So you want to install your own decoders. Actually it is not very hard and after you have done a few, you might even find it is fairly easy. But you might find your first one a little intimidating, here I hope we can help you out.
Before You Start
Find a nice clean area to work on, one with good lighting. One that
you can lay out parts as you remove them and not get lost. You will also
need a few basic tools and items. Small flat and phillips blade screwdrivers.
Tweezers and and a hobby knife are handy. A small pair of wire cutters
and a good pair of wire stripper. Read good as in a quality pair that is
sized for the small wires. A good soldering iron around 25-35 watts, a
temperature controlled one is nice. Good quality rosin core solder of about
1/16 inch diameter or smaller. Heat shrink tubing of assorted sizes around
1/16 inch diameter. Some tape both electrical and double sided foam tape
come in handy. A magnifier or pair of loops really helps. And all the paper
work that came with both the decoder and the loco. If you did not get any,
talk with your dealer and/or manufacturer and ask where it is at.
Basics
There are a few things you need to find out about the loco you plan
to install a decoder in before you start, or even buy the decoder for it.
If it is a new loco, it is always a good idea to see how it runs and break
it in for a little while. You can do this on a DC track or you can use
the analog mode of your DCC system.
One of the first things you need to find out is how to get the shell off. One most locos this is a fairly simple project, but on some it can take a little figuring out. If this is not a simple hold the fuel tank and lift thing, then look in the instruction manual supplied with the loco if there is one. No manual, then look around the underside of the loco for screws, tabs, ect. Some locos you will need to remove the couplers, ect. On some locos all you have to do is lift the rear and slide forward on the shell. Others have little tabs on the side above the tanks that you need to pry out the sides of the shell.
Now you have the shell off, the next stages depends on what you find. Does this loco have a PCB [printed circuit board] or light board as called some. Is this loco DCC ready, that is does it have a NMRA DCC socket. Or may be it has none of the above. But first for all except the PNP decoders you will need some more information about this loco. First thing you might just want to do is contact the manufacturer of your favorite decoders and ask them if they have a recommended decoder for your loco.
You will need to know what physical size of decoder you can fit in here.
Manufacturers make Z, N, HO, Large scale decoders, but these do not mean
that they can not be used in other scales as long as they can supply the
needed current for your loco motor. But what you need to know now is the
physical size and what will fit inside the loco. You can find the sizes
of decoder right from the manufacturers web pages, look in the links pages
here to find their URLs. If need be you can make mockup decoder from wood,
putty, clay, ect to the physical size to help find room in the loco. Some
of the newer locos on the market that are tight in space, the manufacturers
have helped by making removable sections of weights, look close to see
if your loco might fit this category. Most HO and larger scales space is
not a problem for the little decoders. But in the smaller scales Z and
N, it seems even the little decoders are hard to find space for.
But usually with a little thought you will be able to find some place.
But if it is really tight there are manufacturers of replacement frames
that you can use. Such as Southern Digital
for Nscale locos and others. If not then you might have to do a little
machining on your frame to make room, or as a last resort you can put the
decoder in a dummy unit or an attached car or tender. Now we need to find
the locked rotor current [sometimes referred to as stall current] for your
loco. This is needed to see if the decoder can handle you loco motor. There
are several way to go here, and a couple of things you need to know. First
you can use the rule of thumb method, that is for Z and N scale at 500mA,
and HO at 1 Amp. For most of the newer locos on the market and the older
ones that have been re-motored this will usually work. The next and best
way to me, is find the actual locked rotor current. You can find this by
looking in the magazines, they usually publish the stats with the loco
reviews. Looking around on the net for pages that publish decoder installs,
such I have here under projects. Or if you are going to do many installs,
figure it out yourself. To do this you need a 12 volt DC supply, a amp
meter and voltmeter. If you have a power pack with both meters you have
it made. And a section of track to put the loco on. Grab the loco so the
motor can not turn, usually the 
flywheel
is the easiest. Set the voltage to 12 volts. Holding the loco so the motor
can not turn, put it on the track and turn the volts back up to 12 volts
if need be, read the amp meter then quickly remove the loco. You now know
the locked rotor current [stall current] of your loco.
With all the above information you are ready to select the proper decoder for you loco. But there is one more thing you need to know. That is how does the manufacturer rate the decoder current. This varies from one to the other. Some just give you a rating without stating if it is constant or peak. Others just give you the peak rating. Some separate the current from motor drive and functions, others just give the total rating for the decoder, in other words what is left for the functions is minus the motor draw. Here you will have to sort this out yourself and/or ask the manufacturer of you are not sure. The safest way to go, is error on the low side. Here you select a decoder that has a constant current rating equal to or higher then the locked rotor current of your motor. Next is again a rule of thumb. Here you select a decoder that the locked rotor current of your motor falls halfway between the constant and peek current of the decoder rating. ex: If the locked rotor current is 1.5 Amp. select a decoder with a 1 Amp. constant and 2 Amp. peak current rating. Next is the type of protection offered by the decoder, there is thermal, current and overload. Some decoders are almost indestructible, and will actually sit on the rails and blink their lights letting you know there is a problem. With the protection offered in any case, if you should happen to select a decoder with to low a current rating. All that should happen is after a period of running, the loco will stop till the decoder cools down, then resume again.
Ready to start
Now that you have selected a decoder and are ready to start, you really
should test the decoder. There are several reasons for this. One is that
all manufacturers have a warranty on their decoders. But with some it does
not cover mishaps from installations. Besides it is always nice to know
that the decoders does work before you start. If some thing does happen,
without testing the decoder you will always ask your self, did I do something
or did that decoder come that way. Some manufacturers even supply a basic
decoder test kit with each system you buy, such as Digitrax.
Or you can buy a decoder tester that will test the whole decoder, such
as the one
Loy's Toys makes. Another option
is to build one yourself, I will show you how on the projects page.
Once tested you are ready to start. Here neatness really counts. Route
your wires so they are out of the way of any moving parts, such as flywheels,
shafts, universals, ect. Solder all wire joints and use heat shrink tubing
on all joints. It is neat, small and adds strength to all joints. Use tape
to hold wires in place if need be. And double sided foam tape is great
to hold things in place, like the decoder, etc.
PCB with a PNP Decoder
This might be the easiest install of all. Today several manufacturers
are making PNP [Plug & Play] decoders for a lot of these manufacturers
locos. Digitrax and others make
these, some are solder less others require minimum soldering. These decoders
are replacements for the existing PCB, and have the decoder built right
on the replacement. Just remove the existing board and install the new
decoder board in it's place. So by all means check with you favorite decoder
manufacturer and see if it looks like they make a board that will fit this
loco. Even if it is not stated that it is for this loco, look
the decoder or pictures over and see it might fit, especially in the smaller
scales. If it looks close and you are not sure, contact the manufacturer
and ask them. This is the way to go today.
NMRA DCC Socket
If the loco is called DCC ready, then it will have the NMRA socket
to plug your decoder into. These sockets come in three different configurations
pending on the scale of the loco. A small (1x6) 6 pin for Nscale
or larger, Medium (2x4) 8 pin for HO or larger, and the large 4 pin for
O gauge or larger. The first thing you will see is a DC or jumper plug
that is in the NMRA socket. This is so the loco can operate on a non DCC
system. You will need to remove this jumper plug. The next step, you should
always check to make sure there are no shorts 
between any of the pins on the socket. If there are and you plug in
your new decoder and put it on the rails, you might loose that new decoder
is a small puff of smoke. The loco manufacturer will replace the bad board
for you, but this will do nothing for you blown decoder. To check for shorts,
just take a Ohm meter or continuity tester and check each and every pin
against the other pins, you should find no dead shorts any where. You might
find a resistance due to diodes, resistors, ect on the board, but no dead
shorts. If you find any, you will have to either get another board/loco
or remove the short your self. These are usually caused by a solder bridge
on the under side of the PCB, or a stray wire connected to the PCB from
something like the motor connection, ect. If you are lucky the loco manufacturer
has marked pin number 1 for you. If not do not worry, if you plug in backwards,
there is no smoke. If the loco runs, but the lights do not work, or things
seem backwards, just remove the plug and put it in the other way.
| Here is a color chart for the
NMRA DCC Socket.
Pin 3 of the medium socket.
|
|
None of the above
Ok so may be your loco has a PCB but you can not find a PNP decoder
for it. Or may be it does not even have a PCB, like a typical Athearn loco.
All is not lost, just means we have a little more work to do, but alas
tis not all that bad. Again you will need a little more information about
your loco. First thing that needs to be decided is if there is a PCB do
you want to use it, or remove it. In either case PCB or not, you need to
find the leads to the motor brushes. The motor must be totally isolated
from the rest of the loco. Even if the manufacturer says the motor is isolated,
or you read it some where. Check to make sure it is. Do not take any ones
word for this. If it is not, you will more then likely see that puff of
smoke from the decoder. Not a happy thing. If you see two leads going to
the motor, then more then likely it is isolated, if it is not it will be
fairly easy to isolate it. If your loco has a PCB and you feel you want
to keep it, then you will have to do a little figuring or back building
the PCB to see where the motor lead traces go. If there is no PCB and there
are two leads, or you decide to scrap the PCB, cut both leads off from
their connecting points and be sure to leave enough to solder the decoder
leads to. Now take a Ohm meter or continuity tester and check both leads
to make sure they are isolated from anything, wheels, frame, everything.
If there is only one lead, you can almost be assured the other is connected
to the frame somewhere, this is an absolute no no. Here your job will be
to find this connection to the frame and isolate it and solder a wire to
the motor brush and bring it up to where you can solder a decoder lead
to it. This can be simple or it can be a pain. In either case you will
more then likely have to remove the motor from the frame. Again, if there
is an exploded view of your loco in the directions sheet supplied with
it, you can usually figure out the motor removal easy enough. The point
here is after the motor is out, find the contact to the frame, cut it,
remove it, use tape or something to isolate it on the frame. Solder a wire
to the motor at this point and bring it up to where the decoder lead can
be soldered to it. After you have done this, check it with a meter again
to make sure both leads are in fact isolated. To the motor lead that was
connected to the right rail pickup you will solder the decoder Orange lead.
To the motor lead that was connected to the left rail pickup you will solder
the decoder Gray lead. Now you need to find where the power pickups
are, again on the PCB find 
the
points where the right and left rail power is connected. Or if you are
not using the PCB or there is none, then find the wires coming up from
the trucks, or the frame contact points. Here you will be soldering decoder
leads. To the right rail pickup you will solder the decoder Red lead. To
the left rail pickup you will solder the decoder Black lead. At this point
you have completed a basic install and if you like you can test your install
out so far.
You Want Lights
Ok everything's gone fine so far, now you say you want lights. Here
you have a lot options. If you are going to use a existing PCB and it already
has lights on it or connected to it, all you have to do is find the pads
on the board where the lights are connected and isolate these pads, cut
the traces, remove jumpers, what ever it takes. You will also need to know
the current limit of the decoder functions and make sure you do not exceed
them. The decoder Blue lead is the function + common, the White lead
is the front lamp and the Yellow lead is the rear lamp. But you need to
find what lamps are used, or are going to be used. Are they LEDs, or bulbs,
if bulbs what are their voltage and current draw. Do you want to use 1.5
volt lamps with a LM317 voltage regulator, or dropping resistor. Do you
want to wire them up as a full wave or half wave style. Also there are
the new White LEDs which are showing great promise as lamps, they are bright,
actually cast a light, and draw very little current compared to lamps.
Any way all have their good points and bad, the choice is yours, based
on preference and/or what is already there. Here I will give you some starting
points on resistor values for the different lights, but these will have
to be adjusted for your particular system, system voltage, lamp current, etc. Just remember higher resistor values means a dimmer light, and
a lower value means a brighter lamp. The idea is to use the highest value
resistor, keeping the lamp dim as possible, to increase the life. But still
be bright enough to keep you happy. All resistors here are 1/4 watt. 
Starting with LEDs, start with 680 Ohms. 1.5 volt grain of rice,
start with 560 Ohms. 1.5 volt grain of wheat, start with 250 Ohms. For
lamps of 12-16 volts no resistor is required, but when a cold lamp lights
it can draw up to 10 times it's rated current draw. As an example a 12
volt 40 mA lamp can draw 400 mA when first lit. This can and does exceed
most decoder function ratings for a short period. Though it most likely
will not take a decoder function out, it is putting a load on it, and over
a period of time will take it's toll on this function output. So it is
always a good idea to use a resistor to limit the maximum draw it can take,
here a resistor of around 47 Ohms will do the trick, and not dim the lamps
by much.
You Want Functions
These are just as easy as the lamps, in fact they are the same. Think
about it a second, lamps are nothing more then a function. All we are doing
is replacing the lamps with some other accessory. A relay, coil, smart
wire, ect. All current limits hold true here. Also you may or may not need
resistors, pending on decoder and what you are controlling. You will also
want to add the anti-feedback diode across any relay or coil.
All Done
Just about, you have your loco and decoder all wired up and ready to
go. STOP. I know you were very careful followed
the manufacturers direction, double checked everything. But the next thing
you want to do is test the install. You can do this by putting the loco
on a current limited program track. Such as a service mode track, a off-line
programmer track like the PR-1, ect. Check the install by seeing
if you can read the default CVs of the decoder. If you can do this then
you can be pretty sure your install is OK to the point that the decoder
is safe.