i dont think it was me about the diode thing, but i guess it would work.
there is a bit more to it than just replacing the lamps with LEDs.
its always a good idea to look at the schematic first.
here is a link:
http://www.cbtricks.com/radios/cobra/142gtl/graphics/cobra_142gtl_main_sch_a_size.pdf
look in the lower left hand corner just above the antenna socket (labeled ANT).
you will see the two meter lamp symbols. you will notice that they are wired in parallel, which means that one wire of each lamp goes to the 13.8 volts, and one wire from each lamp goes to ground.
you will also notice that the wires that go to the 13.8 volt source go through a resistor R162. instead of trying to account for this 22 ohm resistor in figuring your LED voltage, its easier just to replace it with a jumper wire.
sometimes ill just find my own voltage and ground points and just heat shrink the stock wires.
for now lets do it using the stock lamp wiring.
ok, first step, cut the wires to remove the old lamp. finish one lamp before you start the other. (they are all the same color usually)
cut it within a 1/2 inch or so of the lamps to make sure you have enough length left. (usually there is plenty extra)
be sure to carefully remove the rubber holders from the lamps, as you will need to use them again. if you ruin them, you will have to devise your own mounting system, and that can get "hacky". LOL
now strip off just enough insulation from one wire to allow you to touch a multimeter probe to it. (if you dont have a voltmeter you will have to trace the wires back to their sources, one will go to ground, and one will go to R162. the one that goes to R162 is your positive wire)
ok, with the radio on, (be careful not to let the wire touch the chassis or the board) touch the negative lead of the voltmeter (set to read DC volts) to any of the metal tuning cans you see on the board. they are all grounded to PC board ground.
touch the positive lead of the voltmeter to the wire you just stripped.
you will either get a reading of somewhere around 13-14 volts, or you will get no reading at all.
label the wire accordingly. (the no-reading wire is negative)
time to turn the radio off and unplug it from the wall.
now that you know which wire is positive, and you have placed a jumper across R162; you need to make sure that the negative wire is actually connected directly to PC board ground, and not through any other components. (its different on different radios)
if you are not sure, just remove it from where its source was, and solder it to any convenient metal tuning can. you will notice that all of these metal cans have two tabs on them that, when soldered, secure them to the PC board.
all of these tabs are connected to the main PC board ground foil, and will make a fine place to solder your negative wire to.
ok, time to actually solder the new LED to the wires. BE SURE TO SLIDE THE RUBBER HOLDER ON FIRST!
you will need a 1/2 watt 560 ohm resistor for each LED. (its easier to just tell you the value instead of showing you how to figure it out for now)
the resistors need to be 1/2 watt, and no, 1/4 watt will not work. it would heat up.
looking at your LED, you will notice that one side has a flattened edge.
this is the negative side.
this is also the side where you will solder your resistor on.
i always put the resistor on the negative lead, and again, its just simpler to tell you to do it than to explain why.
ok, you have soldered the 560 ohm 1/2 watt resistor to the negative lead of the LED, and it looks nice and neat and shiny right?
slide some heat shrink tubing down over the negative wire, solder the negative wire to the other end of the resistor, and cover the connection with the heat shrink tubing. (shrink it with a cigarette lighter or whatever)
now slide some shrink tubing down over the positive wire and solder it to the positive lead of the LED. heat shrink the connection.
(never use electrical tape, again, its "hacky")
time to check your work by turning on the radio.
if all went well you should be looking at a nicely lit LED.
(white is my favorite color for the cobra style meters)
if you dont like the brightness, you can add or subract 100 ohms from the 560 value, but dont go crazy. less resistance is brighter.
before you decide that its too bright, try sanding or filing the face of the LED.
this will diffuse the light and it will illuminate the whole meter instead of just one "hot spot".
just repeat the whole process for the other lamp.
there ya go. yes, its seems like ive over complicated things, but once you do it, it will get easier.
i went into such detail because i want others to be able to benefit from this post in the future.
good luck,
LC
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?
when it comes to circutry, any diagrams available?
