Cobra 2000 No voltage at Test points 7 and 8

[guitar_199]

That little green cap... with the black spot......
Is that a tantalum? an NPO ??

It just looks unusual... or at least I have not seen one like that.....

That varies @Brian G - this helps "tune" so be ready with several values - and choose the one that allows for the best low-level SSB power "transfer" - use your voice into your mic, as whispers and low-level ambient sounds work best for this - to find the best transfer peak power.

 

[1iwilly]

it says 47k here is a pic

Great thank you!! Also what is the value of that green cap over near the right edge?

 

[Handy Andy]

47pF - "K" refers to a voltage/tolerance - having a black cap - means NPO - so K then defaults to a 10% tolerance with UNKNOWN working voltage so care must be taken on preventing too much RF voltage across it. A Breakdown issue, not much to worry about in low-power settings...

​

Now it uses a Green body, so that usually tells me the case styles' working voltage is about 16V

Not to worry too much though, the typical working voltage of the unit is between 16 to 25V DC.

You're running about 8 to as much as 10 volts - not much more than that - the Pre-Driver is working with about 300mW Maximum output - just be careful in soldering - a short will do far more damage than no capacitor at all.

Go here if you need more info...
http://www.learningelectronics.net/VA3AVR/gadgets/caps/caps.html

 

[Brian G]

Well I am back. Again there is no Tx power. There is the expected 0.7 into the driver from C157. But on AM Tx the center leg is only showing 0.32 volts. On SSB the TX Volt is 13 volts. And it is 13 volts at the center leg on RX both modes

I don’t think D50 is bad as I’ve swapped it before and it didn’t matter. For laughs there is .67 at the hot side of D50 and 1.2 at the hot side of D49.
Basically same reading at the final. 0.8 volts in. 13 volts SSB TX and 0.3 volts AM TX.
For bias current, I am getting normal at the driver but zero at the final. (But again I get 1.2 volts at D49)

This has been the issue all along. No TX output at all. Help?

 

[Handy Andy]

Nope, when your collectors voltages on AM are that low, you lost either the AM Regulator (TR41 or TR42) or the MODE switch is getting bad. - AM is not working - but your have 13.8 for SSB.

 

[nomadradio]

on AM Tx the center leg is only showing 0.32 volts. On SSB the TX Volt is 13 volts.

Mode selector reached its blown-final limit.

You can roach only so many finals before the AM section of the switch becomes the next-weakest link in the chain.

Okay, I don't really know how many finals this radio has popped so far, but this is what I have come to associate with this failure. The usual sequence is to find a bad final and replace it. Now the radio has sideband transmit only because the shorted final overloaded and damaged it. The contact points for the two SSB sections of the switch are okay, only the AM contact is damaged, most likely.

Had one customer with a bad mode selector who said "Screw Sideband, all I want is AM".

Hot wired around that section of the mode selector. Got him back in the air a good bit cheaper than replacing the selector would have been.

73

 

[Brian G]

Mode selector reached its blown-final limit.

You can roach only so many finals before the AM section of the switch becomes the next-weakest link in the chain.

Okay, I don't really know how many finals this radio has popped so far, but this is what I have come to associate with this failure. The usual sequence is to find a bad final and replace it. Now the radio has sideband transmit only because the shorted final overloaded and damaged it. The contact points for the two SSB sections of the switch are okay, only the AM contact is damaged, most likely.

Had one customer with a bad mode selector who said "Screw Sideband, all I want is AM".

Hot wired around that section of the mode selector. Got him back in the air a good bit cheaper than replacing the selector would have been.

73

So there was a bad solder connection at the AM regulator. I restored that and now all voltages at the final and driver are normal but there is no bias reading at the final at all and still no TX. Like I mentioned before, if I put a volt meter on the hot side of D49 it shows 1.2 volts and .67 at hot side of D50 and I believe those are normal readings? Is the final possibly bad so that's why I can't get any bias reading? Must be some damaged part in there...
Is there an easy way to test the final?

Also I took readings at Tr37 in AM TX:
Base: 1.29 (normal according to Sam’s)
Collector: 8.0 (am should be 2.66 accord to Sam’s)
Emitter: 1.0 (0.67 in Sam’s).


Also the mode switch was replaced yesterday.

 

[nomadradio]

If R181 is damaged, no current will reach the base of TR36 from the emitter of TR37.

So far I don't remember seeing mention of whether or not the final passed ( or flunked) any kind of component test. The voltage at the base of TR36 would be of interest.

73

 

[Brian G]

If R181 is damaged, no current will reach the base of TR36 from the emitter of TR37.

So far I don't remember seeing mention of whether or not the final passed ( or flunked) any kind of component test. The voltage at the base of TR36 would be of interest.

73

 

[Brian G]

If R181 is damaged, no current will reach the base of TR36 from the emitter of TR37.

So far I don't remember seeing mention of whether or not the final passed ( or flunked) any kind of component test. The voltage at the base of TR36 would be of interest.

73

So the final was blown after testing it. I put in a new one and now have normal voltage readings at both the driver and final. But there was still no power output.

In regards to L38, the one in there did not look right.. So I pulled one from an old junk radio and put that in. Now I was getting a small amount of RF (1.5 watts) but that was all I could get even after tuning L47,48,46,45 , 38.and VR10. Getting closer I suppose, but still miles away....
the final and driver are new so no idea why there is so little RF.

As far as the bias, I can set the final no problem. the driver is not right. I cannot turn it down below 50 ma. What could cause the low power output? Could the driver still be bad or weak? Not sure what else to check at this point.

 

[nomadradio]

Did any of L44,45,46,47 or L48 show a peak with the top surface of the slug dead even with the rim of the hole?

This is a visual clue that there has been a failure in that can. The peak position of those skinny slugs should never occur less than one or two full turns below the rim of the hole.

And if they all are peaking with the slug visibly below the rim, the cause lies elsewhere.

73

 

[Handy Andy]

the final and driver are new so no idea why there is so little RF.

Remember where the RF is being measured, the meter is AFTER it goes thru that filter and SWR mess, so if there are issues there, in that strip, it may explain why it seems to eat Finals and Drivers - there may be a shorted, blown or open part.

Caps are notorious for failing and looking dumb in the process.

Even a broken slug or coil that got overly twisted can lift traces off the board and break connections.

Any of the above can interfere with the RF "Tuning" and path - causing a high SWR issue, it can look good on the meter even when you using a Dummy load - the load is fine, the break is further back towards the final - the high SWR is at that location.

It's when they are exposed to RF do they show their "dumb-ness".- they aren't easy to track down - lots of times it's substitution part by part - do you really find the culprit...

Since the radio is so old, and you don't really know it's history. IF the radio ever got struck by lightning or heavy static, or a poorly designed amp it was driving - arced, the radio could appear fine, but one day it just starts popping Driver and Finals and so they stowed it away for another look - another later day.

It wound up on your bench, they just ran of out time and or interest and $$$ to fix it...

 

[Brian G]

Remember where the RF is being measured, the meter is AFTER it goes thru that filter and SWR mess, so if there are issues there, in that strip, it may explain why it seems to eat Finals and Drivers - there may be a shorted, blown or open part.

Caps are notorious for failing and looking dumb in the process.

Even a broken slug or coil that got overly twisted can lift traces off the board and break connections.

Any of the above can interfere with the RF "Tuning" and path - causing a high SWR issue, it can look good on the meter even when you using a Dummy load - the load is fine, the break is further back towards the final - the high SWR is at that location.

It's when they are exposed to RF do they show their "dumb-ness".- they aren't easy to track down - lots of times it's substitution part by part - do you really find the culprit...

Since the radio is so old, and you don't really know it's history. IF the radio ever got struck by lightning or heavy static, or a poorly designed amp it was driving - arced, the radio could appear fine, but one day it just starts popping Driver and Finals and so they stowed it away for another look - another later day.

It wound up on your bench, they just ran of out time and or interest and $$$ to fix it...

Yeah it’s a mess and I’ve been working on this in and off for 6 months. Probably gonna cut my losses with it. I don’t think it’s worth my time anymore. I’ve spent so much time and money on it and have basically gotten nowhere.

 

[Low_Boy]

Yeah it’s a mess and I’ve been working on this in and off for 6 months. Probably gonna cut my losses with it. I don’t think it’s worth my time anymore. I’ve spent so much time and money on it and have basically gotten nowhere.

But you are learning a lot. Keep going you will get it. At least you are now getting output.

 

[Brian G]

But you are learning a lot. Keep going you will get it. At least you are now getting output.

I love your optimism!! Thank you. I just know enough to be dangerous so that's what the frustration is. I agree with Handy Andy, I believe there is still something not right in the driver/final area. I have replaced basically every component between the predriver and L38 (at least once).
I did notice this morning I did had a solder bridge at the base of the final connecting it to the ground end of C154. I removed that bridge and replaced the caps around that area but I have no change still. I suppose the final could have been weakened/damaged (though not completely dead) by this bridge. I was tossing around replacing the final again based on that knowledge. What do you think about that?

I also noticed that the little power I do get come up slowly on the meter. So when you key the radio (no modulation) there is almost no power at first but it slowly comes up to the 1.5 watts or so. after several seconds. I have never seen that. What could that mean and could that be a possible cause? Sounds like a leaky or bad component but don't know what.