Cobra 2000 No voltage at Test points 7 and 8

[guitar_199]

Okay, I hid it pretty good at the top of that last message....

Can you see if you can get me as close of a shot as you can....
of the area that has the white,red,green wire with the orange out.....


This may take a couple of tries/photos....
Because I am having to use YOU for my eyes.....

AND...... the missus had me order dinner and it is on the way....
I may have to stop at one point......
But I would tell you......be ENCOURAGED
I think this is going to turn out well.....

all correct.

 

[guitar_199]

399...I would never have believed it.....

I am planning on making CERTAIN that switch segment is bad before going any further...but I really get the idea that it is.....

But from the pictures he sent....it looks to me like there are AT LEAST one spare switch section...and maybe TWO. There are empty pins in "four pin sets".

I plan on getting him to ring one out first....to make sure the empty section of choice is good...and that it works right....

Then just advise moving the wires.....

Put the mode switch in the AM mode. Trace the ORANGE wire connected to TPs 7&8 back to the mode switch. The three other terminals in the set of 4 where the orange is connected are WHITE, RED, and GREEN. The wires you're concerned with are the RED and the ORANGE. The red wire brings voltage from the AM regulator (TR41) and there should be approx 8VDC on the red wire. If not, put the switch in either USB or LSB and check again. If still no voltage, there is a problem with the AM regulator. If the voltage is there on the red wire, with the switch in the AM mode, check the ORANGE wire for 8VDC. No voltage ... bad switch. Voltage ok ... open connection between the switch and TPs 7 & 8.

- 399

 

[Brian G]

Put the mode switch in the AM mode. Trace the ORANGE wire connected to TPs 7&8 back to the mode switch. The three other terminals in the set of 4 where the orange is connected are WHITE, RED, and GREEN. The wires you're concerned with are the RED and the ORANGE. The red wire brings voltage from the AM regulator (TR41) and there should be approx 8VDC on the red wire. If not, put the switch in either USB or LSB and check again. If still no voltage, there is a problem with the AM regulator. If the voltage is there on the red wire, with the switch in the AM mode, check the ORANGE wire for 8VDC. No voltage ... bad switch. Voltage ok ... open connection between the switch and TPs 7 & 8.

- 399

There is 13.8 at the red in both AM and SSB. There is no power at orange in any mode. And the the 2 legs of the voltage regulator (the middle and the one towards the front of the radio are both showing 13.8 in all modes . The rear leg is showing zero in all modes. That doesn't seem right at all does it?

 

[Brian G]

There is 13.8 at the red in both AM and SSB. There is no power at orange in any mode.

Put the mode switch in the AM mode. Trace the ORANGE wire connected to TPs 7&8 back to the mode switch. The three other terminals in the set of 4 where the orange is connected are WHITE, RED, and GREEN. The wires you're concerned with are the RED and the ORANGE. The red wire brings voltage from the AM regulator (TR41) and there should be approx 8VDC on the red wire. If not, put the switch in either USB or LSB and check again. If still no voltage, there is a problem with the AM regulator. If the voltage is there on the red wire, with the switch in the AM mode, check the ORANGE wire for 8VDC. No voltage ... bad switch. Voltage ok ... open connection between the switch and TPs 7 & 8.

- 399

non related question I think. In the schematic, there is 6.8 volts coming off pin 9 of IC 6 into C175. I get that 6.8 at the plus side of C175 as expected. What should the voltage be on the negative side of 175? Still 6.8 or lower? Getting about 1.7. Reason I ask is because I am getting zero volts at the base and emitter of Tr42 and zero at the base of tr41. Should be about 6.8 all through there. Trying to figure out where that problem is. There is 1.7 at the negative of c174 and zero in the positive side which of course is why I assume there is zero the rest of the way down.

 

[guitar_199]

You should not see the 6.8V DC on the other side of C175.

Capacitors are supposed to block DC voltage....but allow AC signal (audio in this case) to get through.

I don't want to confuse things.....but I do want to "complete" the idea!

Capacitors are born to block DC voltage....but allow AC to go through.

A failure mode of capacitors is..... they leak. They can start to allow tiny bits of DC current through.... which allows the DC voltage to "leak through" to the other side.

Looking at that particular point in your case (CKT # 169) there should not be any DC.

non related question I think. In the schematic, there is 6.8 volts coming off pin 9 of IC 6 into C175. I get that 6.8 at the plus side of C175 as expected. What should the voltage be on the negative side of 175? Still 6.8 or lower? Getting about 1.7. Reason I ask is because I am getting zero volts at the base and emitter of Tr42 and zero at the base of tr41. Should be about 6.8 all through there. Trying to figure out where that problem is. There is 1.7 at the negative of c174 and zero in the positive side which of course is why I assume there is zero the rest of the way down.

 

[Brian G]

You should not see the 6.8V DC on the other side of C175.

Capacitors are supposed to block DC voltage....but allow AC signal (audio in this case) to get through.

I don't want to confuse things.....but I do want to "complete" the idea!

Capacitors are born to block DC voltage....but allow AC to go through.

A failure mode of capacitors is..... they leak. They can start to allow tiny bits of DC current through.... which allows the DC voltage to "leak through" to the other side.

Looking at that particular point in your case (CKT # 169) there should not be any DC.

Radio has so many issues. Getting close to just taking up to the top floor of my house and dropping down to the patio below. If I hadn’t put so much blood sweat and tears into it I would have already.

 

[Brian G]

You should not see the 6.8V DC on the other side of C175.

Capacitors are supposed to block DC voltage....but allow AC signal (audio in this case) to get through.

I don't want to confuse things.....but I do want to "complete" the idea!

Capacitors are born to block DC voltage....but allow AC to go through.

A failure mode of capacitors is..... they leak. They can start to allow tiny bits of DC current through.... which allows the DC voltage to "leak through" to the other side.

Looking at that particular point in your case (CKT # 169) there should not be any DC.

. like to know why I’m getting 13 volts at both the center and front leg of the AM regulator in all modes and zero volts at the rear leg (that is the leg towards the back of the radio with it flipped over on solderside. That’s part of why I was asking about C174/175.

 

[Brian G]

Put the mode switch in the AM mode. Trace the ORANGE wire connected to TPs 7&8 back to the mode switch. The three other terminals in the set of 4 where the orange is connected are WHITE, RED, and GREEN. The wires you're concerned with are the RED and the ORANGE. The red wire brings voltage from the AM regulator (TR41) and there should be approx 8VDC on the red wire. If not, put the switch in either USB or LSB and check again. If still no voltage, there is a problem with the AM regulator. If the voltage is there on the red wire, with the switch in the AM mode, check the ORANGE wire for 8VDC. No voltage ... bad switch. Voltage ok ... open connection between the switch and TPs 7 & 8.

- 399

Does the red wire on the mode switch send power to the switch or from it to the AM voltage regulator. TR41? As I mentioned, I am getting 13 volts in am and SSB at the outer leg of TR41 where the red wire connects. I believe that means the voltage is going to the mode switch. But I don’t think it’s supposed to be 13 volts.

 

[unit_399]

From the regulator to the switch and then to the orange wire and TPS 7&8.

 

[Brian G]

From the regulator to the switch and then to the orange wire and TPS 7&8.

God what a disaster. Thanks I’ll check it out.

 

[Brian G]

From the regulator to the switch and then to the orange wire and TPS 7&8.

Crazy. There’s 13 volts at the red wire. That sound right? Schematic shows the 13 volts with R193 in between but also 6.8v off tr41.

 

[Brian G]

From the regulator to the switch and then to the orange wire and TPS 7&8.

Then that does lead me to believe the switch might be bad because the 13 volts is not getting to the orange wire.

 

[guitar_199]

The way to think of this switch segment is.....

The white wire pin will always be 13.8v (pin 17)
The green wire pin will always be 13.8v (pin 14)
The RED wire pin SHOULD be 6.8v (pin 16)

None of those should ever change... they are the "inputs" to the switch

The ORANGE wire pin SHOULD follow the position of the switch: (pin 18)
USB should be 13.8v
AM should be 6.8v
LSB should be 13.8v

If I remember... you have lifted the orange wire OFF of the switch and STILL pin 18 did NOT start showing voltage out.....

THAT sounds like the switch is bad.....

BUT ALSO..... you are saying that you have 13.8v on the red wire....
THAT sounds like the AM regulator is shorted... or something else may have it
turned on/saturated. That is going to be a separate problem.

Presuming we get this switch problem going... you would possibly see carrier on AM...but no modulation.

Modulation comes down from CKT TRACE #169 through C174 and down to TR42 & TR41 and should be modulating that 13.8vDC on TR41's collector to the emitter... and that is what modulates your final in AM.... essentially..... "wigglilng" the driver/final transistor power supplies. You will have to do some digging to determine if one or both transistors have smoked or something coming IN is forcing them "on".

Then that does lead me to believe the switch might be bad because the 13 volts is not getting to the orange wire.

 

[Brian G]

The way to think of this switch segment is.....

The white wire pin will always be 13.8v (pin 17)
The green wire pin will always be 13.8v (pin 14)
The RED wire pin SHOULD be 6.8v (pin 16)

None of those should ever change... they are the "inputs" to the switch

The ORANGE wire pin SHOULD follow the position of the switch: (pin 18)
USB should be 13.8v
AM should be 6.8v
LSB should be 13.8v

If I remember... you have lifted the orange wire OFF of the switch and STILL pin 18 did NOT start showing voltage out.....

THAT sounds like the switch is bad.....

BUT ALSO..... you are saying that you have 13.8v on the red wire....
THAT sounds like the AM regulator is shorted... or something else may have it
turned on/saturated. That is going to be a separate problem.

Presuming we get this switch problem going... you would possibly see carrier on AM...but no modulation.

Modulation comes down from CKT TRACE #169 through C174 and down to TR42 & TR41 and should be modulating that 13.8vDC on TR41's collector to the emitter... and that is what modulates your final in AM.... essentially..... "wigglilng" the driver/final transistor power supplies. You will have to do some digging to determine if one or both transistors have smoked or something coming IN is forcing them "on".

Yes no power at orange wire in or off the pin.
And I did check and replace tr41 and 42 because I thought same thing. I don’t think they are bad unless I zapped new ones but I don’t think so. But Getting 13 volts at the emitter and collector of 41 in all modes.like you said should be 6.8 volts at 41 emitter so not sure what’s going on there.could R193 be bad? Though it looks fine. Is that what suppsed to drop voltage across the collector to the emitter?

 

[guitar_199]

Hmmmmmmm..... so you have changed TR41 & TR42.....
Two questions then.......

The SAMS shows the voltage at the TR42 Collector to TR41 base to be 6.82v
AND it shows TR42 base to be 7.23,

Can you check and confirm those two?

My suspicion is that you will find the base of TR42 base to be EXTREMELY low if NOT ZERO.

Here is why I think that........

I had to go to CB Tricks and get their schematic to confirm.....as my SAMS copy has a little FOLD right in that area that made me unsure.....but the CBTRICKS schematic DID confirm........ The orange wire OFF of the mode switch is CKT TRACE 145.

Well....CKT TRACE 145 feeds into a bias network the feeds the base of TR42. TR42 requires this "bias" in order to set the voltage out of TR41 to 6.8V. IN OTHER words...... it is a kid of "FEEDBACK controlled "regulation". You HAVE to have voltage on CKT TRACE 145 or TR42/TR41 will not bias correctly .... and you will not get 6.8v on the emitter of TR41. A "chicken and the egg thing".

So my bet is you will find the base of TR42 VERY low......or maybe even zero..... ESPECIALLY if that orange wire is disconnected still. Even if it IS connected.... since you aren't getting anything out of the switch on the orange wire..... TR42 STILL won't be biased right.

IF ALL OF THIS PROVES TO BE WHAT I SUSPECT.... then we have to get the switch problem solved....and this 6.8v versus 13.8 volt thing will solve itself.

By the way...if you ever wondered how your AM modulation works..... this is it!

When you FINALLY have voltage on CKT TRACE 145......TR42/TR41 will be biased in such a way that you will get 6.8V DC on TR41 emitter. When you switch to AM mode..... that 6.8V will be fed down to the driver and final as their POWER SOURCE. So that voltage will be steady at 6.8v..... about HALF of the max of 13.8v.

NOW.......back up to TR42's base.

RIGHT ABOVE it on the SAMS is the TR7222P audio amp. When you key the microphone and talk...that amp is driving your mic signal out pin 9 through C175 down through C174..... RIGHT to the base of TR42. IN effect.... your speech......makes the base of TR42 wiggle to "the speech". TR42 & TR41 are couple as a pretty high gain amplifier..... so the WIGGLE......becomes a much greater "WOBBULATION"....but it is STILL your speech. That "SPEECH WOBBLED 6.8V" serves as the power to your driver and final.....which have the selected channel's carrier coming through...... so that carrier is amplified......but it's LEVEL is controlled by the "SPEECH WOBBLED 6.8v" ..... and THAT is your modulated signal going out.

Let me know what you get from there..... and we will take care of it....