Factory radio/cb recieves but won't transmit

[Chatt_79]

A few owners ago someone installed a tuffy center console in my 79 wagoneer and f'd with all the stock radio wiring to hook up their new radio in the tuffy. I don't use CB much and never over long distances so I thought it would be cool to have he stock unit working. Just got around to jurry rigging the stock radio to see if it works. Got power to it and hooked it to a speaker and the radio and cb reciever work but the CB won't transmit. I bought a 4 pin mic off amazon and it hooked up to the dash connector fine. When I hit the transmit button the speaker cuts out and the ?transmit power? gauge moves like its working but not receiving from my other CB that I know works fine. Any advice? I'm assuming the mic is working fine since the speaker is cutting out, but it is a noise canceling mic so I don't know if that can be an issue. What would you guys look at first? Or would you just go looking for a new radio and separate CB unit?

P.S. Thanks for all the old discussions on factory radio's on here and IFSJA. The wiring was pretty easy to figure out so far. My Factory service manual doesn't come in till next week so I needed all the help I could get.

[Stuka]

Is the antenna still hooked up to the old stereo? The factory AM/FM/CB radio has a dual purpose antenna that is both for the AM/FB and the CB. Not sure if that mic could be the issue or not.

Quite possible that it needs to be overhauled inside. 35 years will make a lot of electronics not work correctly. Could be anything from corrosion to bad electrolytic caps.

[TUDrewser]

Sometimes you have to switch the wires in the Mic. or incompatible mic. I had the same issue with a non oem cb and bought a different mic on Amazon...That fixed mine.

Sent from my SCH-I545 using Tapatalk

[Tatsadasayago]

Adding to what has already been said, I'll add that yes, AMC used a duplexer device with many of those combination radios. The unit consisted of a capacitor and inductor to 'load' the AM/FM antenna so it was resonant at the 27Mhz CB frequency and provided a 50 Ohm 'load'. Those I've worked with had power and switching wires going to the duplexer....a few were of a different design and used no external power or switching.
I would suggest you investigate under the dash, locate the antenna coax cable and follow it from teh radio to antenna base making sure the duplexer is there and hooked up.

If your receiving radio was within a reasonable distance from the transmitting radio, you should have at least heard some 'quieting' on the Rx radio even if the Tx radio's antenna is not connected. I don't recommend ever transmitting without a proper load (A tuned antenna or dummy load) as even though there is a transmit protection circuit, it is often too slow to act and the final amplifier transistors are damaged or ever destroyed on older radios.
If you get some quieting but no audio, then yes, you will probably need to switch some wires around on the mic as was said earlier.
Most CBs run a 4 pin mic. There is a ground, two audio and a PTT wire. You can find the diagrams for just about any radio online if yer good with search engines.

If you verify the antenna is working, by hooking up another known good unit to the antenna, and everything works, you can often find a CB radio repair person at your local truck stop who might be able to sort the radio out for you at a reasonable cost.

I'm hunting one of these radios myself for my Cherokee so if you give up on it, I might be willing to take it off your hands and fix it.

Good Luck!

Jim

[Chatt_79]

Yeah I found the CB/radio connection up under the dash, disconnected the aftermarket radio and plugged/screwed both up to the factory radio before I did anything else. All i saw up behind the dash was the ?pl259? type connector and my radio cb cable fit and screwed on fine. The connection was maybe a foot from the far passenger side of the dash, would the duplexer be past this area? The cable disappears up behind the dash, do I have to access the bottom of the antenna from the cowl? The antenna looks factory. It is receiving so it might be the mic. Looking online there are 3 different 4 pin variations from different manufactures, I'm going to wait for my factory service manual and see what wire does what then try and find a mic that has the correct setup or modify my mic to work. If that doesn't fix it then i'll check out the internals. Thanks for the advice guys.

[Tatsadasayago]

Yeah I found the CB/radio connection up under the dash, disconnected the aftermarket radio and plugged/screwed both up to the factory radio before I did anything else. All i saw up behind the dash was the ?pl259? type connector and my radio cb cable fit and screwed on fine. The connection was maybe a foot from the far passenger side of the dash, would the duplexer be past this area? The cable disappears up behind the dash, do I have to access the bottom of the antenna from the cowl? The antenna looks factory. It is receiving so it might be the mic. Looking online there are 3 different 4 pin variations from different manufactures, I'm going to wait for my factory service manual and see what wire does what then try and find a mic that has the correct setup or modify my mic to work. If that doesn't fix it then i'll check out the internals. Thanks for the advice guys.

It's been a long time since I messed around with those radios, but recall some having separate AM/FM coax and a PL-259 cable that fed the single factory external antenna.
Of the types I worked on, there were those with the 1-into-2, 2-into-1 duplexer and those with dual cables feeding a single antenna but attaching to it at two different points.

If your radio used the duplexer, it should be located between the radio and heater core area behind the dash panel and feed a single coax leading to the antenna.

If you still have problems after getting your service manual let me know and I'll dig out mine from the old days and see if I can help you out.

Jim

[haminawag]

Hey, you got lucky, usually all four pins need to be rewired, but you obviously have the "ground" connected correctly, the "receive" is wired correctly, and the "transmit" is okay, so all you need to do is get the "audio" on the correct pin. You can do this by noting that the microphone audio line is always shielded, so take this wire and connect it to one of the other three pins, test it by keying the mic after each try while listening in your other CB, once you hear yourself you can solder it up and give it a full test, you should be good to go.

[serehill]

Amc never used a duplex-er system they used a match box 2 into one converter. You can disconnect the 259 & put a regular antenna to it. Most likely the mike is wired wrong. The radios had no protection circuits & the match box had a lot to be desired. I have one of those also. They didn't match that well & it usually fried the finals because of it. Tons of these units had bad finals. I have one that has bad finals in the shed stored. I can check the mike & get the pin out next week if you can't find it. I'm betting both issues. bad finals wrong mike. There are 2 switching circuits in the mike just because the receive is dropping it does't mean the mike is wired right. If you had a radio within a few feet & were keying it up it would have seen some kind of signal if it was transmitting. The best thing you could do for it if you get it working is trash the matchbox & put a 50 ohm antenna on it. Finals (Output transistors) haven't been made in years for these.

[Tatsadasayago]

Amc never used a duplex-er system they used a match box 2 into one converter.

I would disagree with you on this because I did quite a bit of dealership warrantee work replacing these units and saw active matching network enclosures referred to by AMC as 'Duplexer' assemblies. In the radio profession these would be correctly named 'Antenna Tuning or Matching Networks' rather than 'Duplexer' since a duplexer isolates two frequencies so a single antenna can be used to transmit and receive at the same moment such as radio repeaters do.
I could have been seeing a factory attempt to solve the load mismatching as these were definitely not an aftermarket installation/add-on ie; untouched/unmolested vehicles.
Admittedly a majority were the Eagle series wagons and Matadors with the Mitsubishi radios.
I never opened one up to see what was inside so I assumed they had a switching and an LC circuit.

Jim

[haminawag]

Actually a duplexer can be used two ways depending on whether you have two radios of differing frequencies and are using a single dual-band antenna, or using one dual-band radio and two separate antennas. The only reason for auto makers to use a duplexer in the first place is so that they can use the car radio antenna for both that use, and as a CB antenna as well, which is what I'm assuming Jeep was doing. Many el cheapo duplexers and diplexers have a "tuning" circuit so that you can null the VSWR down to a tolerable level, but these need almost constant retuning, especially in an off-road environment, which is why so many of these CBs had weak or open finals. It was a low-cost alternative to equiping their vehicles with a proper resonance-tuned duplexer which is the right way to do it, the el-cheapos take up less space too.

[serehill]

Amc never used a duplex-er system they used a match box 2 into one converter.

I would disagree with you on this because I did quite a bit of dealership warrantee work replacing these units and saw active matching network enclosures referred to by AMC as 'Duplexer' assemblies. In the radio profession these would be correctly named 'Antenna Tuning or Matching Networks' rather than 'Duplexer' since a duplexer isolates two frequencies so a single antenna can be used to transmit and receive at the same moment such as radio repeaters do.
I could have been seeing a factory attempt to solve the load mismatching as these were definitely not an aftermarket installation/add-on ie; untouched/unmolested vehicles.
Admittedly a majority were the Eagle series wagons and Matadors with the Mitsubishi radios.
I never opened one up to see what was inside so I assumed they had a switching and an LC circuit.

Jim

\
I suppose but the am/fm never transmitted. Nothing bur a fancy matchbox. that did not work I've been in several of them. The matches were never good. there were impedance & reflection problems they never over came. You can call them what you wish.

Haminawag

Actually a duplexer can be used two ways depending on whether you have two radios of differing frequencies and are using a single dual-band antenna, or using one dual-band radio and two separate antennas. The only reason for auto makers to use a duplexer in the first place is so that they can use the car radio antenna for both that use, and as a CB antenna as well, which is what I'm assuming Jeep was doing. Many el cheapo duplexers and diplexers have a "tuning" circuit so that you can null the VSWR down to a tolerable level, but these need almost constant retuning, especially in an off-road environment, which is why so many of these CBs had weak or open finals. It was a low-cost alternative to equiping their vehicles with a proper resonance-tuned duplexer which is the right way to do it, the el-cheapos take up less space too.

Rather than get off on a tangent of what they can do lets stay with what this one does.That is correct jeep was using them that way. No need for a full blown duplexer since only one was transmitting & ironically the antenna was not designed for the transmitting radio. It was designed for the receive only one. Totally backwards. The basic function of the matchbox was to lie to the cb radio & tell it has a resonant antenna hook up to it. The CB radio needs a much longer wavelength. So the radio operating on an antenna that was a 3rd of the wavelength could not possibly perform well. So if you disconnect it & run a regular cb antenna to the 259 it's a whole new world. Put an swr meter in line you would find most of them @ over 2 swr. Reflected power was always high.. They did need constant adjustment. There's a good reason no one ever heard a Jeep factory radio that really talked.

Apologies for digressing.

Back to the real subject. As stated I believe I have a mike in the shed if I need to dig it out & check the wiring just let me know. Hopefully the radio hasn't succumb to the path they normally took. if you have a friend with a watt meter you can hook it up & see what is going on without guessing.

[Tatsadasayago]

Yes, my apologies for digressing...
As a longtime radio guy in the army then active amateur and CB hobbyist I get sorta jazzed when the subject of radios and antennas comes up.

I'll try to contain my enthusiasm in the future.

Jim

[haminawag]

The basic function of the matchbox was to lie to the cb radio & tell it has a resonant antenna hook up to it. The CB radio needs a much longer wavelength. So the radio operating on an antenna that was a 3rd of the wavelength could not possibly perform well. [quote]

A duplexer is not a matchbox. What Jeep was using is also not a matchbox, it's called a cavity resonator. Based on the impedance mismatch of the AM/FM antenna and the 75 Ohm coax feeding it the signal loss would have been at best 68%, and at worse 89%, and that loss is for receiving as well as transmitting using the supplied cavity resonator. That's hilarious, those are loss figures that are typically seen at 500 MHz, not at 27 MHz. That kind of performance places Jeeps CB radio system squarely in the realm of the Radio Shack "Space Patrol" walkie talkies. A simple "T", or "Pi" matching network, in place of the cavity resonator, could have brought the loss down by better than 45%.

[dmsmcj83]

Where did you get a factory service manual for that radio. I desperately need one