Converting Märklin 3106, DB 78 355, to digital using the conversion set 60760 and an mfx decoder

Updated 06.06.2013

The conversion set 60760 is produced by Märklin.  It is a set with a powerful 5-pole Märklin motor and a simple decoder, remotely programmable.
Another option is to use a motor kit, no. 60941 (same motor as in set 60760), and a separate mLD decoder 60942 (including a circuit board with six function outputs) or mLD decoder 60962, with wire ends and four function outputs. mLD means märklin Locomotive Decoder.  See the Märklin web site for detailed information.
The mSD (märklin Sound Decoder) 60947 (with the circuit board) or 60967 (with wires) may of course also be used, if there is space for the loudspeaker.

This conversion is as if I used the Märklin mfx conversion set 60921, which is no longer produced by Märklin.

Märklin 3106 locomotive, DB 78 355:

The locomotive has a Drum Collector Motor (DCM) and a mechanical reversing unit.  I am rebuilding it to digital, with an mfx decoder.

The locomotive inside before disassembling:

The locomotive inside after the conversion:

Very simple to explain: remove everything.  After removing the pick-up shoe, carefully remove the plastic part under the locomotive, including the brake shoes.  Unsolder the wires to the pick-up shoe contact and to the ground point. Unscrew the two screws on the motor cover, the screw holding the reversing unit.   Then it should be possible to "lift out" the reversing unit, the electronic part and the motor in "one piece".  I carefully pack these parts in a small plastic bag and store them.

I also removed the front and rear leading axels and couplers, to be sure of no accidents during the work.

Cleaning the locomotive's rotating parts
The next step was to clean the locomotive thoroughly, to remove all dust and old oil.  I use dish washing detergent, an old toothbrush and as hot water as I can handle.  After cleaning and brushing, I rinse very well in hot water.  I remove the friction tyres before cleaning the wheels.  Then carefully oil the rotating parts..

Installing the motor:
No problem at all.  Install the new magnet and enter the rotor.  Be careful to hit the inner bearing.  Install the motor cover and secure it with the two 2x12mm screws.  Add a tiny drop of oil on each rotor end.

Installing the decoder:
I decided to use a ESU V3,0 M4 decoder, due to it's reasonable price, programmability and mfx protocol. This was before the Märklin mLD decoder was available.
The decoder is installed in a decoder holder, Märklin part no. 456200, where the reversing unit used to be.

In this case, the wires are a mix of Märklin colour codes and ESU colour codes, which I normally don't do.

Ground point:
I used the screw for the valve gears with a small soldering lug, as a ground point.

The noise suppressor circuit board

I made a small circuit board, to distribute the power to the motor (via the inductors) and the common "plus" to the functions.

The decoder functions:
function 3106, DB 78 3557
f0 f. Light front end
r. Light rear end
f1 f. Light, driver's cabin
f2 f. Not connected
f3 f. Shunting speed
f4 f. Acceleration and deceleration
delay off

Front and rear lights:
original bulbs were 600150, installed directly in the locomotive chassis.  From earlier conversions, I had some bulb sockets in my spare part box.  The fitted perfect in the original bulb holes.

Front light:
I used the original bulb contact as a wire distribution point for the short gray wire from the bulb socket.

Rear light:
For the rear light I installed a small board, as a distribution point for the common "plus" and for the gray wire from the bulb socket.


Cabin light:
A 28 Volt T1 bulb is used for the cabin light.
A two-pole bulb holder is glued to the top of the motor, next to the magnet.

Part list for the conversion:
Part number Pcs. Description
60760 1 Digital high propulsion conversion set, motor and small parts only
276770 2 Bulb socket with wires, from an earlier conversion set 609?3 or 609?4
456200 1 Holder for decoder
756100 1 Screw for decoder holder
ESU 61600 1 Loksound V3,0 M4 decoder with wires
- 3 28 Volt T1 light bulbs
- 1 2-pole bulb holder, cut from a SIL connector
- 1 Home made experimental circuit board, 3 rows, 3 holes
- 1 Home made experimental circuit board, 2 rows, 3 holes

My setting of the decoder:
Decoder settings, 3309 Default Mine
Analog settings:
  Enable analog mode check <
Starting voltage 15,500 <
Maximum voltage 25,500 <
Control settings:
  Swap directions uncheck <
Max. speed 255 95
Min. speed 3,000 1
Acc. time 7,000 8,500
Dec. time 5,250 8,500
Forward trim 1,000 <
Reverse trim 1,000 <
Motor settings:
  Motor PWM-frequency:
Low frequency uncheck <
High frequency check <
Load Control, 5* Märklin motor:
Reference parameter 12,500 11,300
Control parameter K 32 60
Control parameter I 24 128
Control influence 255 <
Special options:
  Preserve direction check <
Persistent acceleration uncheck <
Persistent speed check <
Persistent functions check <

The 3106 function table:
3106 DB 75 355
mfx Light Light
funct. dir. front rear
f0 forward 3          
reverse   3
f1 both     on      
f2 none            
f3 both         on  
f4 both            delay off

The wiring diagram:

C1 = noise suppressor circuit board, C2 = rear lamp circuit board, C3 = front lamp contact tongue

Central Station 1 screen shot:

Future improvements:
I will install close couplers with coupler holders Märklin part no. 286900 and repair the broken hand rails on the front end.

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