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148 gtl keys up but no transmit.

Discussion in 'General CB Services Discussion' started by 776 Long Island, Jan 7, 2017.

  1. 776 Long Island

    776 Long Island
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    Hey guys, This is my first post in here.
    I have an older (March 89) cobra 148 gtl. I've been having problems with it and I'm hoping I can get some help here. When I first aquired this radio it turned on with no transmit and no recive. After messig with it for a while I found the cb pa switch was bad. If you wiggle it around it will revive just fine but, when I try to transmit the radio keys up and the light switches but there is no audio being transmitted. The radio does break the squelch on reciving radios but no audio. Could this be part of the cb pa switch? Can I just jump it out? I've checked the mic and that is fine as far as wiring goes. Any ideas would be appreciated. Thanks.


     

  2. Robb

    Robb
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    Yup

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    If you hear receive audio; then it is not the audio chip per se.
    Does the radios RF meter show power output in AM mode?

    Troubleshooting is the art and science of eliminating possibilities before you can arrive at the point of solution. Don't remember who said it; works for me. Sounds like you are in there and getting it. Don't quit!

    Jump out the PA switch.
    BTW does the PA have ausio output to an external speaker?
     
  3. 776 Long Island

    776 Long Island
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    I get no audio on the pa side either. I'll try jumping out the pa switch, it's used as a base any way
     
  4. Robb

    Robb
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    Yup

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    If it were my radio, I would spray some cleaner into the switch rather than jumping it out. Jumping it out is a troubleshooting thing and should be restored.

    You may have a bad audio chip, or some of the electrolytic caps have dried out and failed. Either one is possible.
     
  5. Crossbow

    Crossbow
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    Robb likes this.
  6. 776 Long Island

    776 Long Island
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    Ok jumped out the pa switch and no difference. dont know where to go from here. How can I tell if the element in the mic is good? I've got continuity from the switch to the pin on all 5 and the switch is good. I don't know if there is anything else to check? In the mic or in general.
     
  7. 776 Long Island

    776 Long Island
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    How do I test the caps and what ones?
     
  8. M0GVZ

    M0GVZ
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    You can see when they're faulty. It is the electrolytic ones you want to be looking at, the round cylindrical ones usually installed vertically. If there is any sign of any gloop around the bottom of it its faulty. If the top of it isn't perfectly flat and is even slightly curved or there are signs of gloop on the top of it it is faulty. Any signs of even the slightest swelling on the top or the smallest bit of residue shows that there has been a catastrophic internal failure which has resulted in a build up of pressure.

    If it has leaked out of the bottom onto the circuit board you need to clean it all off and inspect carefully as it is corrosive.

    This is an article referring to a problem with computer motherboards but its apt for what we're discussing:

    https://en.wikipedia.org/wiki/Capacitor_plague#Visible_symptoms
     
  9. sonoma

    sonoma
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    I would at least look at all the 10 volt caps in the radio.they normally go 1st and kill the 8 volt rail in the 148 radios. have you checked at the wires to the board for the mode switch they should have 8 volts on them when you go to each mode such as usb,lsb, or am modes. one wire is the common 8 volt wire that feeds the switch. put 16 volt or higher in place of the 10 volt caps. those 10 volt are the weak link to a 148 radio.you do not want to just check the caps unless you have the proper test equipment to test each cap. just put new higher rated ones in the old ones spot. do not use old used caps from another radio. if you want to do it right do what needs to be done. i would just put all new ones in the radio to make sure they are good then try to trouble shoot the radio.the 25 and 50 volt caps can use the same voltage that are in each spot.
     
  10. 776 Long Island

    776 Long Island
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    Thanks guys. Seeing as how I don't have the equipment to test looks like I'll be dropping it off to my local guy and let him have a go at it. Thanks again
     
  11. loosecannon

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    many times what happens to these switches is that the switch on the faceplate gets banged against something, and that causes the back of the switch inside the radio to push out. when this happens, the little shorting bar inside the switch becomes unseated and doesn't make good contact, causing the switch to be intermittent.

    776 Long Island- im going to give you the wiring that should eliminate the switch and leave the radio in CB mode.

    i know you said that you already tried to jump the switch, but some of these slide switches work in the opposite direction than you would think, so try what i tell you here anyway.

    first- there is a wire going from pin 3 on the mic jack to the CB/PA switch. remove the end of this wire from the switch. there is another wire that goes from the emitter of TR35 to the CB/PA switch. remove this wire from the switch and solder it to the free end of the other wire. you should now have a wire that goes from pin 3 of the mic jack to the emitter of TR35. you can leave the other wire in place on the switch as it now does nothing.

    second- there is a wire coming from the external speaker jack that is also connected to C166 and C168. follow this wire to the CB/PA switch and remove it from the switch.

    now find the wire that goes from the negative terminals of C174 and C175 and goes to the CB/PA switch. remove this wire from the switch and solder it to the free end of the wire that goes to the external speaker jack.
    you should now have a wire that goes from the external speaker jack to the negative terminals of C174 and C175. again, you can leave the remaining wire on the switch as it does nothing. be sure to insulate these connections.

    now test the radio. if you do not have receive and transmit audio then you have other problems besides the switch itself.

    best of luck.
    LC
     
  12. 776 Long Island

    776 Long Island
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    Got it done and still no transmit. Looks like I'm going to have to send it out to get the caps done.
     
  13. loosecannon

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    do you have transmit audio on SSB?
    LC
     
  14. 776 Long Island

    776 Long Island
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    Nothing. It breaks squech but no audio
     
  15. nomadradio

    nomadradio
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    If you're lucky, its C95, a 47uf 10 Volt electrolytic just to the rear of the AN612 balanced modulator chip. A 100-ohm resistor R108 right next to it may look a bit roasted if this capacitor shorts.

    This will kill ALL transmit power, AM and SSB.

    If you hear anything in the radio you're using to monitor it, the problem is probably farther downstream from this chip.

    A 'scope is your friend here, following the signal stage by stage from what comes out of this chip downstream to the driver and final.

    Never assume that there is ONLY ONE thing wrong with a 27 year-old radio. A shorted C109, 47uf at 10 Volts will kill all your mike audio, SSB, AM and PA all three.

    Easy enough to check. Unsolder one leg of the cap, and see if it works any better. Might have feedback trouble this way, but if it comes back from the dead, you have found a shorted cap.

    A magic screwdriver is worth considering, if you don't know where this radio has been since the last time it worked. Watch the S-meter on the monitor radio, transmit on AM, and see if it shows a peak on the slugs of L44, L46, L47 and L48. Adjusting the slug away from the peak of any or all of these will kill transmit power.

    If you see a peak on the receiver's S-meter, that coil is not contributing to your fault.

    73
     
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