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Cobra 29 ltd classic not transmit
- Thread starter SteveM
- Start date
The only way to test a transistor is by removing it. My bets are still on a shorted C67. That kind of part fails way more often than either of those transistors.
Doesn't mean those two transistors are good, but the odds of a failure are lower for them than they are for that capacitor.
73
Doesn't mean those two transistors are good, but the odds of a failure are lower for them than they are for that capacitor.
73
If you have a voltmeter look at the cobra 29ltd voltage chart for tr17 and tr18 and match it. below is the chart for your radio.
Attachments
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you may not have to pull the transitors out unless you get bad readings from your volt meter
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Greetings!
In answer to your questions, yes. But wait a moment - we have to make sure the supports, the components in the circuit that make the transistor operate, have to be verified they're ok.
This can get tricky, but if you have probe tips that are sharp point style and pay attention - you should have no problems.
The Shining - Don't Wait For The Movie!
Gotta' strong light or flashlight?
Because you'll need to "shine" the traces to find your places to check.
It's basically a process of shining your flashlight under the section of the board that contains the parts you want to check - most of the time on the Cobra 29's (thankfully) they are single clad - (1 copper layer) boards that let some light thru.
Like this, and then just use your finger to find the traces you want to locate by looking at the section and finger on foil side casts shadow on it...
Then while keeping your finger on the W37 location - flip over the board...
Then you can review your schematic and work - remember polarity because the Limiter uses energy, or power, that can swing potentials BELOW FOIL BOARD GROUND - like negative-going AC or Audio. So when you install parts here, keep their polarity...
Hope these hints help!
:+> Andy <+:
In answer to your questions, yes. But wait a moment - we have to make sure the supports, the components in the circuit that make the transistor operate, have to be verified they're ok.
This can get tricky, but if you have probe tips that are sharp point style and pay attention - you should have no problems.
The Shining - Don't Wait For The Movie!
Gotta' strong light or flashlight?
Because you'll need to "shine" the traces to find your places to check.
It's basically a process of shining your flashlight under the section of the board that contains the parts you want to check - most of the time on the Cobra 29's (thankfully) they are single clad - (1 copper layer) boards that let some light thru.
Like this, and then just use your finger to find the traces you want to locate by looking at the section and finger on foil side casts shadow on it...
Then while keeping your finger on the W37 location - flip over the board...
Then you can review your schematic and work - remember polarity because the Limiter uses energy, or power, that can swing potentials BELOW FOIL BOARD GROUND - like negative-going AC or Audio. So when you install parts here, keep their polarity...
Hope these hints help!
:+> Andy <+:
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Hmm. Driver? possible - long shot...
I Like the idea, of the Regulated voltage on mains. Stronger culprit to track down.
Note photos in one post of shining - "buffer resistor" is listed. So if no-go on getting voltages on the Transistors - they may just be (ahemn) current starved...
Which brought me back to the PA issue - so you're onto something.
SteveM - Next on agenda - check board voltages...
If none we may have another place to review...
The power supply to it...why I asked about PA...because this section powers it...
The usual Forensics...
The Scene...
The Suspect
Bring in the Xray...
What Do you Think Bones?
And NO! Colonel Mustard Did Not Do IT With The Candelabra In The Foyer!
(I already drew those cards...)
:+> Andy <+:
I Like the idea, of the Regulated voltage on mains. Stronger culprit to track down.
Note photos in one post of shining - "buffer resistor" is listed. So if no-go on getting voltages on the Transistors - they may just be (ahemn) current starved...
Which brought me back to the PA issue - so you're onto something.
SteveM - Next on agenda - check board voltages...
If none we may have another place to review...
The power supply to it...why I asked about PA...because this section powers it...
The usual Forensics...
The Scene...
The Suspect
Bring in the Xray...
What Do you Think Bones?
And NO! Colonel Mustard Did Not Do IT With The Candelabra In The Foyer!
(I already drew those cards...)
:+> Andy <+:
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To continue...
Look in the photos above - and review the one below...I only aimed the light so I could get some details but it's the same Mic Amp area W37 and all...
Since you still have no audio and PA don't work, you'll need to check for voltages INTO the mic amp circuit.
If you don't get any power, the REGULATOR is possibly failed.
If you get voltage on the right side of the resistor - that means the REGULATOR is feeding power INTO the circuit - you need to VERIFY the heath of the Mic amp circuit is ok, by check the other side that drops power (limits current) a process called buffering - to power the circuit - if you have power then at least you don't have a dead short.
Meter reads 8~9V on Right but 0 or <1V on Left side? Shorted part or more than one part is shorted - Caps, transistor and Diodes fail like this...
Show some but - low voltages like <4V means you have some work to do.
Show 6 to 8 Volts? At least the BIAS of this circuit is ok...
An OPEN in the circuit will show FULL POWER on both side of the resistor - and indicates a failed part blown OPEN. Because the circuit can operate (blown fuse type of operation) but problem is located further up the chain, into the Mic Amp circuit itself.
Why do I do it this way? Experience, because of how a buffer resistor works, it keeps the mains from getting dragged down and possibly being destroyed from a major parts failure in the Mic amp itself.
Buffering allows power to pass as needed by the circuit - and if it draws more than it should the resistor protects the REGULATOR from excessive current draw.
When the circuit is not normal - as in all parts are NOT working like they should - the resistor can reflect that as a larger drop in voltage - parts of the other side of this resistor need/demand more power - like a failed transistor or a leaky cap even bad soldering PULLS down the VOLTAGE across the resistor as a voltage DROP - all of the parts if not right and any have gone out of tolerance or someone's changed them can affect the way the circuit operates and hence the current draw across this resistor will reflect that.
Experience also teaches me that many a person would try to install toys in this area or thru the Mic gain wiring itself - this can pull more power from the REGULATOR from poor wiring practices and bad soldering.
Which is why I made the comment "Many would throw them away" simply due to the fact that tracking down, repairing or undoing and fixing up the radio is not worth it and can cost more to fix than to replace directly - so the cycle repeats.
Hope the above helps!
Regards!
:+> Andy <+:
Sidebar for a moment:
NTE 55 NPN in a TO-220 case style works if you have to replace the Regulator transistor - the 2SC880Y is outdated and Where-Is-As-Is if available. So NTE/ECG-55 or any NPN capable of 3 Amps 60V min is ok to use if you need to replace Regulator Transistor
Not all transistors are pinout - pin-for-pin compatible and drop in in the same orientation - Some are BCE others are ECB - so in the photo above, and on the one of the NTE/ECG package - PAY ATTENTION TO THE PINOUT Of Base Emitter Collector leads on the package. In these photos of the TOP COMPONENT SIDE OF THE BOARD - the Base to top RIGHT (note the silkscreen notch remember Opposite) or towards the Right in the photo
On the FOIL side of "Bones" photo - the BASE is on the LEFT, Emitter is center, Collector on Right.
In both cases - BASE is FURTHEST AWAY HOLE FROM AUDIO AMP....everything else should fall into place.
NTE 55 NPN in a TO-220 case style works if you have to replace the Regulator transistor - the 2SC880Y is outdated and Where-Is-As-Is if available. So NTE/ECG-55 or any NPN capable of 3 Amps 60V min is ok to use if you need to replace Regulator Transistor
Not all transistors are pinout - pin-for-pin compatible and drop in in the same orientation - Some are BCE others are ECB - so in the photo above, and on the one of the NTE/ECG package - PAY ATTENTION TO THE PINOUT Of Base Emitter Collector leads on the package. In these photos of the TOP COMPONENT SIDE OF THE BOARD - the Base to top RIGHT (note the silkscreen notch remember Opposite) or towards the Right in the photo
On the FOIL side of "Bones" photo - the BASE is on the LEFT, Emitter is center, Collector on Right.
In both cases - BASE is FURTHEST AWAY HOLE FROM AUDIO AMP....everything else should fall into place.
Look in the photos above - and review the one below...I only aimed the light so I could get some details but it's the same Mic Amp area W37 and all...
Since you still have no audio and PA don't work, you'll need to check for voltages INTO the mic amp circuit.
If you don't get any power, the REGULATOR is possibly failed.
If you get voltage on the right side of the resistor - that means the REGULATOR is feeding power INTO the circuit - you need to VERIFY the heath of the Mic amp circuit is ok, by check the other side that drops power (limits current) a process called buffering - to power the circuit - if you have power then at least you don't have a dead short.
Meter reads 8~9V on Right but 0 or <1V on Left side? Shorted part or more than one part is shorted - Caps, transistor and Diodes fail like this...
Show some but - low voltages like <4V means you have some work to do.
Show 6 to 8 Volts? At least the BIAS of this circuit is ok...
An OPEN in the circuit will show FULL POWER on both side of the resistor - and indicates a failed part blown OPEN. Because the circuit can operate (blown fuse type of operation) but problem is located further up the chain, into the Mic Amp circuit itself.
Why do I do it this way? Experience, because of how a buffer resistor works, it keeps the mains from getting dragged down and possibly being destroyed from a major parts failure in the Mic amp itself.
Buffering allows power to pass as needed by the circuit - and if it draws more than it should the resistor protects the REGULATOR from excessive current draw.
When the circuit is not normal - as in all parts are NOT working like they should - the resistor can reflect that as a larger drop in voltage - parts of the other side of this resistor need/demand more power - like a failed transistor or a leaky cap even bad soldering PULLS down the VOLTAGE across the resistor as a voltage DROP - all of the parts if not right and any have gone out of tolerance or someone's changed them can affect the way the circuit operates and hence the current draw across this resistor will reflect that.
Experience also teaches me that many a person would try to install toys in this area or thru the Mic gain wiring itself - this can pull more power from the REGULATOR from poor wiring practices and bad soldering.
Which is why I made the comment "Many would throw them away" simply due to the fact that tracking down, repairing or undoing and fixing up the radio is not worth it and can cost more to fix than to replace directly - so the cycle repeats.
Hope the above helps!
Regards!
:+> Andy <+:
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Conclusion...
...at least to this point...
We can only guess so much with the Information we have, but for best troubleshooting technique, a lot can be determined by the operation of the front controls.
Meter lite - even work? Might mean a power failure - not just burned out.
Dimmer function - Meter lite and channel display show Bright/Dim?
No PA - Mic circuit ... but there's more.
RX and TX modes? No receive but hiss, does it TX red? If no red light then power supply failure nearly certain...regarding TX Switch or Regulator (TR21 or TR23 respectively)
Any RX at all change channel - any RX at all? May be a SECONDARY 5 volt power supply failure affecting PLL and Voice Lock controls would confirm this by testing for voltages on RX to ground from pot terminals.
I guess your Voice Lock works...
So the above - we're thinking that since you say...
You have Receiver in patient - RX now works.
Have Speaker for RX,
You change the channel you have RX for that channel...
Just no PA.
Indicates Mic Gain Amp failed...
Because - you have 4 watts TX...means...
Shows PLL has power,
Because your TX light turns RED...
We presume everything else...is ok...
So, for the issue of "remove Thy Limiter" - locate AMC Sense cap, see above, and or D11 - see photos above...and lift 1 leg of diode - does Audio Return? Then the problem is in the LIMITER side of the mic amp...
Proceed at you own risk...
See Above...
Hope this helps
Regards!
:+> Andy <+:
...at least to this point...
We can only guess so much with the Information we have, but for best troubleshooting technique, a lot can be determined by the operation of the front controls.
Meter lite - even work? Might mean a power failure - not just burned out.
Dimmer function - Meter lite and channel display show Bright/Dim?
No PA - Mic circuit ... but there's more.
RX and TX modes? No receive but hiss, does it TX red? If no red light then power supply failure nearly certain...regarding TX Switch or Regulator (TR21 or TR23 respectively)
Any RX at all change channel - any RX at all? May be a SECONDARY 5 volt power supply failure affecting PLL and Voice Lock controls would confirm this by testing for voltages on RX to ground from pot terminals.
I guess your Voice Lock works...
So the above - we're thinking that since you say...
You have Receiver in patient - RX now works.
Have Speaker for RX,
You change the channel you have RX for that channel...
Just no PA.
Indicates Mic Gain Amp failed...
Because - you have 4 watts TX...means...
Shows PLL has power,
Because your TX light turns RED...
We presume everything else...is ok...
So, for the issue of "remove Thy Limiter" - locate AMC Sense cap, see above, and or D11 - see photos above...and lift 1 leg of diode - does Audio Return? Then the problem is in the LIMITER side of the mic amp...
Proceed at you own risk...
See Above...
Hope this helps
Regards!
:+> Andy <+:
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Thank you so much, I will keep at it and let you know where I am with it, as for now, It receives audio from multiple channels, the red TX light does come on and it does show 4 watts on the dosy, the lights all work like they should and the meter swings on TX and RX, the PA does not work and I do hear RX with the PA on.
Good to know, and I wish you the best.
Until we hear from you - I can only do and provide only so much...
But, we'll keep the channel on for you.
CORRECTION:
And if you need - the Limiter itself - that part, is a 2SA733 (TR18) - it's a PNP and NTE/ECG is a 32 - TO-92 style like the one that's in there now. Follow pinout on the package, the Ferrite Bead is on the EMITTER leg of the one on the board now...(goes to ground )...
If REMOVING the TR18 restores audio - then that means there was some other work done to it - it's supposed to protect TR17 from damage - due to excessive modulation - but if someone did some work on it or tried to install something - reversed it out and returned it back to the store - now you know why they say "If there is something wrong with this item - do not return it to the store! Call us first..."
They mean that - because you may have simply "detuned" it by accident - but for you to have this kind of problem from a new radio -means they (someone) "pulled" a fast one on someone else and you're stuck with the problem - whether its' by ignorance of the clerk for the return, or the previous owner attempting to do something more and found out it didn't work and returned it under the radar so it looked normal...
SPEAKING OF NORMAL...
You never know these days...
Let us know!
:+> Andy <+:
Until we hear from you - I can only do and provide only so much...
But, we'll keep the channel on for you.
CORRECTION:
As with everything done up late at night and much to the Chagrin of my XYL - there are some errors in posted graphics.
Yes I'd Fire myself if this keeps up...
On one of the W37 Blue Wire FOIL SIDE photos - I mislabeled two transistors
TR 17 and TR 18 - in the pic shot below, they are correctly identified...
Sorry about that...
Yes I'd Fire myself if this keeps up...
On one of the W37 Blue Wire FOIL SIDE photos - I mislabeled two transistors
TR 17 and TR 18 - in the pic shot below, they are correctly identified...
Sorry about that...
And if you need - the Limiter itself - that part, is a 2SA733 (TR18) - it's a PNP and NTE/ECG is a 32 - TO-92 style like the one that's in there now. Follow pinout on the package, the Ferrite Bead is on the EMITTER leg of the one on the board now...(goes to ground )...
If REMOVING the TR18 restores audio - then that means there was some other work done to it - it's supposed to protect TR17 from damage - due to excessive modulation - but if someone did some work on it or tried to install something - reversed it out and returned it back to the store - now you know why they say "If there is something wrong with this item - do not return it to the store! Call us first..."
They mean that - because you may have simply "detuned" it by accident - but for you to have this kind of problem from a new radio -means they (someone) "pulled" a fast one on someone else and you're stuck with the problem - whether its' by ignorance of the clerk for the return, or the previous owner attempting to do something more and found out it didn't work and returned it under the radar so it looked normal...
SPEAKING OF NORMAL...
For the longest time EVERY COBRA SCHEMATIC I'VE SEEN FROM COBRA (not redraws) SHOW THIS TRACE LEADING MIC AMP SECTION - Ignore it! Doesn't exist even in real world Foil Trace...you'd never hear any audio without some resistor in there...
You never know these days...
Let us know!
:+> Andy <+:
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Well, he did tells us the RED TX light does come on, and if he was in PA, the TX light wouldn't work...I'm thinking/trusting he'd know that, so if PA doesn't work but RX does in the mode - and he still gets RED TX Light - the problem lies in the Power Regulator, Mic Gain or Limiter area. And what tells me even more is that he gets 4 watts and he can receive, meaning at least the PLL and Voice lock have power.
Else he would have NO RX and possibly a blown regulator - but we have to wait for him to tell us that...
:+> Andy <+:
More Stuff!
Else he would have NO RX and possibly a blown regulator - but we have to wait for him to tell us that...
:+> Andy <+:
More Stuff!
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