Cobra 2000GTL "Glowing channel digits" idea

[loosecannon]

Hi all.

Im sure we are all aware of the glowing channel display digits issue that only exists on the 2000GTL when lamps are changed for LEDs.

I know there are different ideas on how to cure this issue from relays to resistors, but im wondering if anyone has just tried soldering in a grain of wheat lamp like the originals across the 13.8 volt rail?
(basically just like leaving one meter lamp in place)

i figure this would be quite a simple solution without the hot resistor, and without the complication of adding the relay.

IDK, just thought of it and figure i'll give it a shot as i am working on one of these radios presently.
LC

 

[Robb]

Interesting; never was aware of that issue.
Soooo, what you are saying is that the digits burn brighter because the incandescent bulbs pull enough current to keep the digits dimmer?
I've replaced several meter bulbs with leds and never noticed!
Learn something every day.

 

[tecnicoloco]

If I recall nomad came with a solution in the past

 

[Low_Boy]

That idea is someplace floating around on the net. Supposed to work.

 

[loosecannon]

Robb,
it's because of the alarm clock function that will turn on the radio when the alarm goes off.
the power switch in the 2000 switches the ground, so there is always 13.8 volts potential at the PC board, even when the radio is "off".

normally, the lamps will drain this voltage down to low enough to keep the channel LEDs from glowing, but LEDs won't do this, and will allow around 2 volts to always be present at the channel switch.
you can even change channels with the radio off and watch the display change.

there have been various cures to this issue, including strapping a bleeder resistor across the power rail, but i have heard that the resistor stays pretty warm all the time.
I've never tried it myself.
MikesRadioRepair has a video showing how to add a relay that keeps the power from going to the channel switch when the radio is off, but i think that is over complicated.

i figured, just having an old school lamp inside the radio might be a more elegant solution, and it sounds like LowBoy is saying that im not the first to try this.

i'll be re-capping this radio soon and will try my idea and post back the results.
LC

 

[nomadradio]

One solution was to rewire the power switch so that a 100-ohm resistor was in the circuit only when the power switch was in the "Off" position. This way a 1/4-Watt resistor worked just fine, since it didn't get voltage put to it when the radio is powered on, only when it's off.

A bit involved, but does away with the hot resistor issue.

73

 

[loosecannon]

well, it's late, but i'll take a stab at it.

im going to guess it involve some sort of PNP power transistor that has 100 ohm resistor in the base circuit where a large-ish value cap is drained through the resistor when power is switched off.

will have to think more on it when im more awake, but if the idea is to let the transistor do the work as opposed to the resistor, i think i might be catching on to what you mean. (i ain't no engineer)
LC

 

[Low_Boy]

Some time ago Nomad helped me with this problem. It was a simple fix but I do not remember what I did to correct the problem. I imagine if I do a search of the site I can find it.

 

[Handy Andy]

This problem is similar to those situations with vehicles that require a bulb to be in place across a circuit that senses the load - and triggers a light on the dash - if the load changes from a load, to say, a high impedance state (no load) because the bulb burned out.

So to let the person know, the system lites up a symbol on the dash telling the operator that they were too lazy to do a pre-trip so you have to get this thing serviced.



Can cost you over $80 dollars to diagnose and clear the code...

So for that mess, to this...

To fix; most people SWITCH to LED, one time investment - cost a little more but for $80 dollars at least it can fix a set of lights so they no longer lite up your dash...

But, does it? NO! Of course not...

IT will just come on again!​

One thing that makes LED's what they are, being solid state, they also require less power, and a lot less maintenance.

• Ok, but it's the Less Power thingy that can make the lite of your dash come right back on...

Can make a typical thermal-relay that looks for bulbs on the line to work and flash the turn signals - (which those whom do the pre-trip would also see this ) the slower or quicker rate of flash. IT should be telling them to check the lights.

Same thing with the Cobra 2000...

So to fix that, you have to use "ballast" or a dropping resistor ACROSS the wires at the bulb.

OR make sure that the package SAYS...

Will not cause CAN BUS problems​

(ok, how does it do that?)

Maybe a little history...

The typical Grain of What? (Wheat) - Bulb -

​

Looks like nearly a dead short on the meter - until power is passed thru it...the filament begidn to heat up changes the resistance as this happens...

And guess what - you get a Cobra 2000 with a power switch that seems to work...

Until you do the LED conversion and now, you have the ghostly glow of a haunting image of The Last Channel You Were On, syndrome and nightmares soon follow...

Here, have two...​

So they offer these...

LED ballast, Dropping resistor...
​

So when in Rome...

May need to install that which was removed from the circuit to obtain the longer life the other lights are giving this radio...

A load...

 

[nomadradio]

I looked for the file. I thought I saved it. Gist of it is to use the normally-closed side of the power switch. The radio is built with a double-throw push button power switch. Only the "On" side is connected. The new resistor goes to the "Off" side of the switch. Only draws current when the radio is powered down, but draws enough to extinguish the channel digits.

Wires also get moved to make it work, so I'll have another look for the file I posted on the old CB Tricks forum. Pretty sure that's where I posted it.

73

 

[loosecannon]

cool.

sounds less complicated than what i was thinking.
LC

 

[Shadetree Mechanic]

Sounds like the thing that is driving the LEDs is floating on the output. A pull down resistor can fix this. Or am I missing something?

 

[Shadetree Mechanic]

I looked for the file. I thought I saved it. Gist of it is to use the normally-closed side of the power switch. The radio is built with a double-throw push button power switch. Only the "On" side is connected. The new resistor goes to the "Off" side of the switch. Only draws current when the radio is powered down, but draws enough to extinguish the channel digits.

Wires also get moved to make it work, so I'll have another look for the file I posted on the old CB Tricks forum. Pretty sure that's where I posted it.

73

I just re read what you wrote. So the pull down resistor gets switched in circuit when the switch is turned off. Or is it a pull up resistor?

 

[Handy Andy]

There are two switches to work with...

One, the main PCB

​

But, on a 2000, you have DYNALERT - the Alarm or Radio Wake feature...Set your channel and volume and sleep tight - for you're going to get blared out of bed when it reaches wakey wakey time...

The other half of the problem is at the Counter board ...

S401 is the Radio power but it lifts Ground to allow Alarm to Wake to Radio
The Counter board becomes it's Ground.​

@nomadradio - I know you posted this at CB tricks but I don't have much of those days left...but I am aware of the work so I'm just putting this out there to help others see that there are several issues of the glow and it's not as simple to fix as it looks. Well, simple solution - but knowing why the glow in the first place...

 

[nomadradio]

This is more than a little weird. I found the pics to do this job on my image host, cropped and resized properly. Just can't find an old forum post that uses them.

Like maybe I dropped a ball. Like 7 or 8 years or more back?
Maybe.

But here's a method to cure the dimly-glowing channel digits when you turn off your Cobra 2000GTL.

First step is to prepare the load resistor. It takes the place of the filaments in the meter lamps. The cold resistance of those bulbs was a part of the circuit that shuts off the channel digits completely, and not just partially. A 100-ohm quarter-Watt resistor usually does the job. One end of this resistor will go to the "AC-DC" switch on the rear panel. Splice a wire that will reach from there to the power switch with a bit of slack.

The resistor lead now goes to that center lug of the AC/DC switch on the rear panel.

So here is the original wiring on the power switch. A gray and an orange wire both connect to one side of the switch. A single brown wire goes to the other side of the switch.

The two wires now move to the left, where the single brown wire had been. The brown wire gets moved to the other side of the switch.

The thin brown wire jumper is not necessary, but allows the switch to "share" the current load across both of the switch's two circuits. Pretty sure it still works with that left off.

Last, the wire from the resistor at the AC/DC switch goes to the front-most pin of the power switch. This pin is a closed circuit when the button is out in the "Off" position. This way the resistor is only in the circuit when the button is switched off. And the resistor's power rating won't be important.

One missing detail here has to do with the clock's alarm feature. If you turn on the radio with the alarm, you'll smoke that 100-ohm resistor.

There are two wires soldered to one side of the power switch. One is orange, the other one either white or gray. The quality of the pics doesn't give me confidence here. One of these wires goes to the main radio circuit board. The other one goes to the clock/counter module's 4-pin plug on the rear side of the module.

Just can't remember which is which. Unhooking the wire that leads to the counter module will remove the 'poof' risk for the resistor, but will also disable the "wake up to CB" feature of the clock's alarm.

Choices, choices.

73