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Cobra 29ltd issue

Discussion in 'General CB Services Discussion' started by Cowboy_Illinois, Feb 22, 2021.

  1. Cowboy_Illinois

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    I have a 29ltd I picked up cheap that I'm trying to get back in service that had a mosfet conversion and they had ran it into a high swr of and smoked the final.... it totally smoked D8... so I replaced D8 and took out the mosfet stuff and replaced with original 2078 and replaced the resistor and cap that was removed....now heres the issue...the output is extremely low and heating up the final...I did notice last night that the voltage on the collector went from 13.5 volts which is what the power supply is providing up to almost 17 volts when the volume knob was increased....I have checked the cap around D8 circuit and the resistor as well as the VR attached ....any suggestions would be greatly appreciated


     

  2. Handy Andy

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

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    When I come across the MOSFET final upgrade, you have to also revert back much of the stuff they did to maximize the MOSFET - you have to rebuild the strip to make the 2078 even work like it used to - you only get 4 watts but hey, to each their own.

    For rebuilding? A Cobra 29 usually has a few things you'll need to put in there, they've removed - so if you are good at copy-catting - the 2078 strip is pretty straightforward...

    upload_2021-2-22_14-37-47.png
     
  3. Cowboy_Illinois

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    I have the conversion back to stock with the 2078 .... asfar as Watts.... its not even peeking out over 1/4 of a watt...the frequency is spot on...and have returned to max output without clipping modulation or anything like that its back to being all stock its just the output isn't even close to what it should be and the final keeps heating up on transmit .
     
  4. Handy Andy

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

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    Then do a tune up on it if possible...

    If you have all the parts and pieces in place, then the only thing that missing is a part they changed, not you - so you may have to dig for it - this means even the DRIVER needs to be investigated...

    some have gone as far as taken away turns of L17 (Tuning Slug) which will hurt the rebuild for you'll need full 5 turn coil to help you return the admittance part back to bring up power. If they did 4 turns, you're safe - but anything less than that you'll have to re-wind L17 to make that tuning slug help the 2078 develop power.

    Hence the tune-up suggestion - this power may return when you do a tuneup...

    Try these links to our Forum to see if they can lead you to your answer...

    https://www.worldwidedx.com/threads...obra-29-ltd-classic.251271/page-2#post-702228

    https://www.worldwidedx.com/threads/cobra-29-ltd-classic-not-transmit.228821/page-2#post-624739

    https://www.worldwidedx.com/threads/cobra-29-wx-st-mod.251721/
     
  5. Handy Andy

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

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    While you went looking for more info, wanted to cover this little ditty you mentioned...

    The "measurement" if it was done with a simple DVM - is normal...

    Why because the Driver turns on, when the Pre Driver gets a trigger - thru Pin 3 - of the Microphone jack/Plug - that opens the Pin 4 Receiver Speaker Return - in turn, being the Predriver into Driver now there is power going into the BASE of the FINAL - to TX power out - using that power that would otherwise be forced to go thru the speaker.
    4-pinPLUG.jpg

    So when you turn up the volume - this same effect is happening only you see it on the FINAL because as you turn up the Volume to hear the speaker, this same line - goes to the Driver AND FINAL - and the SPEAKER - AT THE SAME TIME - only one section works at one time - until you key up and force the Audio that would otherwise have gone to the Speaker, into the Collector of the FINAL and DRIVER - modulating then feeding them Power. Making the Audio into RF.

    How??? because - the Pre-driver supplied the Driver with RF just enough to turn it on, and then apply the Audio you disconnected from the Speaker Return on Pin 4 of the Mic cord, into The Driver and Final - but to turn them on requires RF thru the Pre-driver, into Driver, turning it on by applying RF to the BASE of the Driver. How? By using Ground to complete the Pre-Driver's Power Circuit, done on Mic Pin 3 by grounding it thru to Pin 1 on the Microphone cord this turns on the RF the Predriver makes, just a carrier, but enough to turn on the Driver.

    So why does the voltage go up when you turn up the Volume?

    Because they share the same feed off of the Audio Transformer (Modulation Transformer) so as you turn up the volume to hear the audio - the audio amp puts more power thru this Transformer - you see it as an increase in voltage - it's the spikes of audio power embedded in the DC signal ADDED into and forms a complex vector of power and DC current into both the Speaker winds and the Driver and Final lines - at the same time.

    When you key the Mike - you disconnect a wire thru a set of moving contacts
    MicWiringCloseup.jpg

    Those contacts switch over to connect another pair of contacts - which one turns on your TX mode, (ground your Predrivers Power circuit) and the other turns on the Mic element in the Handset to send Mic audio to be amplified and then put thru that Audio Transformer into the Driver and Final
     
  6. Cowboy_Illinois

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    Makes total sense after you explained that and thank you I'll check the wraps on the coil
     
  7. Cowboy_Illinois

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    Thank you Andy it makes sense
     
  8. Handy Andy

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

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    Keep us posted on your progress...
    FamilyTimeCB.jpg

    Thanks!
     

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