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Wanted: 1 Uniden recieve guru...

Discussion in 'CB Radio Modifications' started by Tokin, Sep 6, 2020.

  1. Tokin

    Tokin 42Zero Tampa

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    I picked up a pc78 that someone couldnt get the "hifi" recieve mod right on. He said the radio has an audio chip hes not familiar with. I get a nasty buzz/hum when the rf gain is turned down and an high pitched whistle when its turned up.
    Id like to have the recieve put back to stock. I dont care for all the highs the hifi mod brings out. Maybe just stroke the stock recieve a little.
    Im no tech but workmanship looks ok and i got great audio reports. It doesn't look botched, its just not right. Im looking for the right tech. You guys rock, thank you! 20200906_093949_HDR.jpg 20200906_094104_HDR.jpg


     

  2. Handy Andy

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

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    Sigh, I don't know where to begin...

    Firstly - this things has a Talkback "Mod" and although the photo shows the rework, the focus is not the best - there are enough details though to say, that the Talkback mod wires are over areas that generate and use IF - so this may be more of a you need to pull-out/remove the talkback mod to see if this can clear up your messy audio and squealing issues.

    This mod, has got to go, you need to redo to stock, then redo to get it what you want...
    upload_2020-9-9_9-50-4.png
    This is another area, there are mods done on the top side of the board, including an install of what looks to be either echo, or a PLL mod - so these RF TX-strip caps are not helping your situation...
    upload_2020-9-9_9-52-47.png
    They had issues -
    because the OEM did not install a 52MHz trap filter for the 2nd harmonic
    so you have been warned...
    upload_2020-9-9_9-59-35.png

    Aboveboard , on top, is a Variable mod that although looks clean,
    you'll need to remove this,
    You can reinstall a better set of values to offer a better set of effects to your audio.
    I cannot' determine if this mod may have opened a pot on the front panel?
    If it did, this generates another headache...
    It may sound good, but seriously?
    You're posting this so work with me here...

    upload_2020-9-9_9-56-42.png

    One more mod,
    You have that "slide into the channel (or Foray mess)"
    It may need to go...
    upload_2020-9-9_10-2-10.png
    This is your "slide in" it looks to be on a switch to the front
    panel so again, work with me...​

    You also have one last mess with this - and it deals with the Audio amp chip - you have what I've seen Uniden do to their PC-68's when they lost their last '7222AP supplier of chips...
    This is a newer 7-pin Audio amp chip...
    upload_2020-9-9_10-10-29.png

    This goes back to a PC-68 write-up I did way back in 2017-2018 thereabouts...
    PC68LTWAudioAmp_TA7257.png


    You can find that write up over another type of Audio Chip and the PDF for it...
    https://www.worldwidedx.com/threads/cobra-148-gtl-china.231704/#post-631640

    With the above being said...

    Look closely - someone added a jumper to a spot that didn't really need a jumper - for they've (OEM did) installed the proper jumper already on the TOP side of the board - so don't know why they added this in...

    upload_2020-9-9_10-8-2.png

    This spot, is a main grounding point for ground in the radio and the wiring is for reduced path inductance that would make or generate, noisy ground planes elsewhere in the radio.
    So the wires you see routing to here, go to the big filter and ripple cap that the Audio amp chip uses to clean up ripple and noise from it's main power supply feed. - so to see a wire jumper added in AFTER the wire was already installed on the top, makes me wonder why...​


    This is all I've got for now...Good Luck!
     
  3. 9C1Driver

    9C1Driver Sr. Member

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    I wish people would just let a nice 11m radio be what it was supposed to be. Give it a nice clean tune-up, maybe add a power mic or desk mic and be done. You see it here from time to time someone wanting to do all these "hifi" mods to a cb. When done the radio has this monotone drone sound to it. Then it gets dumped on e-bay or becomes a truck stop trade-in. If want to sound like a ham rig go buy a ham rig. Good luck getting that one back in order, looks like a headache.
     
    Tokin, NZ8N and Dmans like this.
  4. Tokin

    Tokin 42Zero Tampa

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    I don't like the hifi rx/tx either but thats what im working with.
    This is cb radio and i play in a lot of different cb radio "sand boxes". Some friendly and some not so friendly depending upon my mood or need for entertainment. I have a variety of radios, toys and amps for the different sand boxes.
    This radio does just what i bought it for except the recieve isnt what it should be. The irf520 mod seems to work fine and the variable works fine also. The vfo slide in is a little long, bordering on obnoxious which is just what i wanted to cut thru the channel 19 crap.
    All audio reports with this Uniden are the same as my 29, loud and very clear. I have few complaints with the radio, just the rf gain/recieve issue.
    If someone can straighten out the issues im having feel free to pm me as im a good paying customer for quality repairs and reworks.
     
  5. Handy Andy

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

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    Well, as long as you like the SOUND out of that radio - sorry to bore you...though the "squeals" you'd need to fix were in specific areas they modded to obtain more audio out of the Mic element and in doing so - can make the RX side seem too touchy.

    I'll tell you what, I came across some older Pics back in the CB Tricks day of the PC-78 that had a similar Audio Chip

    To help, I also have ORIGNAL TA7222AP style 78' photos too.

    The radio you have looks like a lot of work was done around the Receive area near the front of the board - so even the DSC (Dynamic-Squelch-Control) Circuit area is affected.

    It's why I'm supplying them, many are ok with the DSC being installed, just not thrilled when the RX side dies from when it gets damaged.

    It is my hope you can do a side-by-side comparison and make some choices on the type of work you want completed - then any tech can do the rest.

    I wish I could help - but XYL has got me on projects since she's seen the "Winter storm" in Colorado - so she's paranoid and pushing me to get these outside yard projects done. - I'm Booked -
     

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  6. Tokin

    Tokin 42Zero Tampa

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    No worries i understand and thank you for all the information as it will come in handy. You are never a bore to me.
    The main issue is when the rf gain is turned down there is a buzz/hum. The only position i have recieve is from 3-5 oclock. Anything lower is an issue. Im sure it has something to do with the hifi rx mod that wasnt done properly. Im sure ill find someone here to put the recieve back to stock/straightened out.
    I appreciate everyones imput, thank you!
     
    Handy Andy likes this.
  7. Tokin

    Tokin 42Zero Tampa

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    20200917_172731.jpg 20200917_172612.jpg Thank you Andy for pointing out the blurry pics. Turns out something in my pocket broke my phones camera lens and back glass.
    Heres a better pic i hope.
     
  8. Tokin

    Tokin 42Zero Tampa

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    Ummm, i dont have a trained eye but wth is this? Screenshot_2020-09-17-17-55-03.png
     
  9. Handy Andy

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

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    Q18 is the Limiter - note how they put the "ferrite bead" instead of it on the Transistor legm they put it on the board.

    The numbering is different because of the different audio chip and a feature they've been using with TDA2005 and subsequent revisions.

    In your board, look at the Audio Chip - Note Q13 - that's what this is similar to...

    upload_2020-9-17_19-15-57.png
    If you remember Galaxy using the 2nd half of the 4558 IC for "Power good"
    This circuit is similar to it.​

    JMO - this circuit started showing up in Unidens after they started to use the different Audio chips including the 7-pin one you have on your board. It simply is a "kill switch" designed to turn off the audio amp if something went wrong.

    President line of radios (under Uniden Umbrella of companies) Quixang (sp?) and others are using this method to "stop audio" if things go wrong - it stops the feedback / bootstrap and the amp shuts down. A fail safe - so to speak.

    This affects both Receive and Transmit and Weather functions - note the 8 volt line, it is a lot like the 148's and Grants "Power good" Pin 6 line from the PLL - as long as it was "locked" and Pin 6 was high, the radio worked normally - similar in function, you have it as TR13.

    upload_2020-9-17_19-24-6.png
    This region also has issues with noise and squeals, the amp isn't at fault but the support parts are not helping the matters any.

    Thanks for those better pics, they do help!
     
    Tokin likes this.
  10. Tokin

    Tokin 42Zero Tampa

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    I was looking more at the puddle of what looks like wax.
     
  11. Handy Andy

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

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    Thats the goo -Epoxy, that is holding the Ferrite to the lead - looks like something else , but yes, that is for assembly. for wave flow - the epoxy was still curing as they did a final press down
     
    #11 Handy Andy, Sep 18, 2020
    Last edited: Sep 18, 2020
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  12. Tokin

    Tokin 42Zero Tampa

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    That looks sloppy as hell hahaha.
     
  13. AudioShockwav

    AudioShockwav Extraterrestrial Admin
    Staff Member

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    I hate sonybond.
    Let me explain, I hate that glue that drys out after years on the board and becomes conducive and can eat leads away.
    It has caused way to many old uniden radios to fail.



    73
    Jeff
     
    #13 AudioShockwav, Sep 18, 2020
    Last edited: Sep 18, 2020
    Tokin and tecnicoloco like this.
  14. Handy Andy

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

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    Which is surprising to see they still used it on later boards.

    Guess they bought out that supplier.- or have no other way to get rid of it.
     
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