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In over my head with a Cobra 29 that I killed transmit on.

silverhead

Member
Apr 4, 2020
5
4
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40
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.
 
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1iwilly

Well-Known Member
Dec 7, 2008
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i would start by disconnecting the wires you solder on the board then take it from there
 

silverhead

Member
Apr 4, 2020
5
4
13
40
If I unsolder them there is no continuity from the modulation to the amplification. I physically cut the trace in the circuit board for my Mauldulator install. That circuit had been working for a few days before this happened. I checked all around those wires for shorts and proper continuity. I'm more curious that when the green wire circuit is complete, I'm getting +5-7 more volts on the collector of TR14 and TR15 than I should be. I'm studying the block diagram of the service manual and trying to understand the way IC3 works with TR16 to the amplifier section.

I have found a sheet with expected voltages off of transistors and ICs, but my radio is newer than any of the documents I've found online and uses a different audio amp, and other random things such as my driver transistor being a C2314 instead of a C1957.

There used to be valid URL links to Cobra's website to download updated schematics and test information, but it appears they have shut it down on their end?
 

silverhead

Member
Apr 4, 2020
5
4
13
40
I may have figured out what potentially ruined the driver and/or final. Tonight I was using a brand new 29 from my pickup as a reference radio. I checked the receive and transmit voltages on IC3, TR16, TR15, and TR14. Everything was pretty much a match between the two except for the collectors of TR15 and TR14. On the broken radio voltage was too high, around 20 volts.

However, when testing my good radio I noticed something. With my cheap Amazon DMM, if I have the probe on the collector before keying up, voltage runs away, like up to 36vdc. So I keyed up first and then probed the collector, getting like 4.5vdc reading off of the final. Something about my DMM seemed to upset these transistors, so I have noted to never use that method of checking voltage.

In my broken radio, I had keyed up, yelled into the mic, etc... trying to get a reference for how much voltage my final was making while transmitting. So if I sent it into runaway mode, I may have very well ruined the driver and final with my DMM. I have a couple of Toshiba replacements showing up next week and hopefully the swap out will get me back in business since everything behind them that I tested matches my working radio.
 

nomadradio

Analog Retentive
Apr 3, 2005
5,305
7,610
573
Louisville, KY
www.nomadradio.com
Are you using a 24-Volt power supply to run the Mauldulator?

Really sounds as if the carrier knob got turned too high just long enough to cause a damaged final, driver or both.

Any more than a 4-Watt carrier makes premature failures more likely.

And if the "asymmetry" control gets turned too high, that will wear things out too soon.

I don't see how you can use the Mauldulator without a 'scope. Let alone set it up to sound right.

That being said, a powerful tool can do powerful damage.

73
 

silverhead

Member
Apr 4, 2020
5
4
13
40
The question I asked was about my radio which hadn't been hooked up to the Mauldulator at the time it was damaged. I have a 30vdc up-converter to power the Mauldulator and have it set at about 18 volts right now.

I'm using a different Cobra 29 with it and I have the carrier set at 1 watt and it swings up to 4 watts. That's all working fine. My broken 29 should be fine once I get a new driver and final installed.

I do own an oscilloscope, as I mentioned in the initial post. I just don't have much experience using it to trace RF issues or audio issues in the presence of RF.
 

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