Hey there, first post, long time random looker...
I have a circa 2015 Cobra 29 that has lost transmit, presumably because of me. I've got a Swiss cheese, generalized knowledge of electronics, but not a great understanding of circuit tracing and fault finding. So here we are with my back story as to how I broke my beloved radio: I've built the MotorMouthMaul Mauldulator and I'm trying to set this radio up to utilize it. I cut the trace on the circuit board between the collectors of the driver/final transistors and the onboard modulation section. I soldered a coaxial cable to the back of the board that will receive my "Mauldulation." I also have a couple of green wires going to the broken trace so I can put the radio back into stock mode with the flip of a switch. Mostly because I still wanted to talk on it until I got the Mauldulator constructed. All of these things went just dandy and my radio was working 100% the way I intended it to.
This radio has no other modifications from factory stock. I've never adjusted it, no swing mod installed, etc..
Sunday night I was doing a simple voltage test at the collector of the final (and driver) transistor(s) just to get an estimate what my receive and transmit voltages were. The next morning I went to use the radio and transmit no longer works.
These are my symptoms:
-Receive works fine, just as sensitive as ever
-PA function works fine, just as loud as ever
-When keying for transmit, the LED on the front changes color, however, the RF power needle falls to zero instead of forward
-I've got an oscilloscope hooked to an RF sampling box and I'm getting no discernible carrier. I'm only using it at the moment to keep track of my modulation levels and I don't have a full grasp of using it to troubleshoot electronics.
-Talking loudly into the mic while keyed up, I can hear my voice coming out of T1, I believe
-I removed both the final and driver (2SC2078 and 2SC2314) and did a basic transistor test with one of those Chinese parts testers from Amazon. The final tested OK as an NPN. The driver showed up as "two diodes" but that could just be this device not knowing
-I did the generic diode test with my multimeter and both NPN transistors passed the shorts, opens, direction, and emitter/collector tests, so they seem like maybe they're OK?
-Nothing popped or smoked and I can't see any burnt parts anywhere on the radio.
I'm looking for help on how to trace the problem back to the offending parts. This is my first venture into trying to learn/understand how these radios work past your general layman's sort of understanding of how a CB radio works and not to transmit without an antenna, etc... I wasn't planning on breaking the radio, but here we are and I'd love to fix it and learn what happened.
I made a youtube video to kind of show what I'm up against if that helps:
Thanks for any help.
I have a circa 2015 Cobra 29 that has lost transmit, presumably because of me. I've got a Swiss cheese, generalized knowledge of electronics, but not a great understanding of circuit tracing and fault finding. So here we are with my back story as to how I broke my beloved radio: I've built the MotorMouthMaul Mauldulator and I'm trying to set this radio up to utilize it. I cut the trace on the circuit board between the collectors of the driver/final transistors and the onboard modulation section. I soldered a coaxial cable to the back of the board that will receive my "Mauldulation." I also have a couple of green wires going to the broken trace so I can put the radio back into stock mode with the flip of a switch. Mostly because I still wanted to talk on it until I got the Mauldulator constructed. All of these things went just dandy and my radio was working 100% the way I intended it to.
This radio has no other modifications from factory stock. I've never adjusted it, no swing mod installed, etc..
Sunday night I was doing a simple voltage test at the collector of the final (and driver) transistor(s) just to get an estimate what my receive and transmit voltages were. The next morning I went to use the radio and transmit no longer works.
These are my symptoms:
-Receive works fine, just as sensitive as ever
-PA function works fine, just as loud as ever
-When keying for transmit, the LED on the front changes color, however, the RF power needle falls to zero instead of forward
-I've got an oscilloscope hooked to an RF sampling box and I'm getting no discernible carrier. I'm only using it at the moment to keep track of my modulation levels and I don't have a full grasp of using it to troubleshoot electronics.
-Talking loudly into the mic while keyed up, I can hear my voice coming out of T1, I believe
-I removed both the final and driver (2SC2078 and 2SC2314) and did a basic transistor test with one of those Chinese parts testers from Amazon. The final tested OK as an NPN. The driver showed up as "two diodes" but that could just be this device not knowing
-I did the generic diode test with my multimeter and both NPN transistors passed the shorts, opens, direction, and emitter/collector tests, so they seem like maybe they're OK?
-Nothing popped or smoked and I can't see any burnt parts anywhere on the radio.
I'm looking for help on how to trace the problem back to the offending parts. This is my first venture into trying to learn/understand how these radios work past your general layman's sort of understanding of how a CB radio works and not to transmit without an antenna, etc... I wasn't planning on breaking the radio, but here we are and I'd love to fix it and learn what happened.
I made a youtube video to kind of show what I'm up against if that helps:
Thanks for any help.