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.
    Dismiss Notice

Cobra 148gtl Basket case!

Discussion in 'General CB Services Discussion' started by Two Pack, Apr 29, 2018.

  1. Two Pack

    Two Pack Active Member

    Joined:
    Mar 21, 2016
    Messages:
    8
    Likes Received:
    0
    So as a new tech I am still learning to repair radios. I am working on a 148gtl side mic that was not functioning when I got it.



    The pll was out of lock and I traced it back to the 11.325 crystal. I was getting no frequency from it all. Replaced it and pll locks!

    Moving on with the alignment I get to where I check ic3 pin 7 on Christmas 19am for 7.8mhz, radio is dead on 7.8. Check either side band for offset and frequency fluctuates between 5mhz and 11mhz.

    I am testing with a 30db wideband, now noise rf amplifier (can be seen in a video by mikesradiorepair) in line to my counter and a 10x probe.

    Also I get a humming/buzz on recieve and transmit on am. The radio keys 4watts and swings to 10watts on am. Receive is dead quiet unless I am transmitting from another radio on the same antenna switch, no static at all.

    I just got it to this point so I haven't gotten too far into diagnosing these issues yet, but t hopefully I can learn something from this post!

    Edit: forgot to add, this radio is bone stock and clean inside!!
     

  2. Robb

    Robb Yup

    Joined:
    Dec 18, 2008
    Messages:
    10,679
    Likes Received:
    2,350
    What about SSB?
     
  3. kaos513

    kaos513 Well-Known Member

    Joined:
    Apr 10, 2014
    Messages:
    1,364
    Likes Received:
    236
    Is the radio recapped?
     
  4. Two Pack

    Two Pack Active Member

    Joined:
    Mar 21, 2016
    Messages:
    8
    Likes Received:
    0
    Ssb receive sounds okishbut I can not clarify in 100 percent. Tx I can not clarify in at all and has some pretty weird background noises in it.

    The radio has not been recapped yet, I'm gathering a list as I have about 5 radios to do!
     
  5. Handy Andy

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

    Joined:
    Apr 23, 2018
    Messages:
    677
    Likes Received:
    409
    A "Side Mic" 148GTL, that's pristine too...

    To some it's a great find - others not so much because it may not be worth much until the details and the squeals get worked out.

    Instance, the Mic, the jack itself can corrode with age, oxides gt inbetween the PCB board and the case - so you're getting "strange sounds" when you key up usualy tells me that the 5-pin wire - there is a poor ground to the case.

    Considering the age, it may need more tlc - how does the thing do when you let it warm up for say...1/2 hour?

    Clean inside tells me that this radio may have been a purchase and never used. That's where much of the problem lies in...the older parts need some excercize to start working right.

    I'd let is go, power up - for 1/2 hour and see if some of your first posts' symptoms correct themselves or disappear entirely - then it tells me that age is a big factor. Dew and drying out - most radios off the line don't always "fire up" right off the bat They need a tweak or two and then they are determined working stiffs or duds for recycling.)

    Don't overthink this, Cobra 148's are like all other manufactured products - they will need "tweaks" after all those years of being powered off...

    So,...you're not alone and neither are your patients' symptoms...

    It can also be from loose case screws and hardware that needs to have it's seal (like front panel case to side panel screws, Mic plug ring - rear SO239 and even board to case - all need to be cracked - backed out - then retightened to see if the board ground and foil ground can start working together again - Retaining Mic Plug Ring may need loosen and hen retightened - any loose corrosion or open oxidizing needs to be cleaned up - board traces may need reheating - especially to the externals - like switch controls, antenna and speaker - power supply (UNIDEN 3-pin power connector to tab - case to board tabs - n' such.)

    It's a lot of work, but it can pay off if you take your time with it.

    Regards!
    :+> Andy <+:
     
    Two Pack likes this.
  6. Two Pack

    Two Pack Active Member

    Joined:
    Mar 21, 2016
    Messages:
    8
    Likes Received:
    0
    Thanks Andy! The radio has wear on the outside but the inside is clean! It was given to me for a "parts" radio for one I repaired that needed a mode switch so the price was right! It's too close to working for me to part out.

    Ive had it on for a few hours while I was diagnosing the 11mhz crysyal and for checking voltages against the other radio.

    What's interesting is the factory 5 pin is 180° out. Upside down, but the flat spot for alignment is in the right spot.

    I am getting an unstable frequency at pin 7 of ic3 during alignment on ssb only. Am is rock solid.

    Can't wait to get it up and running, it can take place of my 2000 while I recap it. I've got time I have another 2000 I've been working on for over a year and a galaxy ssb echo someone dropped a metal screwdriver in!
     
  7. Handy Andy

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

    Joined:
    Apr 23, 2018
    Messages:
    677
    Likes Received:
    409
    Greetings'!

    Good to hear that ...

    Reply internal to quote...:

    Have you recapped that radio yet?

    At this moment, when you check SSB mode, and the unstable - remember that the AN612 will need audio to produce anything on Pin 7 - it can produce a carrier - but between it and the TX ...lies a delicate mess...

    if the FT3 can filter is damaged or one of the Xtals has failed you may indicate some type of failure because of the unusable or bad xtal in that shrink-wrapped package...


    That may not be the most likely case because stability goes back to the PLL loop let alone the 7.8MHz...

    In thinking thru, I'd look back at IC2 TP9/10 and also TP3 - by the primary Conversion for the SSB output by sending a tone or a carrier thru RF from another radio can make the conversion section [TP3] produce some output.

    If RX needs work; fix that first because it may clear up the other issues of the TX side...

    RX first, lot less damaging - then fix TX side after you iron out the RX. If you can see if it's just TX'ing that is not stable {from AN612 thru the FT3] you still need to see if RX stays stable - so RX first...

    Why? If the issue of; is it RX and TX together meaning PLL issues and IC2 is involved too....

    Then to see if the stability in RX is similar - have the Monitor radio produce an AM carrier and you de-tune the repair radios' Voicelock and listen to the tone produced in the radio you're repairing - and don't be suprized to see the PLL and IC2 at TP 9/10 might be affected

    To see TX stability - see if upsetting the Carrier Balance using VR4 to produce a carrier on SSB can provide a "tone" thru a monitor radio as you tap and see if there are intermittent problem in the SSB in TX mode - reset VR4 when done with this step

    Continuing...

    Precaution here:

    The nature of the mic jack beast...You're describing what may be the original problem that this radio donated it's life for. A simple mic jack repair seems simple enough but when someone has to replace the original with something not quite stock is only inviting trouble.

    UNIDEN Grant XL and COBRA 148 uses the same jack - so if they replaced the jack with non-oem, they may have reassembled it wrong not realizing that a ground lug ring from the case is also needed - required due to the Hot audio ground and it's ability to operate in Positive and Negative Ground vehicles at that time - the lug mounted internally and it's lug soldered to a spot on the board that was RF grounding (Chassis Ground for the CASE not Foil Board ground as found via Uniden Power pin Negative) the jack back to the board. - and was separate from the Audio Return on the jacks Pin 2 with Audio being Pin 1
    Back to you...

    Here's what I got for FT3 ...

    Cobra148GTLFT3LAYOUT.jpg

    Here's to help you with that 5-pin Mic wiring...

    5-pinidentificationID.jpg

    This is for anyone to use, not just this moment in conversation - give credit where credit is due - to the respective authors...

    Regards!
    :+> Andy <+:
    QUIK redo of the 5-pin - less confusing
     
    #7 Handy Andy, Apr 29, 2018
    Last edited: Apr 29, 2018
    Two Pack and Shadetree Mechanic like this.
  8. Handy Andy

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

    Joined:
    Apr 23, 2018
    Messages:
    677
    Likes Received:
    409
    Up there, in that quote... There's my reply embedded in your quote - SIGH - learn this sites way to doing things as I go...
     
  9. loosecannon

    loosecannon Sr. Member

    Joined:
    Mar 9, 2006
    Messages:
    2,958
    Likes Received:
    1,194
    two pack,

    while im sure you know that our first tip is always going to be to re-cap the radio before you start trying to nail down problems when a radio is that old; your fluctuating frequency on SSB may or may not be due to an old leaky cap.

    try this:

    align the VCO cans for their 34mhz set points, then connect the freq counter to the output of the radio (don't key a radio directly into a freq counter without appropriate attenuation. i usually just pull TP7 and TP8 so there won't be any real power output, but there will be enough signal to trigger the counter)

    set the radio for channel 20 AM mode. mic gain down.
    key the mic and adjust the AM IF can for 27.20500mhz. if you do the math subtracting 27.2050 from the 34mhz freq of the VCO, you will get 7.8000 mhz.

    now do the same on USB and LSB, but for the SSBs you have to inject a known accurate 1khz tone into the mic since there is no carrier for the freq counter to read.

    on channel 20, LSB, you tune the LSB IF can for 27.20400mhz.
    on USB tune the USB IF trimmer for 27.20600mhz.
    27.20500 + or - 1khz gives you those numbers.

    if the radio will align this way then you know its just a freq counter sensitivity issue or something like that. if they won't align this way then you know you have an IF problem.

    you still need to re-cap that radio, but if you just want to see what kind of problem you are actually experiencing then this is a way to do that.
    LC
     
    #9 loosecannon, Apr 30, 2018
    Last edited: Apr 30, 2018
    unit_399 and Two Pack like this.
  10. Two Pack

    Two Pack Active Member

    Joined:
    Mar 21, 2016
    Messages:
    8
    Likes Received:
    0
    I'm having trouble aligning on ssb, and I think it is because of the squeal on the audio line.

    I've checked the mic wiring and mic jack backwards and forwards and it all appears right, and clean. I've checked the voltages of about every transisor on the board as well as ics and still no smoking gun.

    I've even checked the electrolytics on the audio circuit and they all test at or slightly above capacatance with 3% or lower voltage loss...

    Scratching my head at this one...
     
  11. kaos513

    kaos513 Well-Known Member

    Joined:
    Apr 10, 2014
    Messages:
    1,364
    Likes Received:
    236
    check for any bad solder joints
     
  12. Handy Andy

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

    Joined:
    Apr 23, 2018
    Messages:
    677
    Likes Received:
    409
    Remember the Pin 1 and 2 Audio and Shield, respectively, have their own routing.

    Pins 3, 4 and 5 are separate routings - Pin 4 is grounding for Speaker on Pin 3 and MB3756 Control line (toggles TX and RX) on Pin 5 - Pin 4 grounds between them.

    So, if a squeal, check mic wires for proper routing.

    Check Speaker for shorts or "mods" like loose speaker wires even speaker jack with a corroded contact or a "ring" shorting to case due to assembly - can cause squeals.

    Did you check for loose case to board screws? As in; Back out and retighten?

    In brand new NOS radios these issues with squealing, solder does oxidize. So - I do it more often - repeatedly at times - and even re-flow solder around the screw-holes even rear panel bonding lugs are for-ever prone to oxides.

    As with age, loose hardware and possible dirty switch contacts even the CB an PA switch - you'll need to verify ALL operation.

    Some people rig up a mic that's universal and use it to test radios - that's fine.

    Just remember that even stock 5-pin wiring is touchy on radios like this.

    However, if the testing handsets' not stock, and you haven't even checked PA - you may not have yet discovered the issues that are making a known symptom. Is it due to mic wiring or loom-routing or even those bad caps including the Audio IN caps?

    Electrolytic's are main suspects - tantalums or not - will add shorts and whacky operation to your list of things...

    Regards!
    :+> Andy <+:
     

Share This Page

  • About Us

    The WorldwideDX Radio Forum was originally established in 2001. We pride ourselves on welcoming Radio Hobby enthusiasts of all types, while offering unbiased, informative, and friendly discussion among the members. We are working every day to make sure our community is the best Radio Hobbyist's site.
  • Like us on Facebook

  • Premium VIP Member

    The management works very hard to make sure the community is running the best software, best designs, and all the other bells and whistles. Care to buy us a beer? We'd really appreciate it!

    Donate to us!