Cobra 2000 No modulation

[Brian G]

So the CB/PA switch, you checked, seems to work ok.

I wonder if you even hear a "thunk" from the speaker?

Then look around that area for fresh work as say a diode or wire, here's the thought.

IF they put in a variable key, you'd see work around the Final and Driver section.

You have Transmit, you hear yourself in another radio (correct?) (if so) that means you have Audio going thru there - into the Pass transistors.

You checked and you HAVE NO audio in the PA side, which tells me several things...

Possible...

... they DID do some type of talkback mod to hear themselves - only it's failed. OR they removed it. And did not put it back correctly...

Or you have a bad audio chip - The biggest clue is you don't hear the "thunk" from the speaker when you power up.

Onto... bad soldering - broken wire in the audio amp area. Open circuit.

Just three choices, there are output caps in that area too, look above in the schematic snips I've posted.

So if the Caps --- have they been replaced?

How about that "thunk"? from the speaker when you turn it on.

Does the Audio Chip get hot, or warm?

How about if it's stone cold, Cold - dead it's toast - replace the 7222 Flat-pak with another one. and don't just turn on the power right away, review the traces around it and question any spots - you can power up the radio without the amp in place and observe power flows and voltages to the pins on the side edge - make sure they are not all the same voltage , and you should have power only to one, with little to none on the others.

Hot? Look for bad soldering or panel to part, missing/displaced thermal mica tab - possibly shorting out to side panel - you can check for this by testing for "ground" on tab, as the same value on the side metal panel - if the radio is stock should be 4K to as much as 15K ohmic reading across there.- if it's a dead short, possibly took out the chip - so it will have to be investigated and fixed as needed.

Warm? It' may be in idle - look for the routing of the pins and for open blown traces from it, any surge can pop the trace like a fuse - and even damage the chip so it may "boot" but have no amp inside it. So warm doesn't guarantee of anything just that the amp is biased to work at idle.

IF you don't remember hearing one of the above, and it feels like something isn't right - it usually is...then it may be overdue for some caps and or a chip needs to be checked and replaced.


.

So a small update. When I put the radio into PA mode and key the mike, it’s like it wants to work. If I tap on the mike while it is keyed up, I can “hear” the tapping over the speaker but yet there is no voice when intel into it. And again nothing in the way of modulation when switched to on the air.

 

[Brian G]

So the CB/PA switch, you checked, seems to work ok.

I wonder if you even hear a "thunk" from the speaker?

Then look around that area for fresh work as say a diode or wire, here's the thought.

IF they put in a variable key, you'd see work around the Final and Driver section.

You have Transmit, you hear yourself in another radio (correct?) (if so) that means you have Audio going thru there - into the Pass transistors.

You checked and you HAVE NO audio in the PA side, which tells me several things...

Possible...

... they DID do some type of talkback mod to hear themselves - only it's failed. OR they removed it. And did not put it back correctly...

Or you have a bad audio chip - The biggest clue is you don't hear the "thunk" from the speaker when you power up.

Onto... bad soldering - broken wire in the audio amp area. Open circuit.

Just three choices, there are output caps in that area too, look above in the schematic snips I've posted.

So if the Caps --- have they been replaced?

How about that "thunk"? from the speaker when you turn it on.

Does the Audio Chip get hot, or warm?

How about if it's stone cold, Cold - dead it's toast - replace the 7222 Flat-pak with another one. and don't just turn on the power right away, review the traces around it and question any spots - you can power up the radio without the amp in place and observe power flows and voltages to the pins on the side edge - make sure they are not all the same voltage , and you should have power only to one, with little to none on the others.

Hot? Look for bad soldering or panel to part, missing/displaced thermal mica tab - possibly shorting out to side panel - you can check for this by testing for "ground" on tab, as the same value on the side metal panel - if the radio is stock should be 4K to as much as 15K ohmic reading across there.- if it's a dead short, possibly took out the chip - so it will have to be investigated and fixed as needed.

Warm? It' may be in idle - look for the routing of the pins and for open blown traces from it, any surge can pop the trace like a fuse - and even damage the chip so it may "boot" but have no amp inside it. So warm doesn't guarantee of anything just that the amp is biased to work at idle.

IF you don't remember hearing one of the above, and it feels like something isn't right - it usually is...then it may be overdue for some caps and or a chip needs to be checked and replaced.


.

This is my radio so there was never any work done around the PA CB switch area in raged to talk back. I’m still convinced the issues is around the mic amp area or the audio chip. When is listen on another audio, I just hear a hum on the other radio speaker.

 

[Chipper]

This is my radio so there was never any work done around the PA CB switch area in raged to talk back. I’m still convinced the issues is around the mic amp area or the audio chip. When is listen on another audio, I just hear a hum on the other radio speaker.

Could be the insulator or plastic screw on the audio chip causing it to ground or the wire from the audio chip not soldered on came loose. I’d be willing to bet it’s one of those or the chip is bad

 

[nomadradio]

If you have receive audio, the TA7222P is not to blame for the transmit issue.

And if there is no sideband power, that chip is not on the list of suspects for that fault.

73

 

[Brian G]

Problem fixed. Issue was a bad cap in the rf amp strip. Thanks for the help.

 

[nomadradio]

Rock on! Congratulations for tracking it down.

73