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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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    #16 Brian G, Feb 14, 2021
    Last edited: Feb 14, 2021

  2. Brian G

    Brian G Member

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    Ok so I can now tune L19 (i replaced with a new one). Set to 2.70 volts on channel 40 as spec. . However I am having trouble with L21. It’s supposed to be peaked at about 3.4 pp volts but I can only get it to 1.9 Max. And I cannot tune L22/23/59 on the frequency counter to spec. Not sure if maybe L21 is not good and that is preventing me from peaking to 3.4 volts? Also I noticed that the spec say to peak L20 for max RF or about 190 Mv. But peak on the scope shows about 1.6 volts which is way higher. Should I just put it on 190 mv either way or does that suggest an issue? Thanks.
     
  3. 999

    999 Well-Known Member

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    Check your messages at the top of the page.
     
  4. Brian G

    Brian G Member

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    Sorry I don’t see anything. I’m on my phone so maybe I’ll look using computer later when I get home.
     
    999 likes this.
  5. Brian G

    Brian G Member

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    Update 2: now L21 does not appear to be working at all. There is no voltage at TP10 at all and nothing happens when I try to adjust L21. L20 is working. When I turn L19, the frequency changes but there is no change in voltage reading on meter. Maddening!! Was at least partially working earlier.
     
  6. nomadradio

    nomadradio Analog Retentive

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    The 11.325 crystal feeds into TR29. Since the drive level is fairly high, TR29 produces a lot of harmonic energy. TR29 feeds L21, which is tuned to the third harmonic of the crystal, 33.975 MHz. L21 feeds that harmonic directly into the emitter of TR20. Since the approx 35 MHz VCO signal coming out of IC2 pin 10 feeds the base of TR20, you'll get the difference between those two frequencies at TP10 790 kHz on channel 1, 1.23 MHz on channel 40.

    If the 11.325 crystal shuts down, or TR29 or 20 goes bad, or L21 you'll get this sort of symptom. Until you have a volt more or less peak-to-peak signal at TP10, the 8719 chip has nothing to lock on to.

    L21 will fail to show a peak at TP10 if it's bad or if nothing is feeding into it, or if the TR20 mixer circuit has failed.

    Or if L21 is bad.

    This is the kind of job I can't do without a 'scope.

    73
     
    NZ8N likes this.
  7. Handy Andy

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

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    The L21 is driven pretty hard so the coil can it's components - you've already found in L19 needing to be replaced, can affect L21.

    L19 coil swapped out and now that works, so if this is what is happening, I'm thinking - this is a cascade failure condition - your amp stages to power the Tripler has pressed pretty hard against the coils of the leading and following stages of this section. Yes, they can fail from excessive current and RF energy ringing thru them and losses are as heat - they always do produce. This is not a unique problem, just evident from this design, driving the PLL pretty hard for mods - or the MB3756 regulator - being what it is, isn't perfect so the stresses of age and the type of circuit - a Tripler - it is, can cause the coils to open up.

    Welcome to "Fixer-upper Radio 101" - it's a lot like Murphy's Law...You do have to take it apart and investigate, then put it back together after you've fixed it to recreate the ORIGINAL conditions, now you can proceed to fix that...
     
  8. Brian G

    Brian G Member

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    I’m Leaning towards it being L21 bad. I checked Tr20 and 29 and they are right on. I am going to pilfer and L21 from a parts radio and see what that gets me.
     
  9. Brian G

    Brian G Member

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    Good info, thanks. And you’re right I think!!
    I’m Leaning towards it being L21 bad. I checked Tr20 and 29 and they are right on. I am going to pilfer and L21 from a parts radio and see what that gets me.
     
  10. Brian G

    Brian G Member

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    Also interesting note. When I change channels , nothing happens. The frequency counter stays at the same random frequency. Will change of I turn L19 but other than that the frequency doesn’t move.
     
  11. Brian G

    Brian G Member

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    Also interesting note. When I change channels , nothing happens. The frequency counter stays at the same random frequency. Will change of I turn L19 but other than that the frequency doesn’t move.
     
  12. techradioman

    techradioman Active Member

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    Get a scope and you can have this repaired in about 15 minutes.
     
  13. Brian G

    Brian G Member

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    I have one and am using it. Hasn’t helped solve the problem.using the Sam’s facts synthesizer alignment instructions.
     
    #28 Brian G, Feb 15, 2021
    Last edited: Feb 15, 2021
  14. Handy Andy

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

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    We're here for ya' - you might want to remove L21 and look underneath - it uses a "can" capacitor, that slips, wedged into a little "cubbie" on the bottom the coil - the space between the slug, and the output pins - this cap can fail - affecting the tune which @nomadradio has eluded to.
    upload_2021-2-15_15-24-46.png

    The above is 5MM thin style ones...
    You have the 10MM thick ones...
    upload_2021-2-15_15-27-22.png

    You may need to save the coil and piggy a cap across the leaded output WHEN YOU remount it on the board - the cap solders to the bottom foil side of the board onto those two pins that internal cap was tied across. This setup, having the cap internal - reduced issues with leakage and worked as shielding - you may not have that luxury now if that cap failed - you'll have to install a similar value - on the bottom foil side - by trial and error - caps of close value and NPO quality to make it "ring" and work as a Tripler again.

    Just solder a cap of say 22pF NPO - try and tune the range to make L21 work with L19 and L20 - then if not, try a lower value - adjust slug thru the range - no luck? Then try a higher value - until you can find a balance between the value the Original can cap was, to what you have now. It won't be perfect but it will work.
     
    Klondike Mike and Robb like this.
  15. Robb

    Robb Yup

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    You can also strap a variable cap on to it. Adjust until you find a working value; then test the variable cap and used a fixed value in its place.
     

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