P2K GP38 LED lighting upgrade issues

[NWarty]

Hey guys,

First post and fairly new modeler. I'm finishing an old undecorated P2K GP38-2 and having issues with the LED lights not coming on. I've replaced the incandescent bulbs with 1.8mm LEDs in the headlight and front number board. Each LED is fitted with a 1K resistor. These are wired to the original 8-pin mini board, which then connects to the main circuit board on top. I have an extensive background in RC and scale models, so dinking around with electronics isn't new to me.  

On my DC track (with an old Tech II), when I place the model on it and crank up the speed, both the headlight and number board LEDs, do not light. When I switch the direction to reverse, then the number lights up, but still no front light. I've de-soldered and tested both LED's, and they both work fine when hooked up to a small 9v battery. 

Is this an issue with the old PCB board?

I am upgrading to DCC with a TCS LL8-LED-KA which is en route to me in the mail. My understanding is this may solve my problem. 

Any help or advice would be greatly appreciated and thanks for looking

- Blake

[hon30critter]

NWarty

On my DC track (with an old Tech II), when I place the model on it and crank up the speed, both the headlight and number board LEDs, do not light.

Hi Blake!

Welcome to the forums and to the hobby!!       

With regard to running the locomotive on DC (assuming there isn't a decoder in it) LEDs are polarity sensitive, unlike regular light bulbs. What is happening with your number boards is that the LEDs are getting the correct polarity in reverse but the wrong polarity in forward. What you need to do is install a rectifier in between the LEDs and the circuit board. The rectifier does not change the output polarity when the input changes from +- to -+. Digikey has tons:

https://www.digikey.com/product-detail/en/diodes-incorporated/HD01-T/HD01DICT-ND/278736

The problem with the headlight is harder to explain. Theoretically it should be doing the same thing as the number boards, i.e. off in one direction and on in the other. I suspect that there may be a faulty connection or possibly one or both of the leads are connected to the wrong circuit. If you have a volt meter, I would apply power to the locomotive and see if you are getting any power to the LED  pins (you will have to hold the locomotive in place so be quick with the test). If you don't have a volt meter yet, Harbour Freight sells them for cheap. You don't need anything fancy for model railroad work.

If (when) you convert to DCC the recitifier can be removed or just left in place.

Good luck!

Dave

[NWarty]

Hi Dave,

Thank you very much for the reply!

As it turns out, the LL8 decoder arrived today, dropped in ridiculously easy and installed in about 30 seconds. And...(cue Heavenly sound)...the nose LED came on! Put it into reverse and the reverse LED came on! Yay! The only thing I can't test at the moment are the upper headlight and ditch lights. 

Phew! This decoder in the P2K Geep makes her run like a kitten even on the DC track. 

 

- Blake

[rrinker]

 If you just swapped out the incandescent bulbs with LEDs, and connected it all back to the factory board - there's your problem. The factory board has a pile of diodes on it, which control the directional feature of the headlights as well as drop the voltage to the 3 volts for the incadescent bulbs. The voltage available at the same terminals the existing bulbs were connected to may not be enough to actually light an LED, and certainly don't need a resistor as large as 1K. 1K is the value for full track voltage going directly to the LEDs - Either connect the LEDs va the 1K resistor to the red and black wires (the track pickups) or if the LEDs were connected to a DCC decoder. 

 Combined with the drop across the existing diodes, plus possibly the LEDs are simply backwards, is probably the issue.