Page created on 29/03/2024.
A1A A1A 68050.
Observation
Roco’s latest A1A A1A 68000, released at the end of 2023, finally uses LEDs, but still in the spirit of the old incandescent lamps: single source, multiple light guides. There is, however, one advantage to this arrangement: there is no electrical connection between the chassis and the body. On the other hand, luminosity is really low at the output.
In addition, there is an imbalance between left and right, due to the unequal length of the light guides and the position of the LEDs, not centred.
Examination of the circuits shows that all the resistors are 4.7 kΩ. The LEDs have a forward voltage VF of between 2 and 2.5 V for the white ones, closer to 1.5 V for the red ones. The current per LED is around 2.5 mA.
To try and correct the weakness of the lights, I’m going to replace the 4.7 kΩ resistors with 2.2 kΩ. These are SMD resistors in a 0603 package, i.e. 1.6 × 0.8 mm, so they’re not easy to handle nor to solder.
What’s more, I destroyed the front H2 LED while testing the voltages across the resistors, in order to determine the current in the LEDs. Explanation: resistors R1 and R2 are upside down. All it took was for the multimeter probe to slip, short-circuiting resistor R2 and applying full voltage to LED H2. See component identification below.
Deciphering the circuits
Lights circuit
To make the description easier, I’ve labelled the various components. The resistors have the same number as the LEDs to which they are connected.
Notes:
- The front of the machine is up in the photo.
- Key
H1: Red LED
H2: White LED, hidden under the circuit
H3: 3rd front light LED
H4: Cab LED.
Warning: the colour of the connection wires is not the same at each end.
Wiring diagram
Click on the drawing to zoom in on the lighting circuit.
The dotted lines represent the tracks located under the PCB.
Note: pads C+ and C- are connected to pins V+ and GND respectively on the decoder. This allows to connect a “stay-alive” circuit.
Drawing of the electrical diagram
This diagram explains why the 3rd light comes on by default with the white lights and goes off when a function button is pressed. When terminal A2 / A1 is activated (earthed), but not A5 / A6, the two LEDs H2 and H3 light up at the same time, because they are connected in parallel. If A5 / A6 is activated, LED H3 is subjected to zero voltage, earthed on both sides, and switches off.
That’s why I’ve written NOT 3rd LIGHT on terminal A5 / A6, to refer to the logical negation: the 3rd light is on if the control button is not pressed.
A current is constantly flowing through resistor R3, either through H3 or directly through A5 / A6. Admittedly, this current is small, but this is not very satisfactory.
It would have been simpler to follow the diagram below, in which the 3rd light is independent of the others.
One possible explanation for Roco’s choice is that, as good German people, the designers thought that the 3rd light, imposed at the beginning by the DB (German railways), should be lit by default. This obviously would be an error of judgement.
Changing LEDs and resistors
Changing the H2 LED
Desoldering and re-soldering are carried out using hot air. To prevent the component from moving, it is held in place by crossed tweezers.
On the left, the original desoldered LED.
Click on the photo to zoom in.
Aspect of the solder joints: they may appear too large, but it’s important to understand that the replacement LED is a model designed to be mounted flat. Here, it’s mounted perpendicular to the board.
Click on the photo to zoom in.
Front view.
Replacing resistors R1 and R2
I tried to desolder them using hot air. The first went well, the 2nd stubbornly refused. I’d “protected” the other components with spring clips, but the excess heat caused LED H3 to flatten out; it still works!
The wires left connected also suffered, so I replaced them too. Finally, I iron-soldered the new resistors: one end, then the other, then reinforced with a bit of solder paste. You can see that the soldering paste has created microbeads, a sign that the heating was too rough.
Results
Comparison before (left, cab 2) and after (right, cab 1). With regard to luminosity, for white lights, the improvement is quite clear even if it’s not spectacular. The imbalance is clearly visible on the unmodified side.
As for the red lights, the improvement isn’t very noticeable, even though it’s more noticeable in reality than in the photo. LEDs centred in the lights would be much preferable.
