 458 no receive and carrier on ssb
- Thread starter k9fon12374
- Start date
All three of those radios use the same main circuit board, more or less. This leaves you with the same problem as the guy who wants to take several junkyard cars and make one good car by taking parts off of the rest of them. What he discovers is that the exact same stuff has gone bad on all of them. No amount of swapping around will overcome this fact.
A little history goes a long way. Especially with radios that are 47 years old. The design inside those early 40-channel radios made it too easy to hack illegal transmit channels from them. The FCC said the only CB radios they would approve for legal sale after 1978 had to be locked down better to discourage transmitting on frequencies that aren't legal. CB was crowded back then, so installing channels above 40 and below channel 1 was very popular. And not legal at all.
Bottom line is that radios made for not much more than just one year won't have a lot of donor carcasses in the junk yards. That is, if you can find a CB junk yard.
Most important thing to know is how many attempts have been made to fix any of them. For every failed repair attempt, expect to find added troubles caused by the failed troubleshooting effort. That kind of history goes a long way. Where it's been and what's been done to it are valuable to know.
The reverse-polarity radio will have a list of failed stuff in it. The radio has a part wired across the DC power input called a protection diode. It does nothing at all when the power is hooked up properly. But if it's reversed, this diode acts like a dead short and trips the fuse. But only if there is a fuse. If someone wraps the foil from his cigarette pack around the fuse and puts it back in, stuff blows up.
A "no receive" problem starts with making sure the mike is not broken. Mikes break more often than radios do. Put the radio on an antenna and see if the S-meter kicks around. If it's just dead and stays at zero, that's a receiver fault. If it jumps around on channel chatter and noise, you have an audio problem.
Just narrowing down where to look in the radio is the best place to start.
Service data and diagrams for that kind of radio can be found on the web. How are you at reading schematic diagrams?
Mileage is an issue, but there's no odometer to read for that. Somewhere between 30 and 40 years of age, a whole category of parts called electrolytic capacitors will begin to fail from age even if the mileage is low. They break down sooner in a high-mileage radio that was left on 24/7 for ten or more years. A new-in-box radio may not have any of them gone bad yet. But once it gets put back to work, those parts tend to "remember" how old they are in a week or a month and start to fail one by one.
A member on this forum, Klondike Mike sells packaged kits of all the electrolytic caps in a whole list of radios. Bought one for a similar 1978 radio a month or so back. Had 36 caps in it, if memory serves.
But hey, if it was a 1978 car it would need every gasket, seal, belt and bushing changed out, right?
Doesn't mean you can't make one of these a reliable daily driver. Just means it's not a small job as a rule.
73
A little history goes a long way. Especially with radios that are 47 years old. The design inside those early 40-channel radios made it too easy to hack illegal transmit channels from them. The FCC said the only CB radios they would approve for legal sale after 1978 had to be locked down better to discourage transmitting on frequencies that aren't legal. CB was crowded back then, so installing channels above 40 and below channel 1 was very popular. And not legal at all.
Bottom line is that radios made for not much more than just one year won't have a lot of donor carcasses in the junk yards. That is, if you can find a CB junk yard.
Most important thing to know is how many attempts have been made to fix any of them. For every failed repair attempt, expect to find added troubles caused by the failed troubleshooting effort. That kind of history goes a long way. Where it's been and what's been done to it are valuable to know.
The reverse-polarity radio will have a list of failed stuff in it. The radio has a part wired across the DC power input called a protection diode. It does nothing at all when the power is hooked up properly. But if it's reversed, this diode acts like a dead short and trips the fuse. But only if there is a fuse. If someone wraps the foil from his cigarette pack around the fuse and puts it back in, stuff blows up.
A "no receive" problem starts with making sure the mike is not broken. Mikes break more often than radios do. Put the radio on an antenna and see if the S-meter kicks around. If it's just dead and stays at zero, that's a receiver fault. If it jumps around on channel chatter and noise, you have an audio problem.
Just narrowing down where to look in the radio is the best place to start.
Service data and diagrams for that kind of radio can be found on the web. How are you at reading schematic diagrams?
Mileage is an issue, but there's no odometer to read for that. Somewhere between 30 and 40 years of age, a whole category of parts called electrolytic capacitors will begin to fail from age even if the mileage is low. They break down sooner in a high-mileage radio that was left on 24/7 for ten or more years. A new-in-box radio may not have any of them gone bad yet. But once it gets put back to work, those parts tend to "remember" how old they are in a week or a month and start to fail one by one.
A member on this forum, Klondike Mike sells packaged kits of all the electrolytic caps in a whole list of radios. Bought one for a similar 1978 radio a month or so back. Had 36 caps in it, if memory serves.
But hey, if it was a 1978 car it would need every gasket, seal, belt and bushing changed out, right?
Doesn't mean you can't make one of these a reliable daily driver. Just means it's not a small job as a rule.
73
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What? Really??
I think this would help tremendously!!
As far as caps i buy my own and make my own kits i dont need to buy a ready-made kit. Yes i know its old as i own several old cb sets. Ive been in this hobby 30 year's and yes i can read a schematic. I only posted to find out what i need to look for as far as a part or cuicuit.
No not really. He gave me a generic reply of things i already know. I simply need to know what to look for. I guess if i cant get help here i can ask others.
No Chris offered you very good advice, I don't know what you are looking for?
It's extremely difficult to diagnose a radio from one post on a radio forum via interwebs......
No receive as in only super strong signals and a carrier on ssb. Both of my 458 Navajo bases have the same issue. Im 100% on my own as i have no techs in my area anymore and i cant afford to pay back and forth shipping.
Have a look at the 10mm RF/IF transformers. The ones with the skinny slug. Do any of them show a peak with the slug dead-flush with the rim of the hole?
If so, this is not a resonant peak. It's the inductor's end-of-travel point. That flush position is the coils max inductance setting. It's common for the internal tubular ceramic capacitor to go open circuit. When it does, increasing the inductance will compensate, but not enough to resonate properly.
If the coil's frequency is high enough, soldering a trimmer cap on the solder side of the pcb to the two pins on the resonant side of the coil can fix this without replacing the whole part. Besides, some of the 10mm coils have become hard to find.
This Cobra 142 had a bad case, but the seven trimmer caps restored each tuning slug to resonate well below the max setting and got the radio back to full signal levels.
And if all of them show a proper resonant peak with the slug well below the rim of the hole, this won't help.
73
If so, this is not a resonant peak. It's the inductor's end-of-travel point. That flush position is the coils max inductance setting. It's common for the internal tubular ceramic capacitor to go open circuit. When it does, increasing the inductance will compensate, but not enough to resonate properly.
If the coil's frequency is high enough, soldering a trimmer cap on the solder side of the pcb to the two pins on the resonant side of the coil can fix this without replacing the whole part. Besides, some of the 10mm coils have become hard to find.
This Cobra 142 had a bad case, but the seven trimmer caps restored each tuning slug to resonate well below the max setting and got the radio back to full signal levels.
And if all of them show a proper resonant peak with the slug well below the rim of the hole, this won't help.
73
HEY K9FON -
You ask a generic question, you get a generic answer. You want specific things to check, but you don't provide us with any specific things you've done to diagnose the problem. "No receive" covers a wide spectrum of possible problems. Chris (Nomad) posted a reply that he thought might help. and all he got in return is attitude. That won't get you very far here.
- J.J. 399
Well if you can actually read schematics that should also mean you can tell the difference between "RECEIVE" circuits, PLL circuits, Transmitter circuits, VCO circuits, AGC circuits, Mic Preamp circuits, Receive mixer stages, Transmit mixer stages, Audio amp stages and of course "BALANCED" modulator circuits and, if all that is true then you should be able to tell us what parts to look for? It may be you don't actually want to take the time to learn what to look for? Sorry everyone guess I'm in a bad mood today so I took it out on the newbeeeeeeee!
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Hey K9FON -
A question about the "no receive" problem. Is there no receive in both AM and SSB ?? You said you have receive with loud stations. I would check the receive protection diodes (D64/65) . These are on the solder side of the board and were added as a fix post-production. 1N914 diodes. The only 858SSb schematic they are shown on is the TRC-449. If one or the other is shorted, you will have the receive situation you posted. Also check the uPC1156H audio chip. One of the first things to go out when the rig is hooked up backwards. Chip is NLA, but available on Ebay.
As far as the carrier on SSb, have you checked the carrier balance adjustment VR4 ?? It is used to null out any carrier from the balanced modulator. If VR4 has no effect, check to see if it's open. If it's OK suspect that the balanced modulator chip (IC2-MC1495A) is fried
If this doesn't fix things, let us know.
73.
- J.J. 399
A question about the "no receive" problem. Is there no receive in both AM and SSB ?? You said you have receive with loud stations. I would check the receive protection diodes (D64/65) . These are on the solder side of the board and were added as a fix post-production. 1N914 diodes. The only 858SSb schematic they are shown on is the TRC-449. If one or the other is shorted, you will have the receive situation you posted. Also check the uPC1156H audio chip. One of the first things to go out when the rig is hooked up backwards. Chip is NLA, but available on Ebay.
As far as the carrier on SSb, have you checked the carrier balance adjustment VR4 ?? It is used to null out any carrier from the balanced modulator. If VR4 has no effect, check to see if it's open. If it's OK suspect that the balanced modulator chip (IC2-MC1495A) is fried
If this doesn't fix things, let us know.
73.
- J.J. 399
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Funny you mention the upc1156H I just received 2 of those in the mail as I have a Robyn 510D mobile that oddly enough was given to me 5 years ago the man said he didn't even get to use it as he hooked it up backwards so to speak it looks like a brand new radio but I put it on the shelf and forgot about till last week I seen they were bringing good money so I pulled off the shelf and got it working except for the 1156 chip which I can now replace.
Well for one im NOT a newbie to the radio hobby and of course i can read a profile. If you want to be rude ill ignore you.