Cobra 29 Ltd no transmit

[Rooster74]

Hey guys, sorry to bring up yet another cb no transmit problem, I know this has been covered many times, I know because I've read many threads on the same problem but nothing really seems to match my symptoms or cover specific diagnostic steps, and my diagnostic skills are weak at best, but hopefully you guys can help get me in the right direction.

The problem radio is an older model Cobra 29 Ltd.
The RX seems nice and clear, and PA works well. So far I've tried two known working mics and checked the board for cold solders. There is evidence that work has been done in this thing but I'm not sure what, it looks like a different final at least has been installed, which I've pulled and tested with a multimeter. Based on all the data I found online it tested sat so I reinstalled it.

When keyed the Rx/Tx light turns red but I get no deflection on the S-meter. There is an audible "bump" noise that comes from the speaker, then from there zero transmit, I can't even hear a change in the static on an adjacent radio on the same channel.

Based on the block diagram and advice gleamed from other threads it seems like IC3 is the first most likely cause but I'm unsure on how to test it. The only diagnostic equipment I have is a multimeter. Any help would be greatly appreciated.

 

[kaos513]

Could be a shorted cap if the radio is old and never been recapped.

 

[nomadradio]

If you have another working radio to use as a 'sniffer', it's useful to find out if the radio is producing a transmit signal at all.

If not, it's a waste of time to look at the final and driver.

Yet.

Plug a spare coax jumper into the monitor radio and back the shell of the plug at the far end of the jumper down the coax. This exposes the center pin of the plug, making it a half-inch long "sniffing" antenna.

Put both radios on the same channel and see if the monitor radio can hear a carrier coming from the broken radio when you key it. Hold your sniffing antenna near the driver transistor. If there is any transmit signal at all being generated, you should hear it this way.

If so, next check the receiving radio's S-meter to see if the slugs in L21, L20 and L17 are all set for peak reading. If they have been diddled, this alone can put the transmitter off the air.

If they all show a proper peak on the receiver's S-meter, checking the driver and final transistors would be next.

73

 

[Rooster74]

Well I ended up poking my way through getting that radio transmitting enough to be picked up on an adjacent reciever, I wasn't able to get a radio check from about 2.5 miles away from it though. I wasn't sure if the problem was only partially corrected or if the radio had been set up to push low watts to pair with an amplifier. Either way the radio has been returned and remains mostly untested. Next chance I get off work I'll try to touch base with the guy again to see if I can follow up.
Thanks Nomad for the info on the "sniffer" probe, in all my research I had not ever heard of using that and will definitely keep it in my back pocket for future use as well.
On a somewhat unrelated note since this I did pick up a cheap Astatic Pdc2 swr/watt meter to hopefully aid in future troubleshooting. It seems to function fine to measure Swr however either my bearcat 980 that's never been cracked open is putting out 2 watts from the factory with zero swing with modulation, or the watt function is useless. I got similar results from my own older 29 that I believe is also stock and unmolested. Am I doing something wrong?

 

[Lwr]

New to the forum. Just found this post and I'm having a similar problem. What if I hear a difference on the monitoring radio but there is no reading on the s meter of the radio in question.

 

[Rooster74]

I've had a similar problem with a very old 138 XLR, turned out the mic switch was just dirty after years of sitting, it cleared up after exercising the switch a couple times, but more work might be required based on the condition of your radio. Have you tried a different mic and swinging all the nobs and flipping all the switches? If so obviously I'm still not great at this whole process, thus the original post.

 

[Lwr]

I'll try all that. Thanks for the input. Maybe it is something this simple. Its an older cobra 29 ltd

 

[Rooster74]

Like I said, I'm no master tech, but somehow I've been hopelessly sucked into this hobby. Most of the issues I've seen with radios that aren't incredibly old like over 30 years, have been pretty simple, dirty connections and bad solder joints mostly. Past that shorted caps seem to become more common and whatnot but it's definitely worth it to start with the easy stuff.