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Cobra 200 DX No power output

Discussion in 'General CB Services Discussion' started by Tolteca, Jul 22, 2018.

  1. Tolteca

    Tolteca New Member

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    Hello all,



    Back with another project, unfortunately my last stomper projects ended up getting picked up before I could figure them out. I'm hoping this time around I have a bit more of a casual fault and not such a head scratcher.

    I have 2 Cobra 200 DX radios, in almost mint condition. The issue is that the previous owner ended up over volting them one after the other. Story goes on both occasions what killed them was his power supply failing and sending 18+ into the DC input. Among the things that got fried were the output transistors. He said he sent them to be repaired but not much was done besides changing the 2SC2290 transistors and then they gave up.

    I've picked what seems to be the better one out of the two and have started reviving it. The radio receives and is on frequency. Could probably use a little more reception sensitivity but besides having zero output power it seems to be alive and well. The first issue that I found with it is a bad driver mosfet at the output stage. When measuring the gate voltage I was only getting about 0.5V and after taking the mosfet out and testing it I found that it was leaky between gate and source. I reset the gate voltage to 3.3V and installed the driver mosfet out of the other radio which seems to be good. I can now see a really decent signal coming out of C305 coming from the mosfet stage. It's about 12Vp-p and shows up at the correct frequency and modulated on AM. From here however I cannot find any RF signal at all being applied to the bases of the 2SC2290s. Collector and Base voltages are present and correct just nothing modulating the stage.

    Looking at the schematic I can't exactly understand how the signal coming out of the mosfet driver stage ends up entering the power amplifier. Whatever the case it seems like the signal being put out by this driver stage ends up getting lost somewhere.
     
    #1 Tolteca, Jul 22, 2018
    Last edited: Jul 22, 2018

  2. Martian

    Martian Active Member

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    Excuse the crudity, i have highlighted c200.PNG the signal path here
     
    Shadetree Mechanic likes this.
  3. Shadetree Mechanic

    Shadetree Mechanic 808 On The North Side of Dover

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    My best guess would be a shorted diode somewhere.
     
  4. loosecannon

    loosecannon Sr. Member

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    any chance the coil on that relay could have been damaged by the 18v spikes?
    LC
     
  5. Tolteca

    Tolteca New Member

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    Found the fault. The issue was C641, a tiny SMD capacitor. It's pictured next to the relay on Martian's very helpful schematic of the signal path. On it's input side there was about 8V p-p worth of signal but on it's output side there was barely 50mV. Lifting one end of the capacitor didn't help. Replaced it with a 330pf ceramic disc type and voila! 40Watts output on AM at the moment. Seems like a terrible place to cheap out and use a shitty SMD capacitor.

    Now I just need to go through the alignment procedure. Another thing I'm questioning however is these resistors on the amp stage. They are white sandbar types. As I understand these are wirewound inside, could these cause inductance issues? The amp seems to be working as is. Another issue I found is that the two capacitors attached to them had been replaced with .01uf instead of .1uf so I put them back to .1uf. Could there have been a reason someone changed them to .01? Attached are pictures.

    Hopefully everything aligns right. Will report back.
     

    Attached Files:

    #5 Tolteca, Jul 22, 2018
    Last edited: Jul 22, 2018
    kaos513 and loosecannon like this.
  6. Martian

    Martian Active Member

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    Replace the resistors :)

    I really would replace anything you find to be wrong with what's on the drawings.
    Now that you have output it's probably all of the parts that have been replaced incorrectly that is causing further problems.
     
  7. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

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    Nice detective work Martian!

    Someone's done work to it before.

    Either water damage or high moisture.
    Cobra200Corrosion.png

    Would not be a bad idea to reflow as much solder as you can...not sure of the "plated thru" on this is just as corroded as the rest. You may find some quirks and unstable transmit - it would be wise to look over this when you have rust and oxides - looks almost like "flood damaged" and you're in salvage mode...
    Cobra200CorrosionView2.jpg

    Those two resistors are not part of the original - my brother had me do some work to his - these are not the original - so I concur with Martian. You can see the "solder tint" is different. These would work to a certain degree but are wirewound so they act more like LCR types versus R/C.

    Wow, looking as history here...

    :+> Andy <+:
     
  8. Low_Boy

    Low_Boy Well-Known Member

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    Martian Thank You.
    I have one of these Cobra 200 DX radios sitting in my pile of started projects. My radio shows a watt or two output. It is on frequency and receives. The 2290's are a bit week but good, driver and pre driver are good . Possibly I will put them back in the radio and trace the path you maped out and see if I can get it working.

    It seems working close to 60 Hrs a week really limits any big projects this time of year.
     
  9. Low_Boy

    Low_Boy Well-Known Member

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    Andy we posted at the same time. LOL
     
  10. Martian

    Martian Active Member

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    I borrowed one of the images @Handy Andy posted.

    Cobra200CorrosionView2.jpg

    The screw isn't tightened down and the soldering on the tab of that transistor is quite bad as well.

    My suggestion is to take the board out of the radio completely and give it a good clean.
    Some Branson EC mixed up in a spray bottle or board cleaner.
    You have to try to stop the corrosion that is likely still happening.

    Get the board clean so your not trying to read the silkscreen thru that muck or follow traces on the board to track things down. Just make it easier to see what your doing.

    And not contaminate your new solder joints with that muck.
     
  11. Tolteca

    Tolteca New Member

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    For the other guys having low transmit power I would go and check C641 right away for starters. That capacitor has no business in that part of the circuit. It's about the size of a grain of sand. The rest of the capacitor in that circuit are regular through hole ceramic types up until what I checked. There may be others but man, talk about a bottle neck. I would almost dare accuse Cobra of planned obsolescence with that one.

    The soldering on the 2290s is indeed pretty cruddy, who ever tried repairing it after the damage didn't really try too hard and may have just tried replacing the more obvious things without doing much other fault finding. The driver mosfet had been replaced at least once before as the traces were damaged. I'll look over the board, most of it actually looks okay except for the rear end. The previous owner had them stored away after he killed them so they were probably stored somewhere that wasn't ideal.
     
  12. Low_Boy

    Low_Boy Well-Known Member

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    Same problem. Someone replaced the RD16HHF and screwed up a trace. I since found a very nice unmolested 200 but thought they were desinged so terribly I sold it. Thanks
     
  13. 9C1Driver

    9C1Driver Sr. Member

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    Probably a good move, those 200's were real turds. As expensive as 2290's are now there is no way I would waste a matched pair on one.
     
  14. Low_Boy

    Low_Boy Well-Known Member

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    I always laughed at the radio that had echo but no talkback to set the echo .lol
     
    binrat likes this.
  15. Tolteca

    Tolteca New Member

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    Yeah I don't think I'm gonna end up keeping this. If you guys were to sell a Cobra 200, how much would you guys ask for it?
     

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