Why change from 3-rail AC to 3-rail DC system?

Originally being a Märklinist, one will sometimes recognize that 3-rail AC is a rather limited world and that there is more outside - 2-rail DC.

But there are many Märklin locomotives in your collection at home and you do not want to sell them (either because you won't get the desired price, or because you are in love with the Märklin models, or... )

So you think of combining both system. Wagons are no problem as many manufacturers change the wheel sets at no extra cost with AC compatible ones when you buy a new wagon. Similar with couplers, thanks to the standard NEM pocket you can use any coupler that fits.

But there are some problems to combine both systems, technical and financial problems. You could try to buy rebuilt versions of the DC locos you want. This is normally rather expensive (approx. 25% more expensive than the same 2-rail DC model) and some locos are not sold as AC versions. On the other hand a 2-rail DC loco right out of the box will never run on a Märklin system.


Some obvious problems with 2-rail DC locos on Märklin tracks

1. Both rails are not isolated in the Märklin system. So how to transport the current to the loco?
    
2. The motor normally is one with permanent magnet, so it will not be able to cope with AC.
    
3. The inner distance between the wheels is 14.3mm (NEM) versus 14mm (Märklin)
    

Solutions to these problems

1. Put a slider under the loco and do a re-cabling inside the loco.
    
2. Either use DC for your power supply or any kind of (electronic) reversing relay. Let's forget about DCC for the moment, as I personally want to stay analog.
    
3. Either exchange the wheels or gently press them to the desired distance.
    

Nice, short and easy.

Easy?

As always, the devil turns up in the details. Especially the problem 2) is
a harder one. Electronic relays are rather expensive. And: Especially in
older DC locos there is no room for installing them.

So, what to do?

Why not try 3-rail DC? On Märklin tracks of course. We will not need the relay for the DC model, so less expensive and no room problem.


Let's see what problems arise from here at the Märklin side

We take a standard Märklin transformer and put a rectifier, a pole-reversing switch between transformer and track. A standard Märklin model of the past (3xxx or 33xx) has an all-current-motor, so it will run as good when fed with AC as it will do with DC. Of course the loco will run in the same direction, when you reverse the current with the pole-reversing switch. But of course you can reverse the Märklin loco by the normal 24V pulse. Märklin locos with Delta module will only run, if the direction of the current is right. So by reversing the current the loco will stop. But here also the 24V pulse will reverse the direction (when the direction of the DC current is correct, of course).
I must confess, that I haven't tested DCC models by now, but I expect them to show the same behavior as Delta models.

So, basically there are no (real) problems using DC for Märklin models.

Now the next obvious question pops up...

Why not trying to use the reversing of the DC current for reversing the direction of the loco?


How do I convert a 3-rail Märklin AC loco to a 3-rail DC loco?

As we saw above, problems arise with Delta (and DCC ?) models. So let's start with old standard models. The reversing relay is to be dismounted. De-solder the solenoid of the relay from the main soldering point, then de-solder both cables coming from the motor. Screw off the relay and put it away. Now solder two diodes (type 1N4001 or higher) to the cables of the motor, one to each of these cables. Notice that one diode has to be soldered in one direction (so the ring signing the diode must face the cable), the other in the other direction (the ring facing the outer end). Now solder both "open ends" of the diodes together, bend the legs to a small ring and screw this to the frame of the loco, e.g. at that place, where the relay was before.
The entire rebuilding will take less than one hour for someone who has at least some experience in soldering. And it will cost less than one dollar.

The Märklin AC motor wiring


The motor wiring after the conversion



Now put the loco back to the tracks and try again. The loco will run with DC (but of course no longer with AC). If you switch the reversing pole switch, the loco will reverse the direction. So everything seems to look like normal 2-rail DC.

But there is one difference, and it has to be kept in mind...

If you have a 2-rail DC loco running e.g. running from left to right, then you take the loco of the tracks, turn it around, put it back to the tracks, it will still run from left to right. When there are two 2-rail DC locos on
one track they will always run in the same direction (forget about Kleinbahn locos for a moment.
Now to the 3-rail DC models. Same situation: Loco runs from left to right, you turn it around. and now the loco runs from right to left! So if you have two 3-rail DC locos on one track they can run in opposite directions (having a "nice" crash).

Can we live with that? I personally do so for more than 15 years.

Back to our more problematic Märklin locos:

1. Delta/DCC
    
2. TELEX locos.
    

I personally do not need the electronics in a DCC or Delta model. Remember I want to stay analog. So I get rid of the electronic the same way as with the old mechanical reversing relay - de-solder and dismount it.

TELEX locos keep there mechanical relay for triggering the TELEX solenoid, but the cabling of the motor has to be done in a different way, if you want to reversing the loco by reversing the direction of the supplying current.

So with a little work, we can have our entire Märklin stock running on 3-rail DC.

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Telex wiring will be described later by the author. (Webmaster's comment)
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What have we gained so far?

Having switched to DC on Märklin tracks, we can have a look at "the new world of DC models".

How do I convert a 2-rail DC loco to 3-rail DC?

The easiest way is to buy HAMO locos, remove the current pickup, which contact the wheels on one side, install a standard Märklin slider instead, and we are done. HAMO locos are normally about 10 to 20% cheaper than the comparable Märklin AC model (at least on swap markets here in Germany)!

HAMO motor with slider


But since we now have the entire world of 2-rail DC locos in sight, we would like to adapt them to our 3-rail DC system too....

The theory of converting a 2-rail DC loco to 3-rail DC is rather simple...

1. Connect both sides of the wheels electrically (e.g. by soldering a short cable to the current pickups). And to ONE side of the motor.
    
2. Fix a slider underneath the loco.
    
3. Fix the slider electrically to the second pole of the motor.
    
4. Ready.
    

As stated before, the devil is sitting in the details... All in all, one has to say that there is never an easy way. This is nothing for total beginners. You should be at least familiar with basic techniques/concepts of electrical wiring.

AND: Always use the appropriate tools, since it will help you make fewer mistakes... 

The practical issues sometimes make it more difficult...

1. Sometimes it is rather difficult to follow the electrical lines inside a loco. Either because it is done in a rather complicated way, with lots of cables and platinas, or because the current pickups are "hidden" deep down in the frame of the loco, and you are only able to reach them by destroying (excuse me: disassembling of course) the loco completely. I won't hide, that here a lot of experience, and trial-and-error will help. There is no standard way to do this. Every loco is different and you will always have to examine it carefully to find how to do it best way.
    
2. Another problem: Many DC locos have the gear wheels at the middle of the axle, so they are blocking the slider in a way. Again there is no standard way of coping with this problem. Best is, to have a look under the loco BEFORE you buy it.
   
   Some clues: Try to use standard Märklin (or Roco) sliders. Try to screw them to the frame. If no other way glue them to the frame.
   
   Use special self constructed sliders only when there is really no other way using a standard part.
   
   When rebuilding locos with powered bogies, you may have to "de-power" one bogie, take off the gears, and fix the slider. The traction effort will decrease, but normally it will still be enough for "normal" use.
   Have a look for different positions: May be fixing the slider to the tender is a solution.
   Nearly always you will have to cut away some of the lower part of the frame/chassis of the loco.
   It is most important, that the slider has enough room to move up and down, e.g. when crossing turnouts.
   
   AND: Check, whether the slider has to be mounted electrically insulated of the frame or not.
   I personally have not found any DC model so far, that a slider could not be fixed under... From the nice draisin from Bachmann, to the Köf from Roco, from the BR62 from Liliput/Bachmann via the BR 82 from Piko to the 215 from loco etc...
    
3. Fixing the slider electrically to the motor normally is no big problem, when we reached this point.
    

More things to consider... 

Of course, there are some other problems, and one should not hide this... 

The inner distance between the wheels is 14.3mm with NEM and 14.0mm with Märklin. Only 0.3mm, that make a BIG difference. NEM locos are most likely to derail at the frog of Märklin turnouts. Sometimes it is possible to push the wheels on the axle to 14mm Märklin norm distance. Carefully of course. Also the flanges of the wheels have a slightly different shape than Märklin's. Again a thing, which tends to derail the rolling material.

Many NEM locos are manufactured to run only on large radii, larger than 360mm. The best way is to get rid of Märklin M tracks and use K tracks (not having tested C tracks so far, but this is due to a different reason.). Try to avoid the old 424mm K turnouts, and use the new 900 mm turnouts wherever possible.

So there are some problems, and you have to ask yourself, whether you want to live with these problems or not. But I think it is worth giving it a try.