Rebuilding the locomotive from Märklin train set 26730, SBB Be 6/8 III 13 304, with Märklin 60922 mfx decoder

Updated 23.03.2013

Märklin 26730 locomotive

This crocodile came in the train set 26730, originally fitted with a 6090 digital decoder.

This locomotive already had the five-star high efficiency motor, so no modifications are done to the motor itself.  Two capacitors are removed, they were connected between each brush holder and the locomotive chassis, at the ground tag under the upper screw for the motor cover.  The ground tag is moved to the lower cover screw.

I glued in the 60922 decoder where the old decoder used to be.  The ground screw next to the decoder is installed by me.  It used to be a small plug only.  To secure as good ground connection as possible, I replaced it with a screw, after cutting 2 mm threads.  The newer Crocodiles from Märklin, e.g. the 39561, has this screw originally fitted.

The functions:
f0 = three white front lights and one rear white light, changing with the direction
f1 = interior light
f2 = rear red light, if f0 is on, changing with the direction
f3 = shunting speed
f4 = acceleration and braking delay off

Light control:
The lower left and the upper front lights are illuminated by one bulb, the lower right with another bulb, to achieve the Swiss light change, where the three front lamps and the rear lower right is illuminated.  I wanted the possibility to alter the rear lower right between white and red.  The white light is used when locomotive is pulling a train, the red light if the locomotive is pushing or running on it's own.

Front and rear light:
The lamps in both end is controlled separately with f0.  The original bulbs remains in place.  In addition, a red smd LED is installed to make the rear red light.  The bulb holder makes a good separation of the two lamps, so the red light is only visible in the lower right hand lamp.

If the light change is not synchronised, press f5.

Light control printed circuit board
I have made a light control pcb of a piece of experimental board.   This serves several purposes.  It holds the two relays and their 180 Ohm voltage drop resistors, as well as a diode to "kill" the negative voltage sparks made by the relays when the coil current is cut.  I do not know if the kill diode is really necessary.   The same decoder output is used for Telex couplers in other locomotives, with much more powerful coils than in the relays, without any diode.  But, as I use two relays, I installed the diode to be sure.
I used two relays because they were in stock.  They may just as well be replaced by one two-pole relay.

The board is also used to distribute the lamp current, as well as ground connection (the leftmost strip) and the power supply to the interior light (the rightmost strip).

The photo shows the circuit board before the first test.  After testing, due to erroneous operation of the light, the lower right and the yellow jumper was replaced by1N4148 diodes, the cathode facing right.  See the wiring diagram.

Interior light:
The interior light is controlled with f1.  One 610080 bulbs is installed on the motor, another on the contact tongue for the catenary power supply.  These two lamps are connected in series.  The contact strip between the pantographs is removed and some white paper is installed under the ceiling, for better light distribution.

Later on I will probably install a white paper also in the other end of the ceiling.  The black insulating tape is to avoid short circuit from the catenary contact tongue.  The contact strip between the pantographs is removed.  The screws for the pantograph have been replaced with Märklin part no. 750200  They have bigger heads than the original screws.

The parts I used to upgrade the Märklin 26730:
Märklin no. Pcs. Description
60922 1 Digital mfx decoder
610080 2 Bulbs for interior light
750200 2 Screws for pantographs
- 2 Mini mono-stable relay
- 2 Red smd LED
- 2 1N4148 diode
- 1 1N4002 diode
- 2 180 ohm resistor
- 2 1,5 Kohm resistor
- 1 10x11 hole experiment pcb

My setting of the decoder:
Decoder settings, 26730 Default Mine
Analogue settings:
  Enable analog mode check <
Starting voltage 7,300 <
Maximum voltage 23,000 <
Control settings :
  Swap directions uncheck <
Max speed 255 80
Min speed 4,000 <
Acc time 7,000 8,000
Dec. time 3,750 7,000
Forward trim 1,000 <
Reverse trim 1,000 <
Motor settings:
  Motor PWM-frequency
Low frequency uncheck <
High frequency check <
Load Control
Reference parameter 11,300 <
Control parameter K 60 <
Control parameter I 120 <
Control influence 255 <
Special options:
  Preserve direction check <
Persistent acceleration uncheck <
Persistent speed check <
Persistent functions check <

The decoder functions:
Front light Interior
Rear red light Shunting
End I End II End I End II
f0 f. 3 1     <    
r. 1 3 <  
f1 f.     on        
r. on
f2 f.         if f0 on    
r. if f0 on  
f3 f.           on  
r. on
f4 f.             delay off
r. delay off

The f0 and f2 light functions:
f0 f2 End I End II
Forwards on off
Reverse off

Märklin Central Station screenshot:

26730 locomotive wiring diagram:

The two upper and lower light bulbs for the front and rear light are connected as original in this locomotive.  The 1N4148 diodes between the upper and lower lamps are also originally fitted as shown in the diagram.
The 1N4001 diode on the relay board is there to kill negative voltage peaks when the relays are turned off, to protect the decoder.  Normally not required, but as f2 controls two relays I installed the diode to be on the safe side.

The home made light control pcb:

A printed circuit board is used for distribution of the wires to all the lamps.  It is seen from the copper lead side.  The drawing shows the pcb as if it is transparent.  All the components are installed on the top side (without copper) of the pcb.  The three short horizontal lines are jumpers, the two gray rectangles are the relays.  Finally there are two 180 ohm resistors, two 1N4148 diodes and one 1N4002 diode.  On the prototype in the photos a part of the lower right corner (3x3 holes) is cut off.  This is not necessary.
The left copper strip is used for ground distribution, between the three locomotive parts and the decoder.