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Cobra 138XLR

Discussion in 'General CB Services Discussion' started by unit435, Nov 18, 2021.

  1. unit435

    unit435 New Member

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    DSC_0948.JPG DSC_0949.JPG Hey everyone out there. New member here. I have a Cobra138XLR that I bought new in the late 70s. I heavily modified it shortly after purchase. 120+ channels, Add/subtract switch, open clarifier, peaked power spread through the channels.

    I did the mods guided by a good friend who past away several years ago. The radio was used quite a bit up till about 1995 when it suddenly stopped working. The display lights up but no power out and no receive. I have just pulled it out of the closet and would like to repair it.

    It lights up and the relay kicks in when the mic is keyed. It has no power on am and I believe the same condition on sideband but I did not test that today to be sure. There is no receive either.

    I am very good at soldering and very mechanical. I need some guidance to a starting point for testing. I have a meter and a dumby load. My freq counter was loaned out many years ago and never returned.



    Any help would be great. The radio is worth fixing. All of the modification switches are imbedded to standard front panel locations with the stock knobs. The unit looks completely stock until you try to turn the dimmer or tone control and find a single pole six position rotary switch. The cb/pa switch has been utilized also.

    Thanks in advance
     
    #1 unit435, Nov 18, 2021
    Last edited: Nov 18, 2021
    Shadetree Mechanic likes this.

  2. Lkaskel

    Lkaskel Active Member

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    At 1st thought I would wonder if the cb/pa function is somehow in PA or maybe the cb connection is broken.. That would cause no XMT or RX. Start with checking the wires for that function and make sure that the CB wire is still connected to the appropriate wire. I assume that you just soldered them together when you used the switch for some other function.
     
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  3. Holydvr

    Holydvr Well-Known Member

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    I am sure some may chime in, but it could very well be bad electrolytic caps also.
     
  4. unit435

    unit435 New Member

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    I did not check those cb/pa wires but will later today and I suspect I soldered and shrinked them. I will also post some more pics as I noticed the power transistors looking a little stressed but if one or more of those transistors were bad, would that take out the receive mode also ? There are 5 of them mounted on the back of the casing. TR24 TR25 TR43 TR44 TR45. If any of those went bad, would I have a receive problem ?
     
  5. unit435

    unit435 New Member

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    I guess it may be worth replacing those anyway. Not sure how many there are but I do remember someone selling kits to replace all of them. Is there a safe way to test them installed ?
     
  6. groundwire

    groundwire Sr. Member

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    Possible Dead xtal
     
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  7. unit435

    unit435 New Member

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    DSC_0959.JPG Here is a Pic of what I believe are the wires from the cb/PA switch, one of the pots (tone or dimmer) and a purple wire with a white trace and snipped off or maybe broke off but it looks very clean and not insulated.

    The wires for the pa/cb switch were soldered but not shrinked. I used (dumb 20 year old at the time) silicone on the joints to insulate. One set is exposing the solder slightly so I guess which ever set those are, it's possible they shorted or shorted something else in the area they were tucked in. That white traced purple wire is concerning as that was also in the same area with no insulating component on it. I cannot see anything in its striking distance that it may have come detached from but it's still a possibility even though it looks like it was just snipped and left there. I certainly would not have done that intentionally so I cannot explain it other than probably connected somewhere.
     
    #7 unit435, Nov 19, 2021
    Last edited: Nov 19, 2021
  8. loosecannon

    loosecannon Sr. Member

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    yes, re-capping that radio is a good idea, but be aware that you will most likely need to re-align the radio afterwards, and that takes good test equipment.

    what you are describing sounds like you have lost a voltage regulator.

    so, let's start with what does work.

    the radio lights up, which im guessing means that the meter light comes on, and the channel display comes on.
    if the channel display does not come on, please correct me.

    you said that you can hear the relay click when keying the mic, but that you have no TX power on AM and no receive.

    please do check if you have any SSB audio by talking into the mic on SSB TX and looking at a wattmeter or the radio's meter.

    we will start tracing power, so you'll need a voltmeter set to read DC volts in a range that goes up to 14 volts or so. if you have a probe with an alligator clip on it, use it for the negative lead and clip that lead to PC board ground since you will be making multiple checks and it will be easier to just use the positive to probe around the board.

    any metal tuning can will make a good connection point, as does the negative pin coming in to the back of the radio.
    just don't use the chassis, as it is isolated from DC ground.

    we will use this schematic to trace power:
    http://www.cbtricks.com/radios/cobra/138xlr/graphics/cobra138xlr_sch.pdf

    lets start at the lower right corner where the power comes in to the radio.

    starting at the positive pin, you will measure between 12-14 volts here or whatever your power supply is set at. it should be 13.8 volts.

    what we will be looking for, is either a transistor or an IC chip that has this 13.8 volts (also known as VCC or supply voltage) on one leg, but does not have the appropriate lower voltage on another leg.

    your PLL chip runs on 5 volts, so there will be one transistor that has VCC on one leg, ground in the middle, and 5 volts on the other leg as an example.
    if this 5 volts were missing, your PLL would not work and you would have no receive or transmit.

    ok, follow that positive line through L40, through S406 (the power switch), and notice that the relay coil is connected to this line.

    the other side of that relay coil needs to go to PC board ground in order to engergize the relay and put the radio in to TX mode.

    if the CB/PA switch is in CB position, and you follow it up the schematic, you will notice that it goes to pin 3 of the mic jack. that pin gets grounded when you key the mic, so, if your relay clicks, you can say that you have good power to the relay, and the CB/PA switch is working. well, at least in CB mode which is what we care about.

    ok back to that positive voltage line we are following.

    we know the problem exists in both RX and TX so it's not going to be that RX TX switch that you see.


    so let's follow that voltage line on the schematic up past R113 to where it moves to the left, all the way across the schematic, then up again connecting to R161 which is just to the right of the UPD858 PLL chip.
    measure the voltage on the other side of R161 which should be just a bit lower than the incoming voltage.
    then follow that line up to IC6 which is the 5 volt regulator to the PLL chip.
    if the transistor is facing you and upright, pin 1 is the input pin and is on the left.
    the middle pin is ground, and the pin on the right is the output 5 volts.

    so measure the voltage on the input pin, and then check for the 5 volts on the output pin.
    is it there?

    we will stop there for now.
    please post back the results of your tests.
    LC
     
  9. unit435

    unit435 New Member

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    Thanks LC. I will try to start the testing today. It is turkey day so I cannot say for sure but will try.
     
  10. Shadetree Mechanic

    Shadetree Mechanic 808 On The North Side of Dover

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    Welcome! Those are great radios, definitely worth fixing. Happy Thanksgiving!
     
    tecnicoloco likes this.
  11. unit435

    unit435 New Member

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    please post back the results of your tests.
    LC[/QUOTE]


    Black Friday Update.......

    The channel display is lighting. The meter bulb is bad so that is not lighting.

    The relay is working and working clean with a good snap when keying the mic.

    No power output on AM or Sideband

    No receive on AM or Sideband

    I could not find R113 and will continue to look for it if you think it is necessary.

    My power supply is delivering 13.44 volts to the plug on the unit

    Incoming voltage to R161 is 13.34 volts
    Output voltage of R161 is 9.21 volts

    Input to IC6 is 9.19 volts
    Output of IC6 is 4.78 volts
    I confirmed that a good ground exists on the middle pin.

    Input voltage to IC6 when keyed to transmit lowered to 7.40 volts

    The unit was setup with a power, modulation, swr meter combo and a dummy load.

    Suddenly there appears to be a new problem. When on sideband, the relay chatters when keyed to talk and will blow the 3 amp fuse if keyed long enough. Does not appear to be a direct short and it will handle short bursts but I blew the fuse once while trying to figure out what I probably disturbed while poking around for the needed components. I took a close look at the connections on the am/sideband selector switch which is close to R161 and IC6. Looked fine. I will need to spend some more time to see if something has shorted from me poking around.

    No problem on AM. Relay works fine.
     
    #11 unit435, Nov 26, 2021
    Last edited: Nov 26, 2021
  12. loosecannon

    loosecannon Sr. Member

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    IC6 seems fine.

    HOWEVER!

    you seem to have a very common problem with these chassis, and that is C179, a small tantalum cap located right between those bias wires in front of the driver and final.
    (probably blue but might not be)

    remove this cap, and replace it with a 2.2uF 25 volt electrolytic capacitor, paying close attention to proper polarity.

    now check if that SSB chatter is gone as well as the fuse blowing.

    this most likely has nothing to do with your other problem, but let's get that one done and move on from there.

    the voltage drop on TX seems weird, so try checking the voltage coming in to the radio, and see if it drops on TX.

    after this, we will check some voltages on other IC chips and transistors.
    LC
     
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  13. unit435

    unit435 New Member

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    Great thanks. The sideband chatter did not show up until after I found IC6 and R161. I suspect something I moved or disturbed caused the problem because it did not chatter at all before moving wires etc.

    I thought that voltage drop was funny also. It may be that it is the power supply however I am currently out on the road in a motorhome, plugged into shore power which runs the 45 amp converter charger which works fine however it may be that the 12 volt port and associated wiring to the 138 could be causing the drop. It could have been that I did not have good contact in the outlet. I am now wondering if it may have had something to do with the relay chatter on sideband. I will check all that stuff out tomorrow. I am not sure when I will get the cap. If I cannot find a local supplier here in the Houston TX area I will need to order online.

    I will update tomorrow and many thanks for the guidance.
     
  14. Klondike Mike

    Klondike Mike Sr. Member

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    As already mentioned, this 40 year old radio still has its 40 year old electrolytics. Over 20 years past their designated lifetime. Save yourself and others time and consider replacing them. No reputable repair shop would work on any radio of this vintage without first doing this step. I offer a kit of replacement electrolytics (see link below) for a fair price considering all the time and effort put into making this available. Sure, you can do it yourself but not for the price I sell the kit for. As LC stated, C179 is a tantalum. Tantalums are more expensive than aluminum electrolytics for numerous reasons and as mentioned, an aluminum electrolytic is a suitable, cheaper replacement. C179 is not included in my kit but I could include a tantalum free of charge.

    https://klondikemikescapkits.com/pr...-138-139xlr-electrolytic-capacitor-kit-deluxe

    [​IMG]
     
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  15. Holydvr

    Holydvr Well-Known Member

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    I had C179 explode on a 139XLR and it sent the whole top of the cap crashing into the ceiling with a decent amount of velocity!
     
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: Cobra138XLR

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