I have installed several of the LED lights in Athearn F7's using the Digitrax DH123P decoders. I use anything from 750-1K ohm resistors and have had no problems. I installed the resistor on the blue wire for the headlight. I believe the yellow is for the reverse light.
On the 6V bulb I have no idea what you need but you do need to use a separate resistor for each bulb.
If you haven't purchased the DH123AT yet I would advise you to save a few dollars and buy the standard DH123P. I found that the DH123AT was harder to install because you have to remove the copper motor contacts and attach the ones from the decoder. You chance losing the motor's brush spring when you take the copper clip off the motor.
The wires they have soldered on the copper spring clip retainer broke off so I had to solder them on any way.
I turst my solder joints so after purchasing the first DH123AT it was the last.
This is the installation I did on the first Athearn F7 with the DH123AT. It may help a little.
http://good-times.webshots.com/album/552513071iZyOrv?start=0
Bill
I am installing a digitrax dh123at decoder and im using a 6 volt 25ma bulb and a 3 volt 15ma led. Do I need to install a resistor on either or both. Does it matter which wire I install the resistor on? Any help would be great as the instructions are very vague. Thank you.
Stevin