SSB.... see if any of this nonsense makes sense!
Sorry it is rotated that is just the way the scanner picked them up.
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Cobra 2000 no modulation.
- Thread starter Brian Gosper
- Start date
Sorry it is rotated that is just the way the scanner picked them up.
WE ARE GETTING CLOSE
I just backed up and reread your last update.... R126
Look on those schematic segments I sent!!!
The output of R126 is tied to the AMC circuit.... TR24.
And you said you had audio IN to R126 but NO AUDIO out of it.
Check voltages on TR24 because it sounds like it is either shorted or
something is wrong or misadjusted and TR24 is being TOLD to turn on completely.
Take a good walk through that circuit around TR24 & TR25 and all of those components. Also check soldering on the back. If someone had been doing something in there it could be a solder splash shorting out the audio. Just give the whole thing a once over... I bet you'll fix this.
I just backed up and reread your last update.... R126
Look on those schematic segments I sent!!!
The output of R126 is tied to the AMC circuit.... TR24.
And you said you had audio IN to R126 but NO AUDIO out of it.
Check voltages on TR24 because it sounds like it is either shorted or
something is wrong or misadjusted and TR24 is being TOLD to turn on completely.
Take a good walk through that circuit around TR24 & TR25 and all of those components. Also check soldering on the back. If someone had been doing something in there it could be a solder splash shorting out the audio. Just give the whole thing a once over... I bet you'll fix this.
I would be really curious to know what you are getting on the collector & base of TR24... DC measurement. If that collector looks like it is a hard ground.... I would consider popping TR24 off of the board briefly.... and see if your mic functions suddenly start working.
That collector isn't really "pulled up" by anything..... by design it should be acting like a variable "reistance" ... when the transmit audio is too high... pulling the audio down... when it is not so high.... just letting it float where it wants.
If it EVER goes hard to ground it will kill ALL mic signal.
That is why I am more interested in TR24.
Exactly, What's what i posted in message #34.
So i pulled TR24, I drew a pic of the way it was in the board not completely sure what it looked like under the transistor and found the picture on the board different than what i drew, I tested the radio WITHOUT TR24 in it and AM, SSB and PA now have transmit modulation,
I tested TR24 with my transistor tester and it test ok, But i had a new one so i shot gunned in a new one in it, Now no Transmit modulation,
I pulled C107 thinking it may be shorted to grd and still no modulation.
I pulled TR25 and checked it and it's testing good, I powered the radio and it has transmit modulation in AM, SSB and PA with TR25 removed.
I had another TR25 and shot gunned it and now no Modulation, Still no finding's on why the voltage isn't the same on B of TR25 as it is on the top side of R129.
So i pulled TR24, I drew a pic of the way it was in the board not completely sure what it looked like under the transistor and found the picture on the board different than what i drew, I tested the radio WITHOUT TR24 in it and AM, SSB and PA now have transmit modulation,
I tested TR24 with my transistor tester and it test ok, But i had a new one so i shot gunned in a new one in it, Now no Transmit modulation,
I pulled C107 thinking it may be shorted to grd and still no modulation.
I pulled TR25 and checked it and it's testing good, I powered the radio and it has transmit modulation in AM, SSB and PA with TR25 removed.
I had another TR25 and shot gunned it and now no Modulation, Still no finding's on why the voltage isn't the same on B of TR25 as it is on the top side of R129.
Sounds like you are on the way! I hate to see you have to use those parts.... but it sounds like something is hosed in the AMC/ALC that is "causing" it to think that modulation needs to be cut completely off. At least NOW you know what is doing it!!!!!!!
I'll go back and take a look at the control coming in to the ALC to see if I can "intuit" what might be wrong back that way.
But, again, it sounds like the ALC THINKS that the modulation needs to be clamped.... so we know where to look.
Good work!!!!
Just passing this along.....
It looks like you are already determining that TR25 is telling TR24 to clamp the mic off.
You can pull either transistor OUT and you recover your XMT/PA audio!
So now we are "backing up a step"
Either the components AROUND TR25 are honked.... CAUSING it to tell TR24 to cut off.......
OR
the signal coming into R128 from the right is TELLING TR25 to shut things off.
To that end... here is one more PDF that shows the three conditions that "cut off or cut down" the mic.
One is PLL Out of Lock: this isn't it unless SOMETHING is broken. You are receiving.... so your PLL is NOT out of lock. But this is TR47 and TR53.
One is the SSB ALC. This circuit sniffs the output RF and feeds it back to the ALC control (TR25). The idea here is that ... when you modulate, the RF final will emit peaks. If those peaks are greater than the current ALC Setting says is right... it drives back toward TR25 telling it to roll the mic gain down. Just a feedback loop.... controlled by TR34... the ALC amp.
One is the AM AMC. This circuit is sniffing the modulated DC that comes out of the AM PWR REGULATOR down to the driver and final. If THAT signal is greater than the current AMC setting says is right.... it TOO drives back toward TR35 telling it to roll the mic gain down. Another feedback loop from TR26 the AMC Control.
You aren't even seeing any modulation.... so I would not THINK that it was normal ALC or AMC action causing the problem.... something HAS to be failed along this route. Something is TELLING TR25/TR24 to shut that mic ALL THE WAY down.
In "normal cases" the PLL Out of Lock detection is going to act like a "switch" and just order TR25 to SHUT THE MIC OFF. The ALC & AMC should behave more "variably".... as they are trying to "achieve a balance" where the mic audio is "loud enough" but "not too loud".
In your "failure case".... the mic audio is shut COMPLETELY OFF.
You can take out TR24 and the audio comes back.
THAT says that is IS TR24 killing the mic.
You can take out (ONLY) TR25 and the audio comes back.
THAT says... TR24 appears to be WORKING!!!!! and that
TR25 is TELLING it to clamp the audio.
From here... the trick is to figureout WHY TR25 thinks that.
Is it a failure in TR25's components?
or
Is it being told to by the PDF I just now sent......
It looks like you are already determining that TR25 is telling TR24 to clamp the mic off.
You can pull either transistor OUT and you recover your XMT/PA audio!
So now we are "backing up a step"
Either the components AROUND TR25 are honked.... CAUSING it to tell TR24 to cut off.......
OR
the signal coming into R128 from the right is TELLING TR25 to shut things off.
To that end... here is one more PDF that shows the three conditions that "cut off or cut down" the mic.
One is PLL Out of Lock: this isn't it unless SOMETHING is broken. You are receiving.... so your PLL is NOT out of lock. But this is TR47 and TR53.
One is the SSB ALC. This circuit sniffs the output RF and feeds it back to the ALC control (TR25). The idea here is that ... when you modulate, the RF final will emit peaks. If those peaks are greater than the current ALC Setting says is right... it drives back toward TR25 telling it to roll the mic gain down. Just a feedback loop.... controlled by TR34... the ALC amp.
One is the AM AMC. This circuit is sniffing the modulated DC that comes out of the AM PWR REGULATOR down to the driver and final. If THAT signal is greater than the current AMC setting says is right.... it TOO drives back toward TR35 telling it to roll the mic gain down. Another feedback loop from TR26 the AMC Control.
You aren't even seeing any modulation.... so I would not THINK that it was normal ALC or AMC action causing the problem.... something HAS to be failed along this route. Something is TELLING TR25/TR24 to shut that mic ALL THE WAY down.
In "normal cases" the PLL Out of Lock detection is going to act like a "switch" and just order TR25 to SHUT THE MIC OFF. The ALC & AMC should behave more "variably".... as they are trying to "achieve a balance" where the mic audio is "loud enough" but "not too loud".
In your "failure case".... the mic audio is shut COMPLETELY OFF.
You can take out TR24 and the audio comes back.
THAT says that is IS TR24 killing the mic.
You can take out (ONLY) TR25 and the audio comes back.
THAT says... TR24 appears to be WORKING!!!!! and that
TR25 is TELLING it to clamp the audio.
From here... the trick is to figureout WHY TR25 thinks that.
Is it a failure in TR25's components?
or
Is it being told to by the PDF I just now sent......
Hi Xracer,I am not sure if it was off to begin with or not....
but he did just tell me that... with TR24 IN...... and TR25 OUT....
he STILL has audio. And when he put TR25 back in... he lost the audio
again. That makes it sound like TR24 is doing it's job.....
but that TR25 is TELLING it to turn completely ON and kill the mic.
I am hoping that I can assume that he does have it in right!
Hey Xracerx, Yes TR24 is in the correct way.
I pulled the new TR24 and 25 and reinstalled the original ones.
Still no Transmit modulation,
For what it's worth, I pulled TR26, Tested ok and it DOES have transmit modulation in AM, SSB and PA with TR26 Removed,
So in my eye's it's going down this path
I pulled the new TR24 and 25 and reinstalled the original ones.
Still no Transmit modulation,
For what it's worth, I pulled TR26, Tested ok and it DOES have transmit modulation in AM, SSB and PA with TR26 Removed,
So in my eye's it's going down this path
Check VR12 and D64 for opens. If either of those failed open, TR26 will turn on.
Edit: VR12 has one leg floating, so all it would take is that wiper not making contact to turn on TR26. With that being probably the most commonly fiddled with pot in any radio (and the rarely considered rotational life of the pot, which sucks), its a good candidate.
Edit: VR12 has one leg floating, so all it would take is that wiper not making contact to turn on TR26. With that being probably the most commonly fiddled with pot in any radio (and the rarely considered rotational life of the pot, which sucks), its a good candidate.
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Check Resistor R165 (1.5K) for short-to-ground or resistance change........
If TR26 (NPN) is in the circuit and R165 is shorted or close to ground, TR25 (PNP) will turn on turning TR24 (NPN) on to ground out the audio.
With TR26 out R165 will not make it to turn on TR25.
If TR26 (NPN) is in the circuit and R165 is shorted or close to ground, TR25 (PNP) will turn on turning TR24 (NPN) on to ground out the audio.
With TR26 out R165 will not make it to turn on TR25.
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