Cobra 29 ltd classic not transmit

[SteveM]

Ok, sorry for the delay but work beckoned, but anyways, the voltage on R70 is normal on both sides, the one thing that might help is when I was hooking it all back up on the bench, I forgot to plug in the external speaker and so while I was going over where I stopped last time, I keyed the mic and the modulation meter maxed out, when I plugged it back in, the needle dropped again, it does the same thing when its plugged into the PA jack, also when I key the mic in PA the speaker pops but no sound..

 

[SteveM]

Also, I removed TR18 and it did not make any difference.

 

[Handy Andy]

Ok, good, then lets keep going.

You get an RX noise from the External Speaker...

Any way you can send a photo of what you have on the component side - I'm thinking multiple levels here.

One may be a wiring issue with a broken speaker wire or a failed transformer - the latter be the most unlikely but does happen.

Ok, how is the "temperature" of the board, any sections get really hot fast - like when it's turned on and in less than a minute a side panel is scorching hot - means bad audio chip or is dissipating heat due to component failure.

When you key up the mike and the needle moves up , you know you needed to "set' modulation by calibrating - so I know you did this, you set the "Cal" switch and adjust the knob to set the needle to swing over to "set".

So, that means you get carrier, but audio is not getting into the carrier.

So we have to figure out why the Audio amp sends audio to the to speaker but not to the final and driver - but POWER for the carrier goes to the Final and Driver.

What happened to the Mic amp and what is going on - on pin 3 of the microphone.

Ok why Pin 3? Pin 3 GROUNDS to PIN1 when you TX - it's the only section that needs to make a connection to turn on a transistor as a switch. So, if the mic amp seems to be ok, R70 voltage that is, there is a 4.7K resistor that Bleeds down the Mic Amp so it works. IF PIN3 does not ground properly - like the mic is wired wrong - this can affect how the radio TX's

Pin 4 - comes from the speaker - and GROUNDs to Pin 1 when you RX (mic left alone - unkeyed) - so the speaker lead shorts to complete a circuit then the mic plunger is pushed in the RX side is broken to go into TX mode to force the power thru to the final and driver - TX switches on to do just that. You get a carrier just no audio power.

It's why I posted the pics - so help you find the right wires to make the audio from the microphone - get THAT to the right part of the radio and out to the final and driver.

So with all other checks side, change mics to see if that recreates the problem on the OTHER mic, else that mic may not be wired correctly for that radio - or someone TRIED to re-wire the radio to fit another mic system that I cannot tell without pics...

I hope there may be a goofy microphone or bad wiring - I do know of some MIDLAND microphones that when used on other radios - make these kinds of goofy acts and strange things so we may need to swap a microphone for another wired correctly for a cobra and uniden.

If anyone needs a reference - look for 77-160 ROAD MAX - you'll see what I mean.

Regards!
:+> Andy <+:

 

[SteveM]

Andy....WE HAVE AUDIO!! Lol...it was the mic gain pot..I kept rereading your posts trying to figure it out so I got to thinking about how the audio circuit comes from the mic to the mic gain pot so I jumped the blue and yellow wires and I came to life, so I replaced it with one out of a old donor radio, the audio is scratchy but there..

 

[Handy Andy]

WHEW!

Lock 'n Load!

:+> Andy <+:

 

[SteveM]

WHEW!

Lock 'n Load!

:+> Andy <+:

I cant thank you enough Andy, the knowledge in your head and your willingness to share and help others is amazing.

 

[Squints]

1. NewI have a cobra 29 classic thats constantly transmitting on its own and it wasnt forced nor jarred in any way it just suddenly went to transmit without warning it has nothing to do with the mic orthe mic jack on the radio
   The light indicater shows its still in rx mode but the needle is burried and lights are slightly dim and its showing dead key on watt meter.
   I been through the whole radio looking for anything shorted or grounded out no twisted componant legs mic jack is disconnected as well.
   All i have for equipment is a multimeter

 

[08fatbob]

Andy....WE HAVE AUDIO!! Lol...it was the mic gain pot..I kept rereading your posts trying to figure it out so I got to thinking about how the audio circuit comes from the mic to the mic gain pot so I jumped the blue and yellow wires and I came to life, so I replaced it with one out of a old donor radio, the audio is scratchy but there..

Yep meee toooo! Thanks for all the help!

 

[Handy Andy]

1. NewI have a cobra 29 classic thats constantly transmitting on its own and it wasnt forced nor jarred in any way it just suddenly went to transmit without warning it has nothing to do with the mic orthe mic jack on the radio
   The light indicater shows its still in rx mode but the needle is burried and lights are slightly dim and its showing dead key on watt meter.
   I been through the whole radio looking for anything shorted or grounded out no twisted componant legs mic jack is disconnected as well.
   All i have for equipment is a multimeter

I would recommend that you take the covers off and make sure the speaker wires are not and any other wires along the loom, are not pinching between the case and the covers - this usually tells me the speaker wire may be shorting - if you have not done anything except use it.

This does happen, but just remove the case screws - carefully remove the covers and review the loom and speaker wires to the speaker - see if the "spyder cage" the speaker cone rests in, its braid that heads off to the coil in the magnet - is not touching the spyder cage - if so gently push the wires away from the short and see if the radio will work with the speaker cover open - then if you still have this condition - the bottom panel and the soldering work - it will have to be investigated.

Would not hurt for you to post a pic or two of the top component side and the bottom solder side of the board to help us decipher this condition.

 

[NZ8N]

1. NewI have a cobra 29 classic thats constantly transmitting on its own and it wasnt forced nor jarred in any way it just suddenly went to transmit without warning it has nothing to do with the mic orthe mic jack on the radio
   The light indicater shows its still in rx mode but the needle is burried and lights are slightly dim and its showing dead key on watt meter.
   I been through the whole radio looking for anything shorted or grounded out no twisted componant legs mic jack is disconnected as well.
   All i have for equipment is a multimeter

Most likely culprit is C117 next to L3 is shorted. But, can be any of the other red circled items, too, these are either tied directly to, or low resistance to the PTT line of the Mic socket. last time I helped someone with a Cobra 29 a few months ago, that had TX all the time, it turned out to be a cracked (shorted) C121 up by the mic plug area.


Parts side



Flipped to Solder side

 

[Groundhog KSS-2012]

I've never owned a Cobra 29 however, this one seems to be doing a really good job.