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Cobra 2000 Can’t adjust L19

Discussion in 'General CB Services Discussion' started by Brian G, Feb 6, 2021.

  1. Brian G

    Brian G Member

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    thanks for the help. I am probably going to end this as it only gets worse. I like pitching the radio in the trash. Now the frequency counter is stuck on 92.20000 and there is no voltage readings across TP 1 or 10 for L20 and 21 and only 0.6 volts at Tp 9 for L19 And any adjustments to L19,20,21 have null affect . No idea what happened. Also, i replaced L21 from another radio this morning and that had not made any difference anyways even prior to this latest post. Unless the VCO or PLL is toast that’s the only other things I can think of that would be causing this problem now where it is completely locked up.
    Just frustrating beyond belief how much time and effort and money I have spent on this in the last 2 months for nothing.

     
    #31 Brian G, Feb 15, 2021
    Last edited: Feb 16, 2021

  2. nomadradio

    nomadradio Analog Retentive

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    Bear in mind that it still has market value as a parts-only radio at the very least. Every year that goes by fewer and fewer of the parts in there can be bought new.

    73
     
  3. Brian G

    Brian G Member

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    Thanks. I going to have a cooling down period with it for a few days and then look at it again. Given the latest symptom , would that point to the PLL or VCO possibly?
     
  4. Handy Andy

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

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    I'll just leave this stuff here...
    It's for TOKO...

    upload_2021-2-16_9-47-4.png
    Now for some this seems intimidating, but it's not.

    You have to understand the Manufacturing process, or take a apart a sewing machine once in your life, and you'll never hear the end of it...

    The main problem is knowing how they did this, but once you realize that one brand TOKO or others, all use their own way of doing things, it get's a little easier to understand and to even change the coil to another level or response simply by changing the cap buried inside it. Not all coils are hand made with loving care and consideration for the next radio it goes in. In fact they're shipped in a box neatly lined up but all are supposed to be that same thing - the Radio place will strip them of their identity and slap on a number they use for their own house and throw the leftovers they don't use as saved for repair - into a dark closet corner in a warehouse never to see the light of day ...

    Until one day when the Lady met this fellow...

    upload_2021-2-16_9-45-58.png

    And you think Freddy Krueger was a bad guy, those engineering fellas at the Radio shop - know things that they will take to their grave with secretes never leaving their lips
     

    Attached Files:

  5. Robalo

    Robalo Well-Known Member

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    Handy Andy likes this.
  6. Robalo

    Robalo Well-Known Member

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    you can do it ! be tenacious, it not about the money or time . it about hobby or pleasure like wend you fix that expensive car repair .(y):);)
     
    Brian G likes this.
  7. Brian G

    Brian G Member

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    LATEST ISSUe! There is zero voltage at any pins of either the PLL or VCO. Checked all pins and all show zero voltage!! What the heck! Where does that originate? Help! Where does the voltage come from for that circuit? The channel selector?
     
  8. Brian G

    Brian G Member

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    LATEST ISSUe! There is zero voltage at any pins of either the PLL or VCO. Checked all pins and all show zero voltage!! What the heck! Where does that originate? Help! Where does the voltage come from for that circuit? The channel selector?
     
  9. Robb

    Robb Yup

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    You have begun to learn what is involved with fixing radios. The Hippocratic Oath states: ". . . that you will do your patient no harm". It is better to learn on a lesser, cheap radio, so that mistakes will not be as consequential.

    Perhaps someone will buy it from you, since, as nomadradio stated: "these radios still have value even if for only the parts".

    But to your no voltage question, the MB3756 regulator should be checked.
    Pin 9 on the PLL chip is voltage feed in IIRC
    Regulator:
    https://datasheetspdf.com/pdf/493100/Fuji/MB3756/1
     
    #39 Robb, Feb 18, 2021
    Last edited: Feb 18, 2021
    Brian G likes this.
  10. Brian G

    Brian G Member

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    well I’m not ready to give up on it yet. And yes I checked the voltage regulator. All seems good for readings.
     
    Dmans and Robb like this.
  11. guitar_199

    guitar_199 Sr. Member

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    Just piping in...... if you have voltage at the MB3756 Pin1 and you don't have it at the PLL Pin 9 ... you found the problem here. Just have to make checks on other taps along the route to find out where it is missing. Like ... IC 2 (UHIC007) pin 8 is on the same leg.... do you have voltage there? Just follow the path on the schematic from MB3756 pin 1 and check every place that it touches.
     
    Brian G likes this.
  12. Brian G

    Brian G Member

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    thanks will check a bit more. For reference, this radio has the MB8719 PLL instead of the original 8734. I assume pin out voltages may be different somewhat for this chip. I need to find a data sheet on it so I can confirm.
     
  13. Dmans

    Dmans Sr. Member

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    Pin out voltages on the MB8719 and MB8734 are the same. I believe there is some difference as to the function of pin 10 (MB8734-pin 10 is non-functioning and always tied high- i.e. 8v) between the two but this should not impede your progress.

    If this were easy, everybody would be doing it!

    73
    David
     
    Brian G likes this.
  14. guitar_199

    guitar_199 Sr. Member

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    Pin 10 on the PLL is just an additional divider input that the 8719 (7 bit input) has but the 8734 (6 bit input) does not.

    Honestly I would scale and print as much of that area in the schematic that I could.... grab a highlighter or marker and start at the regulator pin 1.... and mark every place that trace runs with the highlighter..... find it on the board and measure it with a meter.

    The position on the board will most likely not match the flow on the schematic... but it lends organization to the search. Ultimately you will chase it down to an area that does not have the voltage...then it will be down to sleuthing around to find out what is blocking the voltage from that area. The trick is (I think) to organize the search.

    [Edited to add] I am not familiar with the innards of that radio.....but as you search.... look for "jumpers". It could be a dry or cracked solder joint on a jumper causing the failure.
     
  15. Brian G

    Brian G Member

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    So now I have some voltages at PLL but some are off. Here are ones off
    Sam’s / Actual

    pin 4. 3.44 / 8
    Pin 5. 3.44. / 8
    Pin 6. 8. / 0
    Pin 11. 0.5. / 8
     

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