Converted DC GP40 to DCC

[lesklar]

I have successfully converted a GP40 from my "Thunder Valley" starter set. I used a NCE N12SR decoder. I did have to mill out the motor area to isolate the motor contacts and I had to mill a good bit for the decoder to fit into a pocket. I also had to mill a little bit for wire slots to prevent any wires being pinched. Overall, this is not something I would recommend for someone afraid to cut up the loco frame.

Here is my question, or issue, the DC lights for the forward and reverse on this loco is light bulbs that fit into little slots cut in each half of the frame. The light leads were bent back to press fit into this slot and contact each side of the frame. Very clever, but cheap! I milled this out and am going to put in LEDs. The NCE instructions state to use a 1K ohm 1/4 watt resistor in series for each function lead. Does this mean I have to install 2 resistors, one on the yellow lead and one on the white lead? Or can I install 1 resistor on the blue lead (common)? Putting 2 resistors in may mean I have to mill more space (and it is already a premium).

And just to inform those who say that this was not worth effort, I spent $16 for the decoder. My track is now DCC and I am not looking back. So I thought, what the heck! As it turns out, the loco does not run to bad on DCC. I did have to tune the low end speed and limit the top speed (man this bugger could fly at top speed). I apoligize for not doing a video or documenting. I do have an old Bachmann F9A that I may try to update to DCC. I will try to document that one. This one is a good runner, but loud with the brass gears!

[Desertdweller]

I too bought the Thunder Valley set and converted the GP40 to directional lighting, but not DCC.  I replaced the bare-wire light bulbs with conventional bulbs with insulated wiring.  Installed a diode for each bulb, but did not have to modify the frame.

I think if you are using LED's, you can use only one resistor because resistors are not polarity-sensitive.

I modified the paint job on the ATSF loco to resemble the paint scheme these locos were delivered in.  This back-dating puts the loco in the time frame of my model railroad.

Les

[Joe Satnik]

Dear All,

Electrically, it doesn't matter where the series resister is in relation to the LED:

One 1k Ohm in series with the anode leg (+)----WW--->l---(-)

is the same as

One 1K Ohm in series with the cathode leg (+)----->l---WW---(-)

is the same as

One 500 Ohm in series with the anode leg plus one 500 Ohm in series with the cathode leg.  (+)--W--->l---W---(-)

But, why pay for, deal with and solder the second 500 Ohm resistor?  

Hope this helps.  

Sincerely,

Joe Satnik

Edit:  Added "deal with"

[lesklar]

Thanks Les and Joe,
I understand the resistor and non-polarity. What I was wondering, is it possible to have both lights ON and this is why the NCE instructions seem to state that a 1K must be put on "each function lead". I am going to plan on using one resistor on the blue lead, just wanted to make sure I am not missing something in how locomotives use the forward and reverse light (not a big prototypical knowledgable guy and am still learning).

By the way Les, nice name

[Joe Satnik]

LEDs are not current self-limiting, and will self destruct and take the DCC chip with them if current limiting resistors are not in series (somewhere in the circuit) with them.  (Look, Daddy, my locomotive smokes !)

I'm thinking that each function output of the DCC chip is a certain color wire lead, with a positive output voltage (~+12VDC?) when active.   

Each of those function outputs (if used) must have a series limiting resistor when  sent to (the anode side of) an LED. 

To complete the circuit, the other (cathode) side of the LEDs should go to a common return lead of the DCC chip, which is "NOT considered a function output".

Others more knowledgeable, please chime in. 

Hope this helps. 

Sincerely,

Joe Satnik

[skipgear]

You should put a resistor on each LED seprately. It is possible to have both LED's on at the same time. If you have your decoder set up for a dimmed front headlight and the rear light on when in reverse, both LED's would be on. Both LEd's on and drawing through a single resistor would cut the current limiting effect in half and burn up either the LED, the decoder outputs or both.

The safe way to set it up is to limit each and every LED with its own resistor so you know that you can use any combination of the outputs.

[lesklar]

Thanks Tony. That is what I will do so I can utilize any and all lighting procedures. I will have to make a small slot to fit the extra resistor as i only made room for one
No big deal.

[mywhitedog]

I installed a Digitrax DZ123 in a GP40, minor milling to isolate the motor, I did not use LED's I used the stock bulbs, soldered some of the xtra wire from the decoder, and slid the bulbs back into there places. The first one I did, I milled out probably like you did, motor area and a spot for the decoder, made the light area bigger....etc, did not work out so well, I ended up replacing the chassis and starting over. The first decoder was a DZ125, the DZ123 is smaller and a better fit, no need to mill out for the decoder, just isolate the motor. You would need a resistor for each LED, the instructions for the decoder should say so. The decoder fits nicely under the headlight area, I used a small piece of electrical tape to hold the decoder up off the gears below. I may have the decoders switched around, both are small, but one is smaller.

[lesklar]

Thanks whitedog! Yes I milled exactly what you stated. I mounted the decoder in the same place!. I went the LED route and milled a little more on the top for the wires and resistors to fit. Also had to open up the light holders for the LED lights. I probably milled a little more than needed, but it all worked out well.
I used a NCE decoder that does not have BEMF. The DZ125 (the smaller of the two) does have BEMF. Do you use it and how well would you think it works? My GP40 needed quite a bit of "torque compensation" (low speed kicks to keep the motor running smooth).

[mywhitedog]

I have an older GP40 about 5 years old, it runs ok... ran ok on DC so the same on both... It was an el cheapo loco, looks to big compared to the GP7, so I gave it a dirty weathered look.. it looks like it should be retired. Since my main line will be Union Pacific, and it's a Sante Fe loco, it will not be used much, probably sit off the turn table in the yard. I also have a 1970's 4-8-4 that will be used as a museum piece in a park, it's one of the worst 4-8-4's made so I have found via the internet. And I learned a lesson with EZtrack, do not glue it down! I am nailing it through the foam board, too many frog failures. I'm still trying to figure out the BEMF.. will let you know when I do.

[GSXR1000]

Hello,

I have been converting my DC Locomotives to DCC, like you I have some GP40's, GP50's and some F9's.  It would be great if you could take off the shells and take some pics.

Thanks

[mywhitedog]

One point to make here, is that not all DC loco's are good candidates for conversion. If the DC loco runs real good at low throttle positions then do it, if not, don't waste the time, money and milling effort. You will not be happy with the results.

[lesklar]

I completed my F9A and it seems OK on DCC. The BEMF seems a little hokey on this one as it makes the loco run jerky. Will try a little more tweaking of the CV's to see if it will smooth out. However, I think in the course of taking it apart I may have messed up the plastic bearing parts that hold the worm gears. Seems if I remove one of the motor shaft couplers (only one set of four drivers), the engine runs better (and softer sounding). Not sure what I need to fix or how to replace it as it is an obsolete part from Bachmann? Maybe I run it as half driven and leave it as a limited loco for service.
As whitedog stated, maybe it will just sit on a siding for a rust bucket weathering display.

[mywhitedog]

I had pulled the Digitrax decoder out of the GP40 and put it in another loco, and have since replaced it with a TCS M1 decoder, its a tiny decoder and fits nicely under the area where the headlight goes. I am also using the Digitrax PR3 and computer to program the decoders, I have it tweaked just right, the BEMF works great. The TCS decoder is so much easier to set up than the Digitrax decoder, also pulled the Digitrax decoder out of the GP7 and replaced with the M1. Being a gluten for punishment, I put a MRC 1959 sound decoder in my DD40AX, minor milling, and weight replacement in the tank where the speaker goes.

[lesklar]

Very ambitous. Any pictures or Youtube videos?

Love see and hear them in action!