New channel LED install

[Robb]

I read that when installing an aftermarket channel LED, that resistors on that board will have to be replaced. Otherwise the LED will burn out prematurely. In a Cobra 148, there are six 680 ohm resistors on that channel switch board. Are they supposed to be replaced with 2k2 ohm resistors; or is another value used?

I'm putting a blue channel LED and a new blue/red RX/TX diode in it.
Might even put one blue LED in the meter since I have it.

 

[CTStallion]

Hmmmm.... I've done LOTSA CH LED's, some easy, some P.I.T.A. (like the small chassis radios), and I've never changed the resistors.

However, I HAVE changed LED's and found the SAME segments out, only later to find that the resistor bank was BAD, and that's what was causing the segments to be out in the first place.

Stupid resistors.... lol

BTW: the resistor bank I'm referring to is the 'RIVER', a bank or 4 resistors in parallel on the 148 marked: CC1. BUT, I know what you're talking about, the 6 INDIVIDUAL resistors on the CH selector board ITSELF. Quite frankly, I'm not sure; have never heard that one before.

 

[Robb]

This snip of an earlier schematic shows twelve resistors (R401-404 and R406 to 413) that have the 680 ohm resistors and one resistor is R405 which is a 330 ohm resistor.

Soooo; do I replace all twelve 680 ohm resistors or just the 330 ohm resistor?
Or do I replace ALL of them?

The schematic isn't clear as to what goes where on the board; or I wouldn't ask. There must be some members that have done this mod before.

http://www.cbtricks.com/radios/cobra/148gtl/graphics/cobra_148gtl_sch_b_size.pdf

 

[Lil'Yeshua]

LEDs are current driven devices. I'd find out what current rating and voltage rating the LEDs in question and do a little math to find out what value resistors you need. That's what I understand about LEDs as being separate devices however,I'm not familiar with them in a channel display setting.

I don't know if this will help...
http://www.cbradio.nl/cobra/Service_Manual_Cobra_148GTL_ENG.pdf

 

[loosecannon]

Robb, i can tell you that wheny you put a blue LED readout in a 148, you will want to keep the bright/dim switch in the dim position.

sometimes this is all that needs to be done, but if you plan on repurposing that switch, then you'll need to find out how much extra resistance gets added to the circuit in that position, and then add that in series.
LC

 

[Robb]

Thanks.
Isn't there a single resistor that can change what is being fed to the LED?
Like, if I were to put a variable dimmer in it?

 

[loosecannon]

yep, i dont know the value, but its mounted to the switch itself.
LC

 

[sonoma]

the voltage feeds the 2 center bottom legs of the led. that is where it gets its supply voltage from lil yesha left you a part of the schematic for the feed. I know on a cobra 29 it has a 1957 trans mounted to the case and it supplys the voltage to the led read out. if you are putting a blue led in it no matter what resiuster you put in it will be to bright. you almost have to shut the voltage off on blue to get it where it is not so bright. at least that is what I have ran into with it.
the 2 bottom legs of the led's that are soldered together is the voltage into it.

 

[loosecannon]

good point Sonoma.
i just re-read the thread, and i think this is what didnt sound right to Robb earlier.
the 680 ohm resistors are on the ground side of the circuit, not the voltage side.
LC

 

[sonoma]

I was just looking at the scematic for the 148 radio and it looks as if R173 has a 1K resister in the dim circuit. could not make out the bright side on the schematic Rick drew or the stock one. I know when I have one out of circuit to check it I put 1.8 volts to the two lower pins then ground each other pin to get each leg to light up. the blue ones if that is what he is putting in it are very bright no matter how low I drop the voltage. I have an adjustable power supply I use for checking these items and most of the time the blue will go out before it getsw very dim.sometimes if the owner does not like it so bright I out a colored piece of plastic in it to tone it down. I use some tint like you put on car windows to get it toned down some.

LC I knew what you was saying. guess I have read enough of your post over the years to understand what you were saying. been on here too long I guess. HA!HA!

 

[loosecannon]

LOL Sonoma, i was just thinking about that. we do both seem to gravitate towards the same thread.

i always use a piece of 1/8" thick dark blue plexiglass for the channel window.

i got a piece of it years ago that was about 6" by 24".
dont think ill ever run out.
LC

 

[sonoma]

I have a piece of blue window tint and some times have to double it to make it dark enough to satisfy some when they complain it is to bright. if some ones blue I tell them before I ever touch it that it will be bright.

still waiting on the project you are working on with Sue to see how it turns out. bet it will be nice when you get it done.

 

[Robb]

UPDATE:

Got the new ch switch from Ranger today. So, I popped the radio apart and unsoldered the ch switch ribbon cable from the board. Removed the old switch - which has some 30 joints - and put the new one in and re-installed it in this tired, old 148. That fixed the problem. I will end up rebuilding the old switch to have a spare around, since I do work on 148's if/when I get lucky enough to find some - from hamfest to hamfest - so it seems lol.

Thanks for all of the tips, all y'all.

Also purchased a tx/rx blue/red LED and a blue LED for the meter. Gonna dress it up a little bit since the rest of the radio isn't perfect but decent for its age. Will also replace all of the caps too, just to keep it in a decent state of repair. Will put up some pictures when it is done. In no big hurry, just when I get around to finishing other radios that belong to some local ops . . .

 

[sonoma]

glad you finally got it going.

 

[ExitThirteen]

Hey Robb,

Do my asymmetrical modulation mod to it!


~Cheers~