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Cobra 142 gtl meter light upgrade?

Discussion in 'CB Radio Forum' started by Turbogus, Apr 13, 2011.

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  1. Apr 13, 2011 #1
    Turbogus

    Turbogus Light Load Member

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    Oct 16, 2010
    Philomath, Oregon
    0
    I've got this classic Cobra 142 GTL, [​IMG]what with the A 99 on the roof of the building and a modest brick this radio gets out well. Looking over EBay the other day I noticed a guy selling LED's for meters and such. I thought this a brilliant (pardon the pun) idea. So went down to the Rat Shack and purchased a couple of 5mm white LEDs Cat no. 276-0320. Verified polarity then soldered in, trouble was they lit only for a short while. I'm thinking these 3.6 volt LEDs are under rated for that circuit. Any ideas?
    Thanks and a lift of the lynch lid!

    Gus
     
    Turbogus, Apr 13, 2011
    Turbogus, Apr 13, 2011
    #1
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  3. Apr 13, 2011 #2
    xPosTech

    xPosTech Light Load Member

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    Mar 5, 2011
    SETexas
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    I have no idea what the meter lamp voltage for that model is. Did you check the voltage with a meter? If it is indeed 3.6V, you will need to add a current limiting resistor in series with the LED.

    If it's higher than 3.6V, you can add a voltage divider to get it down or simply get an LED rated for whatever voltage it is. Hint: some LEDs contain their own current limiting resistors. 12V LEDs for automotive use come to mind.

    It's easy to set the brightness of LEDs statically but harder to make them variable.

    Was the LED very Brilliant before it died? How long did it take? ie Did you reinvent the flashbulb?

    Ted
     
    xPosTech, Apr 13, 2011
    xPosTech, Apr 13, 2011
    #2
  4. Apr 14, 2011 #3
    Turbogus

    Turbogus Light Load Member

    56
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    Oct 16, 2010
    Philomath, Oregon
    0
    Hi Ted,
    I've learned that that circuit runs between 12-13.8v & I just found out from another forum Rolling Radios sell 12v LED's, might be a simplified way to go, given my limited experience (primarily automotive).

    Prior to the failure they seemed to emit a normal amount of light for an LED but only lasted a minute or two.

    Thanks and a lift of the lynch lid for posting!
     
    Turbogus, Apr 14, 2011
    Turbogus, Apr 14, 2011
    #3
  5. Apr 14, 2011 #4
    xPosTech

    xPosTech Light Load Member

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    Mar 5, 2011
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    I think Rat Shack sells different color 12V LEDs. Remember to current limit via a resistor. You can set the brightness with the same I limit.

    Ted

    Edit: Those are probably true 12V; automotive (Autozone, Advance, etc.) LEDs say 12V on the front of the package but are rated at ~14V. Get them individually wired. I'll take a look the next time I go by one of the auto parts stores.
     
    Last edited: Apr 14, 2011
    xPosTech, Apr 14, 2011
    xPosTech, Apr 14, 2011
    #4
  7. Apr 15, 2011 #5
    Turbogus

    Turbogus Light Load Member

    56
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    Oct 16, 2010
    Philomath, Oregon
    0
    I was just looking for clear and it was my failure to include a resistor, nutz.
    Since I was undaunted I was going to give it another try. I had already got some coming from Washington before it dawned on me to stop by NAPA or Knechts A.P. :biggrin_25526:

    From another forum they mentioned that I'd also have to run a jumper over R162 on the circuit board. I've not done any work of this kind on a PCB so I'm dubious of my skill, perhaps I should farm this out to a shop?
     
    Last edited: Apr 15, 2011
    Turbogus, Apr 15, 2011
    Turbogus, Apr 15, 2011
    #5
  8. Apr 15, 2011 #6
    xPosTech

    xPosTech Light Load Member

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    You've already put one in. If you're leery of the PCB then solder the jumper close to the resister body. Or farm it out.

    But it's not hard to do. The secret is the very best resin core solder you can find. Tin the jumper and resistor leads first.

    Ted
     
    xPosTech, Apr 15, 2011
    xPosTech, Apr 15, 2011
    #6
  9. Apr 16, 2011 #7
    Turbogus

    Turbogus Light Load Member

    56
    2
    Oct 16, 2010
    Philomath, Oregon
    0
    Hi xPosTech,
    While the R162 is easy to spot on the schematic, I'm having a devil of a time trying to locate it on the circuit board. Is it really nesessary?
     
    Turbogus, Apr 16, 2011
    Turbogus, Apr 16, 2011
    #7
  10. Apr 17, 2011 #8
    xPosTech

    xPosTech Light Load Member

    91
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    Mar 5, 2011
    SETexas
    0
    After looking at the schematic I think you can leave R162 alone. It is 22 ohms, current limiting the supply to about 545 milliamps at 12 volts. Since they're wired in parallel, that means about 270 milliamps to each lamp. (Until one burns out, then 545 or so to the other lamp, which burns it out soon after.) I didn't look closely at the PSU. If the DC is 14V, the lamp supply will be 636 milliamps.

    Just add a resistor in series with each led. With a resistor for each led if one does open the current to the other won't double.

    Good luck. Keep us posted.

    Ted
     
    xPosTech, Apr 17, 2011
    xPosTech, Apr 17, 2011
    #8
  11. Apr 17, 2011 #9
    xPosTech

    xPosTech Light Load Member

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    You should check your radio for these two Tech Notes (tech change bulletins) while you have it open. One replaces a 16.2V Zener on a DC Reg./Sw. with an 18.2V Zener. Otherwise the DC Regulator/Switch could fail. The other moves the ground reference to prevent AC hum in an external speaker or headphones.

    You'll also find other documentation on that page. The Tech Notes down towards the bottom.

    Ted
     
    Last edited: Apr 17, 2011
    xPosTech, Apr 17, 2011
    xPosTech, Apr 17, 2011
    #9
    Turbogus Thanks this.
  12. Apr 17, 2011 #10
    Turbogus

    Turbogus Light Load Member

    56
    2
    Oct 16, 2010
    Philomath, Oregon
    0
    Thanks xPosTech for your patience and great helpful responses. :)
    That link is going to be right handy if I ever get more back counter space for the Turner +2 I've been drooling over.

    After first getting this radio several years ago now after adding a brick I quickly learned that the radio output wattage overdrove the amp. Apparently some P.O. had this radio tuned up since '78 (one of my favorite years) Sending in to my friend's shop about five years ago (Accuserve I think it was) with instructions to thoroughly look over the specs. of the radio and add a Remote AM power adjustment pot (seen on the left side of the case)
    What with these past mods I'm believing that these TCB's from '78 have already been addressed as I'm having no symptoms with power/modulation or humming.

    Thanks again for that link, while I'm a longtime responsible user of 11 meter AM, technical repairs are all new to me.
     
    Last edited: Apr 17, 2011
    Turbogus, Apr 17, 2011
    Turbogus, Apr 17, 2011
    #10
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