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cobra 148 unlock PLL

Discussion in 'General CB Services Discussion' started by Robalo, Aug 28, 2018.

  1. Robalo

    Robalo Active Member

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    this radio is from December 92, the board is PC 412AG. This radio is made in the Philippines. It has a lot of resistors 1/8 and 1/4. As you see in the video, the PL is unlocked. When i got the radio it has noise toy echo board and variable power double voice lock and audio mods. I'm trying to put it back as stock maybe the problem is on the voice lock which is unlocked now. I hope you can help.



     

  2. nomadradio

    nomadradio Analog Retentive

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    Unlocked?

    Do you mean that the transmit frequency is fixed by a trimpot inside the radio, so that the receiver can never really "lock" onto the transmit frequency at all? That's the stock setup from the factory. Makes it hard to have a 3-way conversation unless the other two stations agree to slide their frequencies to match your transmit side.

    Or do you mean that the clarifier was modified to lock the transmit and receiver to the same, exact frequency? That makes it possible for 3 or more stations to all talk on the same frequency. Saves a lot of carpal-tunnel risk from having to tweak the clarifier every time a different station keys up.

    Or do you mean that someone has "stretched" the coverage so that the radio tunes almost a whole channel down with the clarifier all the way to the left?

    More than one way to mod a radio. If you'll narrow down what it's doing now, the next step is deciding what you want it to do.

    73
     
  3. Robalo

    Robalo Active Member

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    no radio is not RX and no TX. I want to fix it as factory.
     
  4. nomadradio

    nomadradio Analog Retentive

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    Whoa! You said unlocked PLL.

    I answered about unlocking a clarifier.

    DOH!

    Only way I know to pin down the problem you have is with a 'scope. First to make sure the 10.24 MHz crystal is running properly.

    Next I would look at pin 17 of the 8719 to see that it has at least 1 Volt or more peak-to-peak input signal.

    That would be a very good time to check L21, to see that the slug is peaked for max level on pin 17.

    If either of these signals is missing, that suggests the direction to move next.

    The one thing you can check with a meter is the 8-Volt supply feeding pin 9 of the 8719. If that's missing, the 8719 can't do anything.

    73
     
  5. ExitThirteen

    ExitThirteen Grumpy and Cranky

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    Give L20 about a turn and a half clockwise, see if that locks it in. Looks like it's spun out too far. Give that a try and let us know how you get on.



    ~Cheers~
     
  6. Robb

    Robb Yup

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    Hi Robalo:

    In the video, (video muy bueno!) did I see the VCO voltage @ 6.17v? Isn't it supposed to be >5v? The loop freq is not locked either? Was the dual clarifier wired up correctly? Are the varactors diodes good and the clarifier circuit complete? Just some thoughts . . .
     
  7. Handy Andy

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

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    Well, what I see is the 10.240 is good, the PLL is "locking" but...

    Voice Lock - the TX side needs Diode D52 and D75 installed -

    [​IMG]

    See the pot next to it?
    It should have 8 volts on it when in TX mode.

    Check MB3756 - Make sure it's ok...

    CB PA switch needs to GROUND ( Thru TR 35 2SA945 ) to PIN 5 of the MIC connector (1992 uses 5pin) FROM PIN 5 on the MB 3756. So make sure CB mode is on and you have MIC Pin 5 going thru it to tell the system to turn on the RF Side

    You can manually ground pin 5 of the MB 3756 to Foil ground and see if TX comes on.

    Else PLL or Lock detect circuit TR35 and all diodes (D51 D43) and Resistors (R164 560 ohm) and even Cap C135 (electrolytic) need to be checked...Pin 6 goes high when things are ok, R163 22K by PLL helps Pull-Up Lock line when MB3756 gets a little too close to full load condition...
     
  8. Robalo

    Robalo Active Member

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    You can manually ground pin 5 of the MB 3756 to Foil ground and see if TX comes on. YES IT DOES TX good.

    NO DOES NOT. thank you.
    now pin 5 from mike run red wire to cb/pa and green wire to tr35 and when i press ptt beeep to ground good now i goint to vr5 r187 i have good ground but running to r188 i have no 8.0 v ok now check ic4 pin 8 press ptt (0.22v at the pin and 0.66v at pc board) no voltage ok so bad voltage regulator i hope. also i remove d31 and i have on tr35 C 13.9V BASE 8.3V AND E 8.3V . ok new ic4 and no 8.0v on tx. on pin 8 so what else could be.
    thank you Exit13,Robb and Handy Andy to open my eyes.lol 73's
     
    #8 Robalo, Aug 29, 2018
    Last edited: Aug 29, 2018
  9. Handy Andy

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

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    IF you get 8 volts to the PLL, but no RX 8 volts - then CHECK Pin 6 of PLL status.

    Check at the PLL...D31...Banded side...at the base of the PLL...

    Pin 6 low? LESS THAN 1 volt? PLL out of lock.
    Pin 6 High? Above 5 volts? PLL in lock...

    Now, check its' UNBANDED side - by C135 - right next to it....

    RobaldoPLL.jpg

    Locate C135 - considering age of the radio - can pull PLL lock line low due to shorted old cap - (age failed)

    If that side of the diode is dead flat but BANDED side shows 6 volts - replace C135 - and look for that trace to may have shorted to case by error...

    This may have taken out several parts including the MB3756 due to the error.
     
  10. Robalo

    Robalo Active Member

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    From s403-3 to R152 i have 8.3 v ok now from r152 to de base no voltage this measurements are after new ic4,d31,c135,r152 tr47,c232 and still no tx.hummm whats next.if i short pin 4 to pin 5 on ic 4 i have 8.2v on pin 8off ic4
     
  11. Handy Andy

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

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    R152 goes ACTIVE in AM mode.

    Same for R157...

    MB3756 does now work - Pin 8 is the one that sends TX, so were ok there...the original IC4 may still be good too - but lets' see...moving on...

    Locate TR47 or D43/R164/TR35 - see if the BASE of these gets any power - like 1~2 volts - they are the PLL LOCK signal...

    Then go back to Pin 5 of MB3756 (IC4) and see if goes low on TX.

    Pin 6 is RX

    R162 USES Pin 2 - pulls up Pin5 to help it switch back to RX mode...Pin 2 is straight 13.8VDC.

    MB3756.jpg

    Per Rick Jackson Euro Radio Co - via CB Tricks​
     
    #11 Handy Andy, Sep 10, 2018
    Last edited: Sep 10, 2018
    Shadetree Mechanic likes this.
  12. Robalo

    Robalo Active Member

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    R164 to the base of TR35 on rx 1.52v press ptt base tr35 0.5v .base TR47 rx 5.29v, tx 5.29 but across r225(100k) 0.70v so let me check r223 and tr33 .ic4 pin 6 always on 8.29v not switch it. also voice lock now is back as factory. tr33 C 8.32V E 7.21V B 7.88V
     
    #12 Robalo, Sep 11, 2018
    Last edited: Sep 11, 2018
  13. Handy Andy

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

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    Need to find R162, it sends power to Pin 6...from Pin 2...

    Looks like PLL line still low - not in lock, R225 tells me this - it may be why pin 6 of IC 4 won't change.

    Look for that diode D31 by the PLL, and C135 - if that cap is shorted, that PLL lock line never goes high enough to signal TX line....check that line at C135...see above...
     
  14. Robalo

    Robalo Active Member

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    C135,r162, d31 and ic1 replace two times fresh parts
     
  15. Robalo

    Robalo Active Member

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    FRESH PARTS AGAIN D31,R163,S401,D43,R164,TR35 HAS 0.65V ON EMITER WHY IS NOT SWITCHING TO 8.3V
     

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