1. You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.

Cobra 29 LTD PLL Not Locking for RX?

Discussion in 'General CB Services Discussion' started by LeapFrog, Jul 16, 2016.

  1. LeapFrog

    LeapFrog
    Expand Collapse
    χρυσοποιία

    Joined:
    Feb 15, 2016
    Messages:
    1,339
    Likes Received:
    430
    Hi everybody I'm back playing with the 29 LTD Classic.
    This radio has been my "test subject" for experiments since the beginning, one day I dropped a live component
    (a loose but wired potentiometer for a variable carrier experiment) Inside the radio.
    Ever since that day the receiver has not worked. [Something went short circuit, "heard" a spark]

    The radio transmits fine, the pa function works great, LED for tx/rx switching works.
    No meter movement (Using a known s-meter, and the swr/rf switch is in the correct position)
    Microphone & antenna connected.
    Still no signal gen yet so I even tried transmitting with a good radio in the next room over & the meter never budges.
    Attached 8-digit freq. counter to R61 then PTT and I'm transmitting on frequency
    (close anyway on ch. 28 I get 027.28480 & not 027.28500).

    I don't want to go through life shot-gunning problems, troubleshooting is important to me.
    Last night I stumbled upon Radio Tech's own forum and found:
    "Understanding how CB radio works"

    Following some things I read, I attempted to find a 10.240 Mhz signal at Pin 2 of the PLL
    [With the notch facing west Pin 1 would be at "bottom left corner", like a standard MCU I assume.]

    No dice, the counter reads some strange frequencies but never locks-down solid, most of the time it's reads 0.
    I guess either:
    A) The dc blocking capacitor I used at the end of my counter should be removed.
    B) PLL is not getting the correct voltage during rx.
    C) PLL is bad (or I have the wrong pin?).

    Quote from the thread:
    "The PLL chip has a divider circuit that produces this signal so it is internal to the chip"

    I assume he means that the internal divider has failed?
    Is that possibly my problem: a partial failure of the PLL?

    It's locking for TX & just will not for RX (I think)
    [Where should I probe to confirm the failure, I assume the VCO is okay as the unit will TX]

    As a side note I cannot find TR30 in this radio to save my life!
    If you could provide some input, please let me know what you think.

    Thank You Gents
    -Leap


     
    #1 LeapFrog, Jul 16, 2016
    Last edited: Jul 16, 2016

  2. sonoma

    sonoma
    Expand Collapse
    Sr. Member

    Joined:
    Sep 14, 2005
    Messages:
    1,488
    Likes Received:
    504
    since you have no meter movement I would start by checking TR7 and D4 and D5. you did not say if you have any thing different when you turn the NB and ANL on and off. is the receive light green also.
     
  3. LeapFrog

    LeapFrog
    Expand Collapse
    χρυσοποιία

    Joined:
    Feb 15, 2016
    Messages:
    1,339
    Likes Received:
    430
    Tried NB & ANL made no difference, also on TX the meter moves alright!
    Going to check TR7 now,
    I pulled and replaced D4, D5, TR24, D1, D2, & TR17 from a good unit even though the originals tested ok.
    Phew I'm tired of shot-gunning !! (Never thought I would say that, pun intended)

    Oh & Yes the LED is green for RX and Red For TX, so I assume some switching is happening, the voltages at the FETS are off last I recall. I'm trying to start "at the beginning" of the Receiver, I mean from the coax on to trace where it fails.

    Going to do TR7 after this morning coffee, i'll post my results in a jiffy.
     
    #3 LeapFrog, Jul 16, 2016
    Last edited: Jul 16, 2016
  4. LeapFrog

    LeapFrog
    Expand Collapse
    χρυσοποιία

    Joined:
    Feb 15, 2016
    Messages:
    1,339
    Likes Received:
    430
    TR7 "swapped" same result.
    I have a faint static out of the speaker, internal or external irrespective of the locals transmitting.
    Still no meter movement.
    How can I confirm the PLL is not partially destroyed?
    (If it locks on transmit is it guaranteed to on receive?)
    Short of printing the voltage chart from the service manual and checking all points, i'm a little lost.

    I have an oscilloscope, but no test probes! :(
    It probably is a transistor or diode that I am overlooking.
    Should I record all the voltages and post them here?
    I'm not proficient enough yet to diagnose this just by the symptoms.
     
    #4 LeapFrog, Jul 16, 2016
    Last edited: Jul 16, 2016
  5. sonoma

    sonoma
    Expand Collapse
    Sr. Member

    Joined:
    Sep 14, 2005
    Messages:
    1,488
    Likes Received:
    504
    you will have to trace the receive parts to see which one is bad. here is the service manual and about the 3rd page it has a section that tells you what parts are used after TR7 which you checked earlier. I take it you do not have a signal tracer to see where you have audio at in the circuit. so all I can suggest is start with what the section tells you what parts do what for receive. I think you said you have a scope but no leads. you can also use a scope to trace the incoming signal and where it stops at. you will also need to check the freq in the receive section also. if a diode or trans is bad the mixer will not let the freq through to the next parts. I hate this kind of problem because a shorted radio can have more than 1 part bad. I had a trc-434 last year with a bad trans in the receive section and like to never found it.found 1 mixer freq not getting past the trans and was killing the receive. it would TX just fine just like yours.
    so good luck on your tracking it down. wish I could just tell you what part is out but not such luck. check C48 cap by the audio IC. if it gets zapped it can kill receive. it is in line to the audio transformer. while you are at C48 if it is not bad check the others right by it . a guy on another forum had one bad there causing him a problem. about 4 or 5 in that area and after he changed them he got RX back.

    look for a blown trace also and bad solder joints. a short can pop a trace especially around the audio IC.
    use a good magnifying glass to look for them.
     
    #5 sonoma, Jul 16, 2016
    Last edited: Jul 16, 2016
    LeapFrog likes this.
  6. LeapFrog

    LeapFrog
    Expand Collapse
    χρυσοποιία

    Joined:
    Feb 15, 2016
    Messages:
    1,339
    Likes Received:
    430
    Yeah it is unfortunate that I caused it, but I will be darned if I give up when I fix it I will gain the knowledge so it's not all bad (Forces me to learn)!
    I am glad you got the trc-434 sorted out, a few months ago I almost considered going crazy and replacing all the transistors but that won't teach me a thing about how troubleshooting works.

    I see TP3 mentioned in the service manual (and the thread on RT site) I still need to find it on the board, that's what I'm going to test next. I think 2 or 3 test points with the counter would be all I need to spot where the failure happened in the receiver, but i'm not sure.

    The meter isn't moving so I have a feeling the signal is not reaching the audio ic yet.
     
  7. LeapFrog

    LeapFrog
    Expand Collapse
    χρυσοποιία

    Joined:
    Feb 15, 2016
    Messages:
    1,339
    Likes Received:
    430
    Update:
    16.489 Mhz On TP3 (next to fet1)
    4.99 Volts on Pin 11 of PLL

    Also the thread has an incorrect title it should read Cobra 29 LTD no receive, but I cannot change it.
     
    #7 LeapFrog, Jul 17, 2016
    Last edited: Jul 17, 2016
  8. sonoma

    sonoma
    Expand Collapse
    Sr. Member

    Joined:
    Sep 14, 2005
    Messages:
    1,488
    Likes Received:
    504
    I do not know if you have this service manual so here it is. look on pages 1 to 11 and scroll down to the RX portion and start checking the parts it shows that need to function for RX. forgot to leave it for you on my last post.

    http://www.cbtricks.com/radios/cobra/29ltd_29ltd_st_29wx_st/index.htm
     
    LeapFrog likes this.
  9. loosecannon

    loosecannon
    Expand Collapse
    Sr. Member

    Joined:
    Mar 9, 2006
    Messages:
    2,693
    Likes Received:
    922
    leapfrog, not sure if it was already mentioned in the thread yet, but if you can transmit a carrier, then your VCO is locked.

    if the VCO is not locked, you will not see the RX TX light change.

    you have a "no receive" symptom, and you know that the meter is not lying to you.

    time to build an "rf probe" for your DMM (about three or four parts. find plans online) and start tracing the voltage through the circuit.

    I do not have time to type out that sequence for the next few days, but maybe someone else here can guide you down the receiver strip in order to find where you are losing your signal.

    the first thing i would do is try a tune up on the receive coils using the RF probe on the coil.
    when you find the coil that doesnt change when turning it, you've found your problem area.
    BTW that thread that you found is one of the best tutorials you will find.
    read it through and through and over and over and you will learn alot.
    LC
     
    LeapFrog likes this.
  10. LeapFrog

    LeapFrog
    Expand Collapse
    χρυσοποιία

    Joined:
    Feb 15, 2016
    Messages:
    1,339
    Likes Received:
    430
    I appreciate you confirming the VCO is staying on frequency & PLL Locking, the bit about the LED switching is good to know also. (y)

    I am Building the RF Probe now, just locating 1N34A Germanium diode. :cool:

    On a side note can anyone please explain why two part numbers are listed for Fet 1 & Fet 2, & will a standard 2SK192A work in place for both location? I cannot find a datasheet for anything other than the 2SK192A.
    Cobra 29 LTD Fets Thank You Loosecannon.

    All the best.
    -Sam (aka LeapFrog)
     
    #10 LeapFrog, Jul 20, 2016
    Last edited: Jul 20, 2016
  11. loosecannon

    loosecannon
    Expand Collapse
    Sr. Member

    Joined:
    Mar 9, 2006
    Messages:
    2,693
    Likes Received:
    922
    you can use a 1N60 diode in place of a 1n34. you should have these in any parts radios you have.
    they will be clear glass with a colored band on one end.

    as for the FET, the difference in the device nomenclature has to do with drain to source leakage current. (the current that flows when the switch is off)

    the Y is 3 to 7mA.

    the BL is 12 to 24mA.

    i honestly don't know how much difference this will make in your receiver performance, but i will say this:
    Manufacturers NEVER buy two different parts when just one will do both jobs.
    they get a better deal on parts the more they buy, so it would not be in their best interest to use a different suffix of a certain part if either would work.

    So, I would personally use the original, and the only place you are going to find one is in another 29LTD.

    the good news here is that you can probably buy a 29LTD for cheaper than the parts would cost you if you could find them.


    Did this radio's TX go out on you while you were tuning it?
    or did it go out in the middle of talking on it?

    the reason i ask is because there are a couple of tuning cans in the TX section that will drop the TX to nothing if they are mistuned, giving you the false impression that the TX went out while you were tuning.
    LC
     
    LeapFrog likes this.
  12. LeapFrog

    LeapFrog
    Expand Collapse
    χρυσοποιία

    Joined:
    Feb 15, 2016
    Messages:
    1,339
    Likes Received:
    430
    Honestly I mosfet swapped it, then tip120 variable carrier, everything was working , I decided that I didn't need the variable anymore so I went to pull it out long story short I still had the power supply connected and the radio switched on. I drop the pot inside and shorted something then the receiver whent quite.
    Yes as incredibly dumb as that sounds it is the truth I was pulling a component out while the machine was on and running, obviously I didn't have my sharpest wits about me at that particular point in time.
    ( I also had to add some capacitance to increase the carrier and i followed everything in the pdf for the mosfet conversion )

    The radio transmits fine.
    I never fiddled with the 54 megahertz trap, never touched anything in the vco, I never adjusted any of the metal tuning cans, I did use a proper ekl component part and a vishay irf520n I did remove one turn off the inductor by the final and then put the slug back in it after it was reinstalled.
    But yeah like I said the radio seems to switch fine between transmit and receive. TX works great, but no receiver.

    LC i appreciate the tip on the component designation, I'll have to look for the originals, and I'm still waiting on my component tester so I can actually test the transistors in question.

    I saw an RF Probe on Amazon for 15 bucks they say it will work with a digital multi-meter so I may just order one of those. if I get desperate I'll build one I'm just tired of feeling like I'm half-assing everything because I don't have proper test equipment but when I can save some money and build something reliable I tend to do that.

    All the best.
    -Sam
     
    #12 LeapFrog, Jul 22, 2016
    Last edited: Jul 22, 2016
  13. loosecannon

    loosecannon
    Expand Collapse
    Sr. Member

    Joined:
    Mar 9, 2006
    Messages:
    2,693
    Likes Received:
    922
    i would not start with those mosfets if i were looking for culprits.

    the most sensitive parts in any radio are the semiconductors, so we do want to start there.

    since you have transmit audio, we are going to assume for a second that the audio chip is ok.

    the most obvious place to start looking for a bad part is in the region of the radio that you dropped the pot on.

    if you don't already know how, learn how to test a transistor using your DMM. there are tutorials all over the internet and it's easy.
    you must remove the transistor from the PC board before you can accurately test it.

    start with TR6, then TR7, then FET1, FET2, TR8, TR9, and TR10.
    if you don't find anything amiss there, check D4 and D6.


    let us know what you find.
    LC
     
    LeapFrog likes this.
  14. LeapFrog

    LeapFrog
    Expand Collapse
    χρυσοποιία

    Joined:
    Feb 15, 2016
    Messages:
    1,339
    Likes Received:
    430
    Is a VTVM prefered for alignment, & how about for use with the RF probe?
    I am off to the internets to see about testing transistors with a basic dmm!

    Aaannnddd here is where I confess to needing proper desoldering braid or even a handheld pump..
    I mangaed to lift I a trace off the board, good God another noobish mistake!

    Thanks LC
    All the best.
    -Sam
     
    #14 LeapFrog, Jul 22, 2016
    Last edited: Jul 22, 2016

: PLL, Cobra, no receive

Share This Page