Cobra 140 no audio in CB mode but ok in PA

[Brian G]

Radio is receiving normally but there is no audio out of speaker or external speaker when in CB. There is audio in PA mode so I don’t think it’s the audio chip or TR36. What else should I check?seems like it should be something stupid.
thanks.

 

[nomadradio]

Only thing besides the external-speaker jack to come loose would be the PA/CB switch itself. Not many pieces in line with the CB-side speaker audio that aren't also in line with PA.

73

 

[red97k1500]

Has this cobra 140 gtl been sitting for 20 or 30 years? On one of my cobra 140’s when I first got it, I had the same exact problem as you do, I sprayed some deoxit. In the cb/pa switch and worked it back and forth a bunch of times and it fixed it.

 

[Brian G]

Has this cobra 140 gtl been sitting for 20 or 30 years? On one of my cobra 140’s when I first got it, I had the same exact problem as you do, I sprayed some deoxit. In the cb/pa switch and worked it back and forth a bunch of times and it fixed it.

I haven’t used it in 2 years or so. Been in my trunk so maybe that’s it. I will spray some cleaner in that switch and see if that fixes it. Thanks.

 

[Brian G]

Only thing besides the external-speaker jack to come loose would be the PA/CB switch itself. Not many pieces in line with the CB-side speaker audio that aren't also in line with PA.

73

I haven’t used it in 2 years or so. Been in my trunk so maybe that’s it. I will spray some cleaner in that switch and see if that fixes it. Thanks.

 

[red97k1500]

Also check to see if the switch is broken, as in something bumped the switch and pushed it in and it doesn’t make contact anymore in cb mode. I’ve seen that happen before.

 

[Handy Andy]

Just to quick check - before I delve in further...

Radio is receiving normally but there is no audio out of speaker or external speaker when in CB.

How do you verify the Radio Receives normally?

Ok, not hearing Receive Audio - yet to say it receives normally - let's clarify that so I can help direct you...

But with the above in mind, you have PA audio - just not CB.

People are asking you to check that switch - I agree with them...

To help the diagnosis...

Power Off NB ANL off, AM mode - Squelch off (all the way down)

Turn on Radio - do you hear a thunk from the speaker in CB mode?

You say you have PA audio so - you'd hear a trunk when you turned it on in that PA mode?

• Why? because of the audio chip ...

But, do you hear a thunk (I think - so therefore I thunk) when you turn it on in CB mode?

If it passes - We can rule out Mic for the moment because you'd wouldn't have PA audio UNLESS Pin 5 wire was open

• - which if Pin 3 and 4 weren't working would silence the speaker permanently in RX mode,

But when you key up does Pin 5 and 4 work? Yes, so HEY We Have Audio

BUT - do you have receive ?

• - you'd know that once you fixed the mic...make sure Pins 3, 4 and 5 are FOR the Switching of the speaker....

Mic Wiring for Cobra 140...​

Pin​

1. Mic Audio (See above - thru Switch)
2. Audio Shield - Microphone Ground
3. Receive (Speaker Return from Speaker)
4. Board Ground for Switching and Speaker Return
5. Transmit (Switches MB3756's toggle function to TX mode)

So, that means Pin 3 short to 4 IN RECEIVE - verify...even at the Mic connector ok? Follow all wiring back to main harness switch and to board - because Pin 3 goes to the CB PA switch ok?

Pin 4 should be Board ground - check continuity from Pin 4 on Mic Jack on Radio - to a Can coil case which is the shield for itself and is shielded-connected-soldered to Board Ground..

Pin 5 Grounds to Pin 4 - turns on TX, but in PA mode, turns off the Switch that turns on the MB3756 (TR37) TX mode.

Why is this important?

Because, hearing a thunk in both modes means the speaker wiring is ok, Mic Pin 4 and Pin 3 communicate...you may have something else wrong which is why you don't hear the audio - could be a talk back mod onto a broken speaker wire onto a faulty open squelch pot forcing the audio to mute in RX mode.

Secondly - old radio - old parts, if you haven't worked on or looked at in for a while - caps can fail - so you'll have that hanging over our heads - to have us say I told you so - so be ready for those moments of "I told You So"...

Ok one switch - you'll have 6 wires to it.

The Switch has 2 sets of contacts, with a common - When in CB, the radio SENDS Audio from the Audio chip to one of two jacks, then onto the speaker inside - if the radio has had any sort of Talkback mod done to it, it'd show up in the speaker jack wiring.