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Cobra 148tl no am tx

Discussion in 'General CB Services Discussion' started by red97k1500, Mar 10, 2019.

  1. red97k1500

    red97k1500 Active Member

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    I recently purchased a Taiwan made cobra 148 gtl.From a member here that sold it to me for shipping cost, which I really appreciate. It apparently had been hooked up backwards at some point in its life.The hash chokes were burned up, and the protection diode blown. I got that fixed, and changed a 4.7 uf electrolytic cap that was corroded by the brown glue. Then fired it up and cautiously have done some testing on it.

    The radio has full output power on ssb, with excellent sounding audio,it is even on frequency. The radio does have receive,but the green receive led is not light,,,but it does change to red on tx. The radio does not apear to have any mods that I can see, channel, clarifier etc even the limiters appear intact. The only thing wrong with it is there is no carrier on am, no modulation no nothing. The the light does turn red, but I get nothing on am. I do plan on doing a full recap just have not had time yet. What can cuase this on a 148 gtl? Any advice is very much appreciated I will not give up on this radio lol.
    It will never end up in the scrap pile.
    Thank you


     

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  2. 338_MtRushmore

    338_MtRushmore Sr. Member

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    loosecannon likes this.
  3. red97k1500

    red97k1500 Active Member

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    Thank you, I hadn’t seen that thread, I will check it out.
     
  4. nomadradio

    nomadradio Analog Retentive

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    The AM modulator transistor in a Taiwan 148 was probably a 2SC1419 from the factory.

    The list of parts that will replace it is pretty long. We always replace the 2SC945 transistor that drives it, just in case it got stressed when the bigger part broke down.

    The part in question is bolted to the side rail, at the very rear near the power socket. Only radios from the late 90s and onward have it bolted to the rear panel.

    We use a TIP41 transistor to replace it. It's rated for about twice the current of the original part. Lots of other types will do the job just fine.

    73
     
  5. NODENS

    NODENS New Member

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    Did you check carrier oscillator area? Have different coils/diods for am/usb/lsb. Can be in that area too.
     
  6. red97k1500

    red97k1500 Active Member

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    Not yet, I ordered some transistors today and some nylon screws, and some thermal paste. Once the parts arrive, and I get a chance to mess with it, I will report back.
    Thank you to everyone for the good advice.
     
  7. 338_MtRushmore

    338_MtRushmore Sr. Member

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    You aren't just throwing parts at it, are you?
     
  8. red97k1500

    red97k1500 Active Member

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    I’m going to throw an am regulator at it. But other than that no. By going ahead and ordering it, it will be here by the weekend when I will actually have the time to mess with it. So if it is indeed bad I will already have it.
     
  9. nomadradio

    nomadradio Analog Retentive

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    Easy enough to check if you have a DC voltmeter.

    Ground the black lead to the radio's negative power supply lead. Touch the positive probe to the metal tab on the final transistor. Just don't cause a bridge to the head of the mount screw. Key the mike in SSB mode with the mike gain turned all the way down. Should read around 13 Volts DC.

    Now select AM and key the mike. If the reading falls to zero, the AM modulator transistor is likely the culprit. If the reading holds around 6 Volts DC, then the fault lies elsewhere.

    73
     
    Robalo likes this.
  10. red97k1500

    red97k1500 Active Member

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    Ok I finally got a chance to carry out the tests u described nomadradio. In side band I get about 13.6 with the mic keyed, in am I get 0 volts with mic keyed. And about 3 volts with the mic unkeyed. So it looks like the am transistor could indeed be bad?
     
  11. nomadradio

    nomadradio Analog Retentive

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    Yep.

    73
     
  12. kopcicle

    kopcicle Sr. Member

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    TIP41
     
  13. red97k1500

    red97k1500 Active Member

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    I replaced the c1419 with a tip41, then tried it and still no am. So I replaced tr42 with another c945 and still the exact same voltage readings, with no am tx. I am sure that I did not create any solder bridges, and the tip41 tab is not shorting against the chassis.
     
  14. nomadradio

    nomadradio Analog Retentive

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    Check voltage at the wiper pin of the carrier-set trimpot, the pin towards the rear of the radio. Worst-case is that a shorted TR42 could damage the trimpot. Should be able to get 4 or 5 Volts on the trimpot's wiper (output) terminal with it set around middle of its range.

    73
     
    red97k1500 likes this.
  15. red97k1500

    red97k1500 Active Member

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    I figured out why am didn’t work.what I originally thought was a superficial scratch had actually cut two traces and might even be a crack in the board. so I thought no problem I’ll just remove the solder mask on the traces and dab a little bit of solder on them and get continuity again, it worked but now I have a short to ground. I still measure about 68 ohms from after where I made the last cut in the the trace, and removed the solder mask to any board ground this is driving me crazy. I hope this makes sense. It’s the trace that jp1 connects too. I guess the board might be no good? See second picture
     

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