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Cobra 25 LTD Classic, no voltage change tuning

Discussion in 'General CB Services Discussion' started by Mark A Bradley, Jun 4, 2021.

  1. Mark A Bradley

    Mark A Bradley Active Member

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    Cobra 25 LTD, the voltage does not change when tuning the receive and transmit on the Cobra 25 LTD Classic. All I hear in the external speaker is the humming of the radio, and getting no response on Oscilloscope when trying to do any adjustments, seems like the pots are not responding to my turns of the cores, I installed a new Audio Amp, IC1, ( TA7222AP ), because all I hear is the speaker noise, like the radio is trying to put out audio ,and replaced the mixer, IC2 ( TA7310P ) to cover my behind, no 10 volts on pin 9 when keying up, but now I have the 10 volts on TX, pin 9.
    Checked the Diodes, the Diodes seem to be OK, but a few have no voltage through them, I cannot remember which ones, but in the audio section, some Capacitors and Resistors are dead, no juice going to them. I think I possibly have a dead Electrical Part.
    All I know is that I aligned the Receive, worked for a short, then quit?. This is a stumper, should I do a Signal Trace in the Audio Section, I do not have either Transmit or Receive, and the green and red indicator light is inoperative, it was working when all was working on the radio, when the light went out, the radio to a shiznik. Where to start, I never had a issue like this before, working then all of a sudden, the thing just up and stopped.
    No RX or TX, not Green or Red indicator light, speaker slight humming, no change on millivolt change on Oscilloscope, turn the pot slug, nothing, this is on all of the pots, nothing, no change at all. Any help is definitely appreciated.
    Any more info, let me know, I am home all day and will watch the post for any more information if needed. Enjoy, and Thanks. Mark


     
    #1 Mark A Bradley, Jun 4, 2021
    Last edited: Jun 4, 2021

  2. Low_Boy

    Low_Boy Sr. Member

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    First thing I would do is go over your alignment. Especially if the radio was working before the tune. A good guess the VCO is misaligned if you lost TX and RX. Also if the RX TX light is not changing you are probably out of lock.
     
    Shadetree Mechanic likes this.
  3. Mark A Bradley

    Mark A Bradley Active Member

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    Hello, thanks for the possible repair, the only thing is, on the Oscilloscope, I do not get any voltage or peak readings, the peak may move 1 Millivolt, but the rest 9f the cans, both RX and TX, when u go to adjust that can, no voltage change. How do I align this when you have no voltage change?
     
  4. OldTech03

    OldTech03 Sr. Member

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    Do you have schematics and alignment info?
    No disrespect intended but what is your level of understanding of how this radio or any radio works?
    And then we can figure out what we need to do to guide through the process.

    not every part of in radio will necessarily have a voltage or signal that would be relevant to your issue.
     
    Crawdad and Shadetree Mechanic like this.
  5. Mark A Bradley

    Mark A Bradley Active Member

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    Yes, I do, I tried to tune this last night with no avail, I will give it a try again, but with no voltage change, I watch my Peak to Peak last night and went off of that, and nothing, I even turned it on this morning and still nothing. You van hear the hum or a sound of a speaker when it is just on and no sound. Will try and post back, Thanks Mark
     
  6. Low_Boy

    Low_Boy Sr. Member

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    You lost me.
     
  7. Mark A Bradley

    Mark A Bradley Active Member

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    Hey, the radio must be in lock, I cannot get my 3.2Volts for the VFO, I have like .261 volts, thats all I have, and it will not come out of lock it seems. I am going to peek at the Schematics and see if I can see anything down the line. What I meant on the last post is I did try to align it the day before yesterday, and I did not get any voltage, especially when I put the generator on for my receive tuning, it seems it all went down hill after turning it on. I know it does not make sense to me, but this is what this thing is doing. I will keep digging with ya, I will not be home tomorrow, one of my best friends from the old neighborhood, his Mother passed away a few weeks ago, and going to a luncheon memorial, if not to tired when I get home, I will work on the radio again, talk later. Any ideas how to unlock, or am I missing my 5 volt line, I may have to look into this.
     
  8. Handy Andy

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

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    IF you're radio is old, then more than likley it's got a lot of old caps that when worked after sitting for a while, can fail.

    There are several caps in and around the PLL, so even if you have a display - you'll have to check and make sure the PLL is getting enough voltage to work too...

    The power supply in that radio is three different voltages, and needs all three of them to work.
    • your typical 13.8V DC battery (mobile) supply) to start,
    Then breaks down to two others...

    One an 9V (approx) regulated thru a Zener, by Mic and Audio Chip...TR17 (Main regulator)

    Second is the ~8.3V to 9.1V (more tightly regulated) that Feeds the PLL and VCO section with 8 volts interfacing to it thru their respective output amps using the 9V side of TR17..

    It has two regulators

    The 2ND one is not a big regulator - its a voltage feed buffered and uses a BIG ~220 to as big as 470uF Cap to filter out and smooth the spikes and switching noises from the PLL as well as external stuff from the voltage feed it gets from the TR17.

    Due to the nature of the 25LTD PLL chip - its more like the 91099106 PLL chip from olden days - they kept it's design function to keep it simple for many years.

    Now, you may have a more exotic design that is a spin off of the original - if you do, you're on your own.

    Usually you don't find the newer ones dead...

    Older ones can be brought back to life with a little TLC and replace the old parts that dried, wore out - and will be serviceable and useable for another lifetime.
     
    NZ8N likes this.
  9. Mark A Bradley

    Mark A Bradley Active Member

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    Hey Handy Andy, I may have to recheck the Caps, I did replace them all, it is possible that one may have or be shorted out. I cannot set the voltage as there is no voltage upon setting the 3.2 Volts on the first procedure of the alignment according to the Cobra service manual.
    I shall hopefully find time today to re-look those caps over, to me it seems that a component went bad on me, I will have to get into those Caps around the Pll, I replaced the Cap to the right of the PLL, I believe it was C104 and C103, the 2 larger Caps, now there are C101 and C102 to check, let you know. I will check over the Caps and report this afternoon or tomorrow, today is going to be a long day, I have a Memorial to attend for one of my Buddies of 40+ years, as his Mother passed. Thanks for the heads up, will get back to you all.
     
  10. Mark A Bradley

    Mark A Bradley Active Member

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    I have a decent amount of knowledge of the basics of a radio workings, I am a bit green on the subject, but I am learning as I go. I am not a dummy, I have the basic repair knowledge of how a CB radio works. I understand that the radio does not have the voltage in all places, but I have aligned a handful of radios, and repair at least a good handful. I will be honest with you, I don't know everything on the repair of these Cobras, but only around 1 year have I been messing around with these radios, and basically learning as I go. I started to learn, read, and hands on since the lock down from this Covid, but I have been interested in working on the radios, since I have back issues, and have a hard time repair Automobiles. I have repaired Cars and Jeeps for 30 years+, and I wanted to do something different and less back breaking on my retirement than breaking my back doing repairs on cars that nobody else wants to do, like dropping a Transmission on my back. I had enough with the surgery's on my back, as I am fused from my tailbone, half way up my back, and according to my MRI, my neck is starting to go next, as I now have to Disk's starting to protrude into my spinal cord, and when the disk does hit whatever, it almost's put me to the floor. This is why I chose to play and repair a few radios, way easier on me, and my Wife does not want me out in the garage working on cars or trucks till 2 or 3 in the morning, as I also did after work for many, many years. Doing that kind of work takes a toll on a person, up and down, climbing on your hands and knees, lifting heavy things that is only knee high, when it is easier when the car or truck is face height. So, I decided to try fixing CB radios t keep me off of my knees and rolling on my back.
    Overall, I am a quick learner, that's why I post these questions when I have a issue, I know how to do the major involvement, in a radio, checking to make sure we have our primary frequencies, voltages at certain areas, voltages upon transmission, ect. So, bare with me, you tell me to do something on the radio, I will do it so I get the knowledge and experience doing the repair and or diagnostics. So, I am no dummy, but willing to learn more from all of you Techs and knowledgeable individuals, this helps me out, and gives you the satisfaction of helping us learn and repair our own stuff. So, no dummy here, willing to listen to you and learn the ropes of the repair of a radio, as I do have the equipment of repairing the radio. Thanks for listening, as everyone else who needs help on repairs, they are willing to listen and learn from the Techs on this site.
    All is appreciated from you Techs, and the public, being a wiz or a dummy, we all are willing to try to repair our own radios, as long as we work together, all will be great. Willing to listen to you, learn, and try to repair what you ask us to do, as I am willing to learn everything I can, as this is the only thing that my body and mind will let me do any more, working on a CB takes the pain away. Thanks for listening, and I hope we can work together and get this radio screaming again, as I have around 20 Cobras in my shop that needs repairing, this is where I will learn on how a CB radio works, reading, and tips and tricks from the pro's on this website. Thank you, no dummy here, just a broke down ,retired Auto Tech who wants a change of pace, and a non back breaking thing to do in my spare time, but I am still out in the garage till 2 or 3 in the morning working on these radios. Peace,, talk soon.
     
  11. Handy Andy

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

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    The eaiset ways are to simply observe and so do a simple Voltage check.

    When you key up, or unkey - the old '25, the PLL will attempt to "shift" the frequency it uses in the VCO - down or up (that 9109 or 9106 variant from KEPC 25LTD) 455kHz so it can now have 3 IF's - meaning it uses 3 different frequencies it can gather from the VCO and the 10.240MHz Xtal.

    The main frequency, is your BASE frequency, 10.240 - that is the PLL's timing.

    upload_2021-6-5_10-55-15.png

    The 10.240 MHz signal is important, it is the timing, loop reference and 2nd IF (FET2) signal used to make 455kHz 3rd IF.CF 2 (455kHz Black Box) L6 (TR2 TR3 TR4)

    So to help keep this simple, L15 generates your Approximately 16MHz signal thru your channel Selector - the BASE timing frequency, your 10.240 - is mixed together in that at the TX mixer to make 27MHz.

    L15 then has to "shift" gears to make the 1st IF RX frequency so you can even hear the channel - you can see that "jump" at TP2 - R59.
    • (To help you understand this, it's still around 16MHz, just designed to be "off" a set amount so the radio can do the correct mixing in the IF sections and the TX mixer - done properly so others can hear you (TX), and you hear them (RX).)
    The 10.240 frequency is used - because L15, thru SUBTRACTION (image processing) makes a 10.7MHz image the radio amplifies. 10.7 is the end result of the 1St IF - MIXING - so 10.7 is the 2nd IF )

    Then 10.240 is mixed in with it, your 10.7, thru SUBTRACTION, to generate another Image to make 455kHz - your 3rd IF

    Hope this helps, because L15 is the main reference for all this, the PLL shifting the 455kHz is done thru T/R pin at Pin 8.

    When the previous posters asked if you've done the tune up - hearing a "buzz" is one thing - IF noise or hiss/White Noise - is different it's not a buzz or a hum. Locate VR2 - the RX IF main gain adjust pot, it may be open - causing a "dead" receive - especially when you have TX but no RX - it may be VR 2 need to be adjusted.
     
    #11 Handy Andy, Jun 5, 2021
    Last edited: Jun 5, 2021
    Robalo likes this.
  12. Handy Andy

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

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    Ok, broke this down into another step because you may need to look at something the Radio uses to determine it's "health"

    PLL to make sure things are working - will be happy enough to send out stuff, but if one section isn't working - it designed to shut down - that's where Pin 4 comes into play.

    The loop from the VCO is one thing, but it also needs to know signal is getting out, so it looks to two sense diodes - designed to detect, and rectify a small RF signal presence into a current (power) the PLL can see this and utilize it as a means to monitor it;s health and prevent unwarranted emissions - A.K.A. - out of lock and something's wrong stuff the engineers put in there...

    So Pin 4 is ACTIVE when the PLL is working and RF is being used (also known as IF and RX - as well as TX stuff)

    Those didoes are D12 and D18 - in the old '25s - you'd see power (about 7.5V max) all the time on Pin 4 of the PLL.

    upload_2021-6-5_11-26-46.png
     
  13. groundwire

    groundwire Sr. Member

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    No offense intended but the cobra 25 is so cheap, just throw that thing away and buy a new radio.
     
    TheRealPorkchop likes this.
  14. Handy Andy

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

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    There are things in this world that if we don't do something to redirect our efforts.

    The pain keeps returning and will never leave, unless - until, we find a way....

    You should try and take on someone else's pain...like mine - I've got a lifetime of suffering from it.
     
    sunbulls likes this.
  15. sunbulls

    sunbulls Sr. Member

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    I don’t put a price tag on the pleasure I get (eureka effect) from solving a difficult problem or the grin I get all day afterwards that pains my cheek muscles.
     
    Holydvr and TM86 like this.

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