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cobra 2000 receiving and tx but channel not changing.

Discussion in 'General CB Services Discussion' started by wildwinds, Feb 12, 2020.

  1. wildwinds

    wildwinds Member

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    Hello first to say what a great forum youn have here, i have been reading the articles trying to fix a cobra 2000 i bought new in the eighties. My father and i used it a lot in them golden dx days. Anyway it was put away in a room where it stayed for many years untill recently when i thought i would give it a try again.
    the radio had mods done to it way back and the pll was changed to mb8719 it all worked fine.
    I have a fair understanding of the workings and electronics has been a hobby here for many years, but i am by no means professional.
    It worked for a while but then died so i replaced the electrolic capacitors but not the tantalum.



    So the radio is receiving and transmitting on one frequency but there is no channel selection, I put the radio on 27.405 and checked the signal on a close by sdr, and it was way off at 27.580 or around there.
    would i be right in thinking the vco pll is not locking?

    So I have checked tp 10 but there no rf there also there is 4.6 volts dc at the tp 10
    PIN 17 of the mb8719 has 3.6 volt
    PIN 6 has .02 volts
    Tp 9 is at 6.7 volts and not changing with the channel selector.
    So is there any way i can get it up and running, and ideas where to look for the trouble.
    Once again thank you for the great forum
    Best regards
    jim
     
    357magnum likes this.

  2. Handy Andy

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

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    Well, try this and get back with us...

    This is to check to see if the Rotary channel selector is not causing this fault...

    Turn on radio, look at channel display - note the channel..

    Turn off radio and unplug it, to discharge the power supply and memory (if any for clock)

    Turn the channel knob a few clicks left or right, don't matter...

    Plug radio back in, turn on radio - does channel still show previous or is it a new channel?

    Check frequency...TX - all that jazz.. A new channel - new frequency?

    IF this causes the channel and frequency to change, and you have to do this in order tom make the channel change that means there is a problem with the BCD rotary knob in the channel selector and display. The PLL looks to see if a small switch in the rotary knob assembly lifts and then makes contact again, with an internal detent (the click and engagement you feel and hear when you turn the channel selector) this is supposed to do this whenever you change the channel with the power on. The Detent changes it's state telling the PLL to hold and then whenever the Detent finds another resting spot, the new programming of the Binary Coded Decimal (the BCD programming to program the PLL to the new frequency) tells it to got to this new frequency and lock in.

    By what you describe, this sounds like the most likely problem, a frozen rotary assembly or dirty detent switch inside the channel selector can do this - forcing you to turn off power then make the change to new channel by your count then repowering the radio - it can also mean some of the previous work done to the radio is causing this problem - in other words if it's no longer OEM - and the chips been worked on, then this fault may be due to wiring onto other problems that require you to remove the work done to it so you can rebuild it back to it's stock condition and restore normal operation.
     
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  3. wildwinds

    wildwinds Member

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    hi Andy thanks for the quick reply,
    i have tried what you said, ithe display shows the new channel.
    I tried a short tx on channel 20 and i can receive it audio and all on the sdr but the frequency is 27 405 regardless of the channel number displayed.
    THanks again
     
    Handy Andy likes this.
  4. Woody-202

    Woody-202 Sr. Member

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    Welcome to the forum (y)
     
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  5. wildwinds

    wildwinds Member

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    thanks Woody
     
  6. 357magnum

    357magnum Sr. Member

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    Welcome to the forum ! I can't but I'm sure someone on here can get you through this (y) I run a Cobra 2k also . ;)
     
  7. wildwinds

    wildwinds Member

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    Thanks Magnum.
    A great radio in its day
     
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  8. 357magnum

    357magnum Sr. Member

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    A great radio in it's day ? :whistle::LOL: Mine is awesome now w/ 650 watts behind it . :)
     
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  9. OldTech03

    OldTech03 Sr. Member

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    Do have a license for that power???????? Well do you!!!????
     
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  10. wildwinds

    wildwinds Member

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    650 watts...tube?
     
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  11. wildwinds

    wildwinds Member

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    ok so i have made a discovery:sneaky:
    I was checking out the capacitors and i found that c 135 was not connecting properly, i have re soldered it and now i find
    1 that the rx audio is much lower....i can hear stations
    2 its not switching to tx no output
    if i unsolder the c135 again it all works except there is no frequency changing

    also i have checked the binary voltages are changing with the channel selector

    thanks j
     
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  12. BJ radionut

    BJ radionut Supporting Member and 6m addict

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    Cap may be bad. Replace with new and see what happens
     
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  13. nomadradio

    nomadradio Analog Retentive

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    357magnum likes this.
  14. Handy Andy

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

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    @wildwinds - if you can get to that site, grab both the 2000 GTL as well as 148GTL MOBILE radio info too.

    Why? They pretty much share similar boards and since you mentioned the 80's - the number of parts you may have to replace in the Electrolytic caps alone is quite a bunch.

    Having a direction to go and information to work with on similarly made boards can help you in finding out stuff a single service manual will not always tell - or show, you in what to do or where to go in locating parts let alone what they are.

    Since you mentioned the AGE of the radio - it was purchased back in the 80's - that usually means the radios' internal parts like those Electrolytic caps - can dry out and corrode internally from age and lack of use (your storage no matter the effort - this corrosion event will occur) these Electrolytic caps contain a weak acidic solution for the purpose they're designed for - polarized capacitors. Ordinary humidity and air contains moisture that the caps can "Breathe in" and release gasses that accelerate this process in time. This is really a normal process, and the longer the radios is USED the caps can stabilize and last a long time. It's when they are discharged and in a relaxed state does the Electrolytic action turn into a GALVANIC reaction and the plates corrode faster due to their internal resistance and their locations on lines that supply power within circuits.

    You may be experiencing this now and it simply requires a simple unsolder cap, swap out and solder in new ones to make the radio work well again.
     
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  15. wildwinds

    wildwinds Member

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    Good morning, bluster day here in Ireland thank you all for the comments.
    Yes i have changed all the capacitors except the tantalum ones.
    As regards c135 when i remove it the radio transmits a and receives but does not change frequency when i turn the channel selector.
    Now i have replaced c 135 there is a noticable lowering of the received audio and the transmit is not working
    i have checked the voltage regulator voltages
    these are
    1 8.37
    2 13.5
    3 8.37
    4 00.0
    5 13.5
    6 8.3
    7 00
    8 8.5
    no change when i press the transmit key

    THe pll voltages
    1 6.9 10 8.3
    2 3.5 11 .03
    3 .06 12 8.31
    4 8.3 13 8.31
    5 8.3 14 0.02
    6 .18 15 .03
    7 4.3 16 8.2
    8 4.3 17 3.5
    9 8.6 18 0.12

    Thanks again for the help.
    I have download that schematic as suggested and checking out posts on the 148 also
    there appears to be stuff on cbtricks but the sites not working.
     
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