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The class 18.4 (Bavarian S 3/6) sound module

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End of 2001 märklin offered their second Premium Digital Starter Set, number 29855, that contains a class 18.4 steam loco, and a class E 40 electric loco. Both locos have sound modules built in.

On this page, the sound module of the class 18.4 is analysed. This sound module is configured to produce a steam whistle (triggered by digital function F2) and bells (digital function F3). Below I present a schematics plan.

This sound module has the same sound IC as the sound module of the V160. Also, the circuit is nearly identical. I have used the same component labels in both schematics. So, with this sound module you can do all the modifications that are also possible with the V160's module. For audio samples and descriptions of modifications please refer to that page.

Technical Details

The digital decoder in this loco is identical to the 60902, but instead of only two functions it contains another transistor to offer three digital functions: f1 for the smoke unit, and f2 and f3 for sounds.

Here is a photo of the sound module's printed circuit board (PCB), component side:
photo of sound pcb,

And a photo of the solder side:
photo of sound pcb, 27kB
fotos courtesy of Hanno Brüninghaus

It is a double-sided PCB with components on both sides. The size is approximately 74 mm × 32 mm × 14 mm.

The Schematic

Here is a schematic of the sound module: schematic of 18.4 sound, 32kB
click on the image to download a high resolution gif picture of the schematic.

The component side placeplan
decoder, 39kB

The solder side placeplan
placeplan sound, 41kB

The circuit is nearly identical to the one from the V160. New is the use of two small, separate voltage detector ICs, IC1 and IC5; and the change of R17 to 27k. The input pins of the sound IC are connected differently. Component R14 is missing on my board (by mistake?), which does not matter under ordinary operation conditions.

Supply comes from J1(Vcc) via diode D1 into buffer capacitor C5. The components T1, R15, IC4, R20, R16 build a voltage regulator. IC4 is a "configurable zener diode". It adapts its cathode voltage such that the control pin (to the right) has 2.5 Volt, i.e. the voltage divider R20, R16 provides 2.5 Volt, which is the case if T1's emitter output is 3.9 Volt. If voltage divider R8, R9 delivers a voltage below 3.9 V to the voltage detector IC5, its output pin goes low, which activates the audio amplifier via the shutdown pin 1 of IC3. So, if the supply voltage rises too high, the audio output is shut off (probably to avoid thermal damage of T1). Similarly, if voltage divider R11, R12 delivers a voltage above 3.3 V to voltage detector IC1, its output pin goes low, which lets T2 close the path from emitter to collector to deliver supply to the soundchip, IC2.

IC2 is märklin's sound IC. Pin 4 is the audio output which is not lowpass filtered but outputs a "staircase" signal. The sound goes through the potentiometer P1, decoupling cap C7, through a lowpass R17, C11, to the amplifier IC3 which provides bridge outputs to the speaker. The components R18, R19, C8 form another lowpass filter. This becomes clear by considering that IC3 is an inverting op-amp from input pin 4 to output pin 5.

The soundchip, IC2, produces audio output at pin 4 after it has been triggered by one of the trigger inputs, t1 to t7, pins 7-9 and 18-21. These pins are internally pulled high, and activated by driving low, i.e. applying zero (ground) voltage at the input. Unlike the other unused inputs t2, t3, t6, t7, the input t1 is also externally pulled high via a 47k resistors, R7. If this resistor were missing, the circuit would still work the same way. If you want to quickly check what sounds are available on the sound IC, simply take an ordinary resistor, value in the range 3k - 22k, hold one end with your finger, and with the other end touch the appropriate trigger pin of IC2. Your body capacity will pull the appropriate pin to low voltage. Of course, the loco must be on powered track.

The connected input pins, pin 18 and 19, are routed via a 3.3k resistor, R1 / R2, to the decoder's function output. A cap to Vcc, C1 / C2, lowpass filters the trigger voltage to prevent false triggering from spurious voltage spikes potentially occurring on the line. Resistor R21 sets the sampling rate (i.e. playback rate). Lowering the value increases the sampling rate and all sounds will come in a higher tone, higher pitch, and vice versa. It is as if you were accelerating a vinyl record on the turntable. Audio examples indicate that märklin also uses the trick of playing sounds at slightly different rates to offer basically identical sounds in different models; listen to the sound bits below.

Audio Bits

This sound module can play more than just the two whistle and bell sounds. The available sounds are determined by the sound IC. Here it is a MKL 502 601, which is also on the sound module of the V160. For a table with all sounds please refer to that page.

Possible Modifications

This sound module has the same IC as the sound module of the V160. Also, the circuit is nearly identical. I have used the same component labels in both schematics. So, with this sound module you can do all the modifications that are also possible with the V160's module. For a description of modifications please refer to that page.

Building into the tender of a BR 44

I have built this sound module into the tender of a class 44 steam locomotive, no. 37880. I have replaced the decoder by a 602758 - a 60902 equivalent with two functions. One is for the smoke unit; the function f2 I have used for the steam whistle. The width of the sound module is perfect for the tender's upper shell, the length, however, is too long. I have cut off the amplifier part at the notch, the right part in above pictures, and reconnected the two parts via cables. The sound module is connected to the decoder via three cables: the orange and violet cable for supply, and one for the whistle trigger. Decoder connections should become clear from the graphic.
connections, 14kB
original foto courtesy of Hans-Günter Heiserholt