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Cobra 2000 No Tx.

Discussion in 'General CB Services Discussion' started by Brian G, Mar 16, 2021.

  1. Brian G

    Brian G Active Member

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    Replaced driver, final and various other suspected parts. Absolutely no tx power on am or ssb!


     

  2. nomadradio

    nomadradio Analog Retentive

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    So, did you test any of these parts? If they were all good, this suggests a problem somewhere else in the radio.

    It's time to find out if the radio is producing a 27 MHz signal in the first place. When a power stage fails, the weak signal driving into it will be audible in a second radio on the same channel, with a coax jumper plugged into it. The shell on the far end of the jumper gets backed down over the cable to use the center pin as a half-inch long sniffing antenna.

    If the second radio can hear a weak carrier from the 2000, that tells us that the mixer chip is producing a carrier on the channel frequency.

    And if you can't hear that, there's no reason to look at the power sections of the transmitter. They can only amplify what's driving into them.

    Establishing this issue becomes a dividing line. If you can hear a carrier, everything leading up to IC5 is probably okay. If so the problem is between IC5 and the antenna socket.

    And if you can't hear even a weak carrier, there is a problem upstream from IC5. That chip, BTW almost never fails. If there is no transmit carrier at all, putting a 'scope onto one, then the other of its two inputs would be next. Odds are one of the two input frequencies is AWOL. Odds are the chip is okay.

    73
     
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  3. Brian G

    Brian G Active Member

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    there is definitely carrier I can hear on the other radio. So I think the problem is definitely in the power section.
     
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  4. nomadradio

    nomadradio Analog Retentive

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    Next thing would be to watch the S-meter on the monitor radio. Move the sniffing antenna to get a low reading on the meter, below 4 or 5.

    Now see that L45, L46, L47 and L48 each shows a peak when adjusted.

    If all four of them reveal a peak, have a close look at the position of each slug. If any of them have the end of the slug DEAD EVEN with the rim of the hole, that's worth noting. This is a fault that's probably not causing the "dead" problem, but you'll need to come back to it when you can see power on the wattmeter. And if all four of them show a peak with the slug visibly below the rim, that's a good thing.

    73
     
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  5. Brian G

    Brian G Active Member

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    all those coils seem to be peaked fine best I can tell. I know they were all operating fine before the power failure.
     
  6. Brian G

    Brian G Active Member

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    . here is a question, I replaced the 2 bias diodes by pulling them out of another radio that I know was working properly when it was in operation. I am 99% sure I put them in properly when I moved them over. But on the outside chance I reversed them, would that cause the power problem or no?
     
  7. nomadradio

    nomadradio Analog Retentive

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    The bias diode works by pulling current away from the base (input) of a driver or final transistor. If it's reversed, it won't pull any current at all. The transistor's bias current should shoot way up, beyond what's safe.

    Using DC-meter measurements alone only helps when the fault is in a DC circuit of some sort. Have you pulled loose the two bias-test jumpers, put a current meter in line and tried to set the bias current on both?

    Poking a 'scope probe onto the collector of TR39, TR38 and TR36 would serve to narrow things down.

    Every time a part gets replaced there is the opportunity for error. A solder bridge or some such. Best reason to replace only parts that appear not to be working. Reduces the risk of adding unintended problems.

    If the bias current can be set normally on the driver and final, this suggests that they were installed correctly and the problem lies elsewhere.

    And if they won't set properly, it's time to find out why.

    73
     
  8. Brian G

    Brian G Active Member

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    just to clarify, when I meant possibly “reversed” I meant D49 switched with D50. Would that possibly cause the no power issue ? And I tried to bias both the driver and final. I have not been able to properly do so. One cannot be tuned down enough and one cannot be set high enough which is why I asked the question. There is no way I can see to tell those 2 bias diodes apart by looking at them. I was able to find replacements for them but they look like regular diodes so I don’t know how to wire or mount them if that becomes necessary.
     
  9. Brian G

    Brian G Active Member

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    now I just burned up L39 again. And when it had that surge something else has happened. The radio will not transmit. When you key the mike, the receive light stays on and very bright. When I flip the CB PA switch to PA, the receive light stays on bright. When you key the mike then, the receive light stays on but dims while keyed.

    I took the mike off and tried to key the radio by touching the pins on the mike jack which I have done before. It sparked really bad at the jack and the clock and frequency counter blinked out for a second. That’s definitely not normal.

    Finally the frequency counter is stuck in the same channel (26.51 approximately).

    Any ideas where to check?

    i checked voltage readings at the voltage regulator and they seem to be ok there and other places. So I think I toasted another component. Hope it isn’t the VCO or PLL!
     
    #9 Brian G, Mar 24, 2021
    Last edited: Mar 24, 2021
  10. Brian G

    Brian G Active Member

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    Update. I replaced the voltage regulator because i didn’t have voltage at pin 5 where I should have had 13 volts into TR35. I could get 13v at the pad if I unsoldered the pin but not with it soldered. As soon as I turned back on , L39’started to burn up again so I think that is what popped the regulator before. What can I do to stop that burning up so I can work on it at least? What could the issue be over there?

    also how do you tell the difference between the final and driver bias diodes? One has a green dot and one has a white dot. Want to make sure I didn’t reverse them but I don’t know how to tell which is which?
     
    #10 Brian G, Mar 25, 2021
    Last edited: Mar 25, 2021
  11. nomadradio

    nomadradio Analog Retentive

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    Two possibilities. Simple one is that the final transistor is shorted collector to emitter. Easy enough to find out with the ohms side of your meter.

    Less-simple would be that the bias setting for TR36 is turned way,way too high AND the transmit-only 8 Volts is active all the time.

    I have come to depend on Occam's razor. The simplest explanation is usually the most likely.

    But if TR36 turns out to be shorted, the next question is to find the boo-boo that croaked it. Don't want to install another one unless it's going to survive. If the fault that caused the final to fail is still there, a new final won't last long.

    If I'm right, you'll need to remove TR36. When you do, power up the radio and see if there is any DC voltage on the foil pad where the base lead was soldered. Should read zero. If you see a DC voltage any higher than about 0.3 Volts, that could cause a new final to commit suicide pronto.

    73
     
  12. Brian G

    Brian G Active Member

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    Forgive me, how do I check it with the ohm meter? And when you say shorted, you mean the leads are shorted together or an internal short? and since my L39 is nice and black and smelled up the room, where do I get another one? Is there a reasonable substitute I can buy? No idea what the rating is on it..
     
  13. nomadradio

    nomadradio Analog Retentive

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    Oh, and about the bias diodes. One end of each is grounded. Should be the side with the color dot. Take the other wire loose at the pc board. Clip the black lead of your meter to the circuit-board ground. Set the meter to diode test and probe the now-loose lead of the diode. If it reads around 0.6 Volts, that's the driver diode. If it reads about double that, or 1.2 Volts, that's what the one on the final transistor should read.

    And if the reading is not one or the other of those two, there's trouble.

    73
     
  14. nomadradio

    nomadradio Analog Retentive

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    Oops. Looks like we cross-posted. If you don't have a general-purpose Volt-ohm-Amp meter on hand, this severely limits what you can do. It's pretty much THE basic tool. For my money the most-basic would be an analog 'scope, but if you need to learn to use a meter to measure resistance, and look for a short?

    Hmm. Gotta be stuff on YouTube at this level of instruction. Pretty fundamental stuff. Not really my specialty.

    I suppose this points out the hazard of just changing parts because someone's opinion was to "just try that". There are two main risks:

    First, is that you'll replace a perfectly-good part that isn't bad to start with. And the radio is still broken.

    Second is that some small installation error, like a tiny hair of solder shorting two foil pads together will create a new fault that wasn't there to begin with. When this happens, you now have a radio with more faults to fix than it started out with.

    I don't have a high opinion of the "Replace This And See What Happens" method. Far better to make non-destructive tests first and replace parts that cause bad meter readings.

    Not the perfectly-good parts.

    73
     
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  15. Brian G

    Brian G Active Member

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    thanks. I do have a good VOM meter so I can do it-just wanted to make sure I knew how but I can google it. . And I do have a scope too, though my scope expertise is limited.
     

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