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Cobra 2000 No TX power then whole radio died

Discussion in 'General CB Services Discussion' started by Brian G, Feb 27, 2021.

  1. Brian G

    Brian G Member

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    I have very little Tx. I replaced the driver and final thinking one was bad but still nothing. I’ll get a few watts out of it and then it drops off while you are keyed. And the SWR seems to be jumping around as well. I cannot peak the tX more than a couple of watts max. And I have a really bad squeal from the mic as well. Not sure if that’s all related.
    Help!


     

  2. nomadradio

    nomadradio Analog Retentive

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    Umm, okay. Any time someone says "Cobra 2000" I immediately assume that it's the very-oldest 1978 radio with 250,000 hard miles on it.

    Of course, it could be the newest (last) 1993 radio, but the odds are small.

    First thing I try to keep in mind with a radio that old is that you will nearly always find more than one fault.

    A newer radio that ran every day and then failed all at once might have a single fault, just one failed part. But the older they get, the less and less likely this becomes.

    Changing a part because it somehow seems logical is not a method with a good track record. Do you have a way to test a transistor? A multimeter? Dummy load, wattmeter?

    Did the old driver test bad? What was the before-and-after difference when you changed it?

    I really think you have more than one thing wrong with it. Just for starters, has someone wired a mike with a 4-wire cord on it to this radio? It has a 5-pin socket because the correct mike has two separate ground circuits. A 4-wire mike cord has only one ground. This can cause squeal issues on transmit, even if the radio is working perfectly on its own.

    A weak transmitter also suggests checking the "bias" current.

    I'll skip the procedure for that right now. And if you already know how, I'm curious what the current reading is for the driver and final, both.

    Also, there is one fat tuning slug next to the final transistor. Can't remember the schematic callout right now. This one should be peaked for max modulated power, not for AM carrier. The position where this slug shows a peak can offer a clue, as well.

    73
     
    NZ8N likes this.
  3. nomadradio

    nomadradio Analog Retentive

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    Whups! Just noticed the phrase "whole radio died".

    I would pull out the two bias-test wires each from the pin where they attach to the pc board.

    An external DC power supply will bring it back to life if a blown fuse is what has shut down the AC power.

    And if you don't have a DC supply to power it, that's a handicap when your radio has power supply trouble.

    A multimeter on low-ohms gets the black lead clipped to the radio's DC-power ground. Probe the end of each bias-test jumper wire and see if either one of them shows a short or low resistance. If so, that's the one that shorted and shut the power supply down.

    73
     
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  4. Brian G

    Brian G Member

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    Ok I’ll check it out. The fuse is not blown. I will check it on DC only tomorrow.
    I actually did check the final bias right before it died and it was only a few milliamperes as I recall. When it should be 50 I believe. I do have decent multimeter.
     
  5. Brian G

    Brian G Member

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    Before it died, I noticed the wattage would start at 3 or 4 then drop down to under 1 while the radio was keyed. I could not turn up the watts through VR10.
     
  6. nomadradio

    nomadradio Analog Retentive

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    If the bias trimpot for either the driver or final acts unstable, it needs to be cleaned. Just like a scratchy volume control. And a badly-oxidized pot might cause the current setting to shoot up to an unsafe level. That would be consistent the the power dropping off as the final overheats before melting down permanently.

    If the final now shows a short from emitter to collector, this is probably what happened. If VR8 looks visibly rusty, it will probably not be stable where you set it.

    Like Indiana Jones once said, "It's not just the years it's the miles, too.

    73
     
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  7. Handy Andy

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

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    @nomadradio - I know you're working with @Brian G - but the symptoms I'm seeing here reminds me to have him check...

    upload_2021-2-27_8-38-38.png
    I've also known of people that remove the Bias diodes and swap in resistors - Epic fail...
    upload_2021-2-27_8-40-12.png
     
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  8. Lone Ranger

    Lone Ranger New Member

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    Because of the age of the radio, I would start by cleaning ALL controls including the trimmer resisters... Then Recap the radio as the electrolytics start to leak after some time... Especially with lots of milage... The cap kits are cheap enough and also a good place to start with those two things on old stuff....
     
  9. Brian G

    Brian G Member

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    Ok here is an update and more information. I thought I had added onto the original post but I guess I didn’t save it. I went downstairs this morning and now it turned on. Found a bad solder spot in the bridge rectifier that was causing the short and not to turn on. Voltage on the board seems to be restored. However at IC4 , the pins that should be at 8volts are only showing 6.8 which seems low? What would cause that? The other ones that should be at 13.8 are ok actually showing a little higher than that.

    I rechecked the TX. It will key up with the near normal amount of watts dead key and then slowly fade down to under 1 watt. Then if you unkey and wait a couple of minutes and key again it will do the same thing. I think this is where the issue is but would be causing that to fade down like that? The driver and final biases are really low and I can’t raise them I checked Tr37 and 39 39 seems to be ok but 37 has voltages of .5, .5 , 1.2 all in TX. The one connected to c153 says it should 2.67v on TX but it’s only .5. Could that transistor or the cap be bad causing the low bias voltage?
     
    #9 Brian G, Feb 27, 2021
    Last edited: Feb 27, 2021
  10. Handy Andy

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

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    upload_2021-2-27_13-20-11.png
    Make sure your Bias diodes are in there...
    Installed the right way.

    Feel your back panel by Driver and Final...
    IF it's hot - the Didoes may have been fried...
    Just put in new 1N4148's with Banded ends to Board FOIL ground
    and mount the diodes to the rear panel.​
     
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  11. Brian G

    Brian G Member

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    I never touched those so I assume they are in correctly. Whether they are bad or not I’m not sure. Would that cause the symptom I described above where the TX power starts off “ok” and then within several seconds fades down to less then 1 watt and little driver or final bias readings? Just don’t want to go replacing things that I don’t have to. I doubt I have replacements for those diodes anyways.
     
  12. Brian G

    Brian G Member

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    I replaced those 2 from an old Cobra 140 I’ve been using for parts so they are exact replacements. No change. Wattage drops within seconds after keying the mike and the bias currents are very low.
     
    #12 Brian G, Feb 27, 2021
    Last edited: Feb 27, 2021
  13. Handy Andy

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

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    The check is simple, just measure the DC current across Test point LEAD to Test point Plug....

    upload_2021-2-27_14-21-54.png
    Done in SSB mode, No Audio - into a 50 ohm dummy load.

    upload_2021-2-27_15-7-29.png

    What one of those Test Point Jumper posts (jacks) look like
    upload_2021-2-27_14-40-43.png

    Install Clippie...

    upload_2021-2-27_14-42-5.png

    Other Test lead from DVM goes to Test lead Wire with the plug end...

    upload_2021-2-27_14-43-28.png

    Set up your DVM...

    upload_2021-2-27_14-44-19.png
    Done this with POWER OFF so you can be safe...
    But note the DVM is set...
    DC A at the 200mA scale...
    TX - measure and adjust the Bias Pot for respective DRIVER or FINAL - DVM is set for 200mA scale - in SSB mode either USB or LSB.

    Can't find the pot, try one or the other - shown above, if they are not adjusting - STOP and recheck your connections. THIS IS WHY YOU PULL BOTH WIRES OFF THEIR STEMS - PREVENTS ERRORS IN ADJUSTING WRONG TP TRIMMER Power doesn't go to the Driver and or Final unless you connect thru the DVM and it should be fused so if you go too far, it will blow it's fuse and protect your investment....

    In TX mode, adjust TP trimmer bias to match 50mA - do this for one TP at a time - once completed TURN OFF POWER REMOVE TEST leads reconnect Wire to STEMS and begin to finish up the work...

    Verify the Radio does work -

    Reapply power,

    • - set MODE to AM
    • - while on Dummy LOAD
    • - key up and verify the S/RF meter shows RF power - if not - verify your work

    Once you've set RF power settings by verifying Scale of Meter and Scale on external power meter and you're satisfied they are good, then close up the radio...

    But you asked about BIAS DIODES ....

    The above check is for this - if the Bias diodes are bad, you won't get any mA reading because the DVM fuse could have blown already - it means you have to locate one of two things...

    A short in the Final and or Driver - Blown shorted...or OPEN No power flow or blown DVM fuse.

    OR

    Bias diodes blown - cannot adjust mA reading to proper mA readings...

    To fix the Bias diodes that easy enough - remove the Bias boards - one at a time unsoldered from the main PCB

    You
    can bend the leads of the 1N4148s' to 90 degrees down - at the body - and then unsolder remove the old diode and blob solder the other one in to replace it. Position it carefully but the body can fit in there. If you aren't comfortable with this - apply the diode directly to the back panel using a simple adhesive and rewire it back to the main PCB following proper polarity - see below...

    You follow the install with the Diodes UNBANDED end towards the respective Trimmer Pot - the BANDED end goes to Foil Board Ground and you then remount the new diode (either on the bias board or directly) thru to the back panel. On either the board, or as affixed to the rear panel using a simple adhesive to make the body contact the metal of the rear panel for thermal contact.

    Ok, also caught you already swapped out those parts from another radio - well while the old ones are out, check them to make sure they are still diodes and only work one way. Keep them and use them back in the 140...

    Now as far as the output - did you check for heat?

    This also means the SIDE panel by the AM regulator - you may have to recheck the install and make sure you are going directly to the TEST POINTS shown in the above in this post.


    Else you may have a failed part in the output network - re-check the work in the Final to output tank and beyond into the output network - it can be a solder "strand" or a blown cap onto a bad antenna connector jumper wire - or the SWR section can have a blown diode making your day...

     
    #13 Handy Andy, Feb 27, 2021
    Last edited: Feb 27, 2021
    Shadetree Mechanic likes this.
  14. Brian G

    Brian G Member

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  15. Brian G

    Brian G Member

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    yes thanks. I know how to check those
    2 biases. The driver one was waaaay to high though I don’t know why because I didn’t touch it. The final bias was ok. The max wattage a can get on a dead key is 3 through vr10. And I peaked all the other coils. .There something keeping the power low. I used to be able to get a lot more watts out of the dead key through vr10 though I kept it low for swing purposes. But something is not right. The wattage will slowly drift down as you keep the radio keyed. Yes the heat sink gets warm/hot with the radio keyed. And I keep calling back to the fact that the voltage regulator (ic4) has only 6.8 v at pin 1. The ground side of vr10 should be 8 volts and it’s of course only showing 6.8. Could that be reducing the watts coming out of that ? And again leads back to my original question about why there is only 6.8 volts from IC4.

    And to add one more thing. The “high end” voltage going into ic4 at pin 2 is 15.2 volts which seems to high. Would a higher voltage in lead to a lower voltage out for some reason? Interesting because the voltage is to high at pin 2 by basically the same amount as it’s to low at pin 8. That is the same voltage (15.2) measured at the ac/dc switch on the back.that traces back to TR401. The “blue” and “yellow” out on that are both over 15 volts but the red “in” is normal at 18.5.
     
    #15 Brian G, Feb 27, 2021
    Last edited: Feb 27, 2021

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