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Cobra 2000 No voltage at Test points 7 and 8

Well THAT would seem to imply that SOME switching voltages are working... because you wouldn't have that without the 8.15 #12 source working.

I am afraid it is getting close to scope time..... and I know I would start back at IC5.... looking for the 7.8 MHz and 34 MHz inputs...and seeing if 27 MHz is coming out.....

A freq counter would sure be handy..... as the frequencies into and out of the TX Mixer (IC5) not only have to BE there.... they have to be RIGHT.

Brian G said:
Yes there is definitely 8 volts all through those points.
Click to expand...
 
guitar_199 said:
Well THAT would seem to imply that SOME switching voltages are working... because you wouldn't have that without the 8.15 #12 source working.

I am afraid it is getting close to scope time..... and I know I would start back at IC5.... looking for the 7.8 MHz and 34 MHz inputs...and seeing if 27 MHz is coming out.....

A freq counter would sure be handy..... as the frequencies into and out of the TX Mixer (IC5) not only have to BE there.... they have to be RIGHT.
Click to expand...
I do have an external frequency counter. Just gotta see where to hook it up. I’m not very proficient at using the scope but I can manage.
 
If you could have both scope and freq counter available..... I would cut it right in the middle..... the base of TR39. The SAMS is calling for a .08 v peak to peak....and it SHOULD be at your current channel transmit frequency.

Driving a stake right in the middle can tell you whether you need to work back....or forward in the circuit.

Do be sure that you key the mic when measuring. All of this depends on the XMT voltages being there.
 
guitar_199 said:
If you could have both scope and freq counter available..... I would cut it right in the middle..... the base of TR39. The SAMS is calling for a .08 v peak to peak....and it SHOULD be at your current channel transmit frequency.

Driving a stake right in the middle can tell you whether you need to work back....or forward in the circuit.

Do be sure that you key the mic when measuring. All of this depends on the XMT voltages being there.
Click to expand...
So at the base of tr39 frequency is spot on agreement to the radios frequency counter (27.189). The p to p reading is jumping all around. Between about 0.15- 0.23 volts. The p to p reading at the base of the driver is 7.5 volts. The Sam’s says should only be 0.6! I get 0.7 with my VOM meter which is the “raw”‘reading to shows in the Sam’s. But why is the p to p high?

also in regards to c157. I get 8.2 volts at the side where it connects with that coupling coil. But the other side of it is showing 0.7 —which is continued in to the base of the driver. That sound correct?

. Should I get the same frequency reading as I move to and past that coupling coil and c157? Because it’s not there. Getting readings over 45 MHz in that area. What should it be at base of driver?
 
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i did not see anything on the schematic so here is one if needed
 

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  • cobra_2000gtl_sm-good copy.pdf
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Very frustrated. Can’t figure this out. Voltages seem to be ok through the circuits but there is no current. I just don’t know enough to know where to check. And when I do check, I’m not always sure if the reading I get is good or not (per my earlier post). Just to many potential places I guess where the fault is happening.
 
I am going to be "in and out" today.

One thing I forgot to ask or specify..... I am not sure what mode you were in when you did that test but I would think it easier to fight AM battles first. One reason.... in SS that base is liable to be jumping because you won't have anything there unless you key AND MODULATE. So.... if you keyed.....and the mic was picking up random audio in the microphone.... that level you were measuring WILL be jumping all over the place.

Here is why.....

Down that TX strip we looking at......
When we are in SSB (either side) I THINK that the 7.8MHz coming in to IC5 is from the double balanced mixer.... shifted for the particular sideband you have selected....but the important thing is... nothing will be there (or shouldn't be) UNLESS you are modulating.... like with a tone or a whistle. If you are not.....then the mic will just be picking up random room noise so the "mixed 27 MHz" will come and go with the modulation.

In AM the audio modulation is done by the orange wire circuit to the driver/final. So what comes in at the base of TR39 SHOULD be a nice solid 27 MHz carrier signal.

When using the scope... assuming your probe has a ground clip... be sure that ground clip is clipped to a good ground on the radio to give the scope a good solid "reference". Without that, the signal you view may be unstable. ALSO... depending on the condition of solder joints in the area.. make sure that your probe tip (which is usually pretty sharp) is "punching in to the solder" good. Older solder joints can oxidize and be tough for a probe to get the signal. just make sure you are bearing into the solder joint good and solid.

With all of this in mind...... make sure that you were on AM and if you weren't repeat your checks while switched to AM.

but I will say this....... if you can see RF at the base of TR39..... but something is wrong with it at the TAP on L44.... something seems funny right there. Once the 27 Mhz (27.189 MHz on your radio as it is) comes OUT of IC5 it should STAY 27.189 MHz all the way through the RF strip right up to the driver and final. THAT is your carrier frequency.

Mull this over and let me know what you think. As I say... I will be doing some other things today....but let me know... I will see it at some point.




Brian G said:
So at the base of tr39 frequency is spot on agreement to the radios frequency counter (27.189). The p to p reading is jumping all around. Between about 0.15- 0.23 volts. The p to p reading at the base of the driver is 7.5 volts. The Sam’s says should only be 0.6! I get 0.7 with my VOM meter which is the “raw”‘reading to shows in the Sam’s. But why is the p to p high?

also in regards to c157. I get 8.2 volts at the side where it connects with that coupling coil. But the other side of it is showing 0.7 —which is continued in to the base of the driver. That sound correct?

. Should I get the same frequency reading as I move to and past that coupling coil and c157? Because it’s not there. Getting readings over 45 MHz in that area. What should it be at base of driver?
Click to expand...
 
guitar_199 said:
I am going to be "in and out" today.

One thing I forgot to ask or specify..... I am not sure what mode you were in when you did that test but I would think it easier to fight AM battles first. One reason.... in SS that base is liable to be jumping because you won't have anything there unless you key AND MODULATE. So.... if you keyed.....and the mic was picking up random audio in the microphone.... that level you were measuring WILL be jumping all over the place.

Here is why.....

Down that TX strip we looking at......
When we are in SSB (either side) I THINK that the 7.8MHz coming in to IC5 is from the double balanced mixer.... shifted for the particular sideband you have selected....but the important thing is... nothing will be there (or shouldn't be) UNLESS you are modulating.... like with a tone or a whistle. If you are not.....then the mic will just be picking up random room noise so the "mixed 27 MHz" will come and go with the modulation.

In AM the audio modulation is done by the orange wire circuit to the driver/final. So what comes in at the base of TR39 SHOULD be a nice solid 27 MHz carrier signal.

When using the scope... assuming your probe has a ground clip... be sure that ground clip is clipped to a good ground on the radio to give the scope a good solid "reference". Without that, the signal you view may be unstable. ALSO... depending on the condition of solder joints in the area.. make sure that your probe tip (which is usually pretty sharp) is "punching in to the solder" good. Older solder joints can oxidize and be tough for a probe to get the signal. just make sure you are bearing into the solder joint good and solid.

With all of this in mind...... make sure that you were on AM and if you weren't repeat your checks while switched to AM.

but I will say this....... if you can see RF at the base of TR39..... but something is wrong with it at the TAP on L44.... something seems funny right there. Once the 27 Mhz (27.189 MHz on your radio as it is) comes OUT of IC5 it should STAY 27.189 MHz all the way through the RF strip right up to the driver and final. THAT is your carrier frequency.

Mull this over and let me know what you think. As I say... I will be doing some other things today....but let me know... I will see it at some point.
Click to expand...
I think i tested it in both modes. Will recheck in AM only including recheck at l44 to driver. I do think there is an issue there somewhere. I had to replace l44 from a salvaged radio. I assume it is installed correct and not damaged. Not sure what else there could be a problem. I’m heading out for most of the weekend later today but I will get back you as soon as I can look. Thanks again for help!
 
guitar_199 said:
I am going to be "in and out" today.

One thing I forgot to ask or specify..... I am not sure what mode you were in when you did that test but I would think it easier to fight AM battles first. One reason.... in SS that base is liable to be jumping because you won't have anything there unless you key AND MODULATE. So.... if you keyed.....and the mic was picking up random audio in the microphone.... that level you were measuring WILL be jumping all over the place.

Here is why.....

Down that TX strip we looking at......
When we are in SSB (either side) I THINK that the 7.8MHz coming in to IC5 is from the double balanced mixer.... shifted for the particular sideband you have selected....but the important thing is... nothing will be there (or shouldn't be) UNLESS you are modulating.... like with a tone or a whistle. If you are not.....then the mic will just be picking up random room noise so the "mixed 27 MHz" will come and go with the modulation.

In AM the audio modulation is done by the orange wire circuit to the driver/final. So what comes in at the base of TR39 SHOULD be a nice solid 27 MHz carrier signal.

When using the scope... assuming your probe has a ground clip... be sure that ground clip is clipped to a good ground on the radio to give the scope a good solid "reference". Without that, the signal you view may be unstable. ALSO... depending on the condition of solder joints in the area.. make sure that your probe tip (which is usually pretty sharp) is "punching in to the solder" good. Older solder joints can oxidize and be tough for a probe to get the signal. just make sure you are bearing into the solder joint good and solid.

With all of this in mind...... make sure that you were on AM and if you weren't repeat your checks while switched to AM.

but I will say this....... if you can see RF at the base of TR39..... but something is wrong with it at the TAP on L44.... something seems funny right there. Once the 27 Mhz (27.189 MHz on your radio as it is) comes OUT of IC5 it should STAY 27.189 MHz all the way through the RF strip right up to the driver and final. THAT is your carrier frequency.

Mull this over and let me know what you think. As I say... I will be doing some other things today....but let me know... I will see it at some point.
Click to expand...
So did quick check at L44. There are 3 legs on it. The one that connects over to C157 is the one that is wrong. That is showing weird numbers around 45 MHz. The other 2 are good, right in line with the channel frequency and from Tr39. So perhaps something weird there. Not sure what problem could be though. Not much there besides that and the capacitor.
 
More odd but relevant questions.......

What is the make and model of your scope?
What is the bandwidth?
Were you able to get the display synced and get a good clear view of the waveform?
 

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