Question......How to light up MTH LEDs in a recently purchased body shell when my chassis and boards are too old to support that feature. The chassis dates from 1996 and the body shell from right before the change to Proto 2. Can I do it and how ???? Thanks !
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Are you saying you already have MTH LEDs (from, say, a PS3 engine which uses LEDs) or are you just wanting to install LEDs into an MTH engine? Just a headlight? Or an interior light, cab light, number boards, back-up lights, etc. The more info you provide about what you have, the better answer you'll get!
First off, what electronics are in the locomotive now, PS/1, PS/2, TMCC, what? The driving electronics will determine the exact wiring for the LED replacements.
I'm talking about an MTH shell from around 2000 with LEDs already aboard (and their plugs). These are marker light LEDs, all other lighting on this installation will be provided by the L 57 bayonet bulbs....it would be neat to get the markers to work, otherwise I have to knock the LEDs out and install clear plastic for lenses and paint them on the inside.
If you're talking about the standard MTH LED's, they are two red LED's in series. Consider their operating voltage to be around 4 volts and work from there for the resistor value. If you're running with TMCC light outputs or conventional from track power, try a 1k ohm resistor and a series diode in the red lead, band point toward the LED's. That should light them fine and give you plenty of operating margin.
Of course, using LED's for the headlights would result in a much nicer look IMO, but since you are going with the bulbs...
Was MTH using LED marker lights in 2000, or did someone add these more recently?
MTH used LED markers back then, at least in all the PS/2 stuff from 2000 and on. Pretty sure they used them in some of the PS/1 stuff as well.
I have two factory PS1 locos with LED markers, and this shell is from that time period. These LEDs are wired in parallel going into the plugs. There is no LED tap on a PS1 1996 board set ! This will have to track power stepped down to whatever these can safely handle.
Look closer. 
Right near the plug, under the heatshrink, they wire the two in series. However, there are also some that are indeed in parallel, it's useful to know what you have. If you have the parallel ones, try a 470 ohm 1/2w resistor for track power. The PS/1 Premier models had a 6V power board with one or two plugs with series resistors for LED's.
My set-up is the one on the left, in your photo.
OK, they're indeed in parallel. That's an older PS/1 setup I believe.
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