How to dim channel display led on 148gtl?

[red97k1500]

i recently picked up a 148 gtl made in the Malaysia 1992, for to cheap to pass up. All the wiring for the switches is hacked up. Extra channels on hi/lo bright/dim.

I installed a cheap blue channel display from eBay, I keep reading you need change out some resistors or it will burn out the display.
There is about 10 or so resistors on the channel selector pc board. Which ones do I need to change out? The schematic I have only shows r401,r413, and r403 which is a 1/2 watt 330 ohm, all the rest appear to 680 ohm. Do I only need to change these 3 resistors?

 

[red97k1500]

Or do I need to change every resistor on the board

 

[sunbulls]

I look for the common LED lead and place a 5K pot in series with it. Once the desired brightness is found, I measure the pots resistance and place an approximate valued resistor in its place. Looking at your schematic, the common lead is connecting the emitter of TR28 to the LED display.

 

[red97k1500]

I look for the common LED lead and place a 5K pot in series with it. Once the desired brightness is found, I measure the pots resistance and place an approximate valued resistor in its place. Looking at your schematic, the common lead is connecting the emitter of TR28 to the LED display.

Thanks for the info, that’s something I can do for sure. I have been google searching for two days now. The only other thing I came up with was to change all the resistors on the channel switch to 2.2k, but that just does not seem like a good idea to me.

 

[loosecannon]

to add to what sunbulls already posted, have a look at the schematic where the BRIGHT/DIM switch is.

you will see that it controls both the meter lamp and the channel display.

when it's in the bright position, it jumpers out that 100 ohm resistor on the ground return for the meter lamp.
so, if you want to change the brightness of the meter lamp (this needs to be changed if you use an LED in the meter instead of the incandescent lamp) here are the resistors you would change the values of:

for meter lamp-
Bright position- change R169 (pay attention to the wattage you might need)
dim position- change R418 (remember that if you change the bright position resistor, tha the dim position will be that much lower also)

now follow that DIM switch position line over to TR28, and you will see that when the switch is in the DIM position, R173 is added in, which turns R137/R173 in to a voltage divider, thereby lowering the input to TR28 and lessening the brightness of the channel display.

so, with the switch in the BRIGHT position, the channel display brightness is determined by R137.

if you want to change how bright it is in the DIM position, you need to change R173.

and just like the meter lamp, changing R137 will affect what R173 does to the brightness since they work together.

remember to pay attention to the wattage level of the resistors you are messing with, and that if you raise the value, you are asking the resistor to drop more current and may need to step up the wattage value.
ohms law is your friend here.

hope this helps.
LC

 

[sunbulls]

In some cases that common connection is ground or negative, in others that common connection provides the positive voltage to the LED segments. It all depends on whether the LED is a common cathode or common anode type. In either case, adding series resistance to that connection will limit the current going to all the LED segments.

 

[OldTech03]

Mean-Well makes some really nice LED driver modules that can be configured adjustable voltage while maintaining current at a fixed/variable setting of your desire and they maintain a constant current and won't degrade over time. And they are very reasonably priced like 5 to 6 bucks should take a look at them My Grandson and myself use them in custom XBox mods for strings of LED backlighting and they would work great for LED channel displays and other ideas as well.

Mouser # 709-LDD-1500L this is 1.5 amp module they make them down to a 300 milliamps and higher.

 

[red97k1500]

Thank you to everybody, this has been very very helpful. I wish I had studied the schematic more before I posted, but I wasn’t quite Sure what I was looking for.

A channel mod has been installed on the bright dim switch, and the meter led is wired straight to the board.I can’t even find the wires they took of the bright dim switch.When I got the radio the only switch on the front that did anything was the cb/pa switch.someone had done a channel mod but only one wire was hooked up to the
Hi/lo switch.the other wires were connected to pins 10 11 and 12 of the pll chip. So I rewired it before I even fired it up. I also put the pll chip on a socket and popped in one of those 5 dollar mb8719s you see on eBay from Taiwan, just to see if they really work, and they do.

I’m getting sidetracked lol, seeing as I can’t even find the wiring for the brite/dim any more. Changing out both r173 and r137 should still work.also what would happen if i left r137 and r173 alone, and put a resistor where j42 is?

 

[loosecannon]

i can't tell from the pic what J42 is connecting, and the schematic doesn't show jumpers.
what is on either side of it?
LC

 

[red97k1500]

It is connected to the emitter of tr28.and then goes to just one single trace.that goes to the channel switch...Which I can only assume is the common lead going to the led display that sunbulls mentioned. I was able to see this by shining a flashlight through the board.

Edit: see photo I marked where j42 is, I’m doing this from memory but I’m 99% sure it’s correct

 

[loosecannon]

ok i gotcha.

so, the answer is yes, you can put a resistor there and it will lower the voltage going to the channel LEDs.

HOWEVER!

you have to ask yourself why they are using a regulator transistor there in the first place.

i mean, they could have just taken the 8 volt source that feeds the base of TR28, and connected it right to the channel display through a resistor, and then dimmed it the same way they do now, IE by adding in a parallel resistance (R173).

so why did they bother putting that big regulator there?

the reason is current!

looking at TR28, you have a constant 8.1 volt source connected to the base, you have VCC (13.8 volts) on the collector, and you get 7.5 volts out of the emitter in this configuration.

what this allows you to do is control the amount of current coming out of the emitter lead by varying the current available to the base.

transistors are used like this all the time because you can control large amounts of current by varying small amounts of current.

that is why they can use 1/4 watt resistors for R137 and R173. if they powered the channel display without the regulator, those resistors would be required to pass all the current the channel display sucks up.

so, you can do the math to figure out what wattage resistor to use in place of that jumper, or you can change the values in the base circuit to suit your needs.

one reminder though, there will come a point where you have increased the values of either R173/R137, or both, that they are having to drop so much current that you would want to step up to a 1/2 watt resistor. (ohms law is your friend here)

if i were going to use a variable resistor first, in order to figure out what exact resistance i needed, i would choose a pot the size of the ones that get chassis mounted, and not just a small trimmer resistor because of the increased current involved.

yes, i am making you do some of the work here, but i think you will be much more satisfied when you work it all out yourself and get your radio exactly like you want it.
LC

 

[red97k1500]

Thank you loosecannon, for taking the time to explain things. It has helped me understand what it is that I’m actually doing lol. And thank you to everyone else that contributed.

 

[OldTech03]

Here is simple little fundamental constant current circuit primitive as it is it works fine I have used this in many radios to give the user variable control over displays and other things but works great with LED displays. Sorry if the ink is messy I did this in a hurry. There are many transistors that would work for this the one below is one I have a bunch of in stock.

And you don't need to use the variable resistor bottom left of the image but you can use it to get the brightness you want and then sub a fixed value or you can use it..