The 602749 sound module

Table of Contents

• Preface
• Technical Details
• The Schematic
• Audio Bits
• Options for Improvement
• Turn on the Diesel Motor!
• Spare Parts

In spring 2000 I bought a modern Epoch V traffic-red class 216 diesel with high-performance propulsion, märklin number 37744. This model has a sound module built in. In its original form it produces two different horns which can be triggered by the digital functions F2 and F3.

This document describes in detail the sound board that is built into the just mentioned class 216, and into the V 160 locos of the first premium starter set, number 29845. I provide a schematics plan, audio samples to listen to, and description on how to modify the module to get other or further sounds.

Technical Details

After removing the body shell the inside of the loco looks as follows:

The large PCB (printed circuit board) you see on the picture is the sound board. The decoder is clipped on the back of the sound board into a plastic holder. The sound board PCB is a double-sided board but the rear side is a ground plane only, except for the mounting of two large electrolythic capacitors. The ground plane is probably necessary to shield off any unwanted high-frequency interference from the digital decoder.

The digital decoder is - as far as I could see and re-engineer - principally identical to the 60902, but differs in the digital function outputs. Function F1 is readily available, but unused. Functions F2 and F3 are connected to the sound board, their corresponding driving transistors are smaller types (less collector current),

Here is a photo of the decoder:

And a photo of the sound board:

The size of the sound board is: area 58 × 22 mm; height 4 mm where no elco caps are, additional 10 mm where the elcos are.

The current consumption is 12 mA idle, that means with no sound active. Playing sound t3 (see table below) draws about 140 mA at maximum volume, sound t2 draws about 110 mA. Playing sound t1, the diesel, draws about 50 mA.

The Schematic

Here is a schematic of the 602749 sound module:

click on the image to download a high resolution gif picture of the schematic.

The decoder side placeplan

The sound side placeplan

The function of the circuit should be quite clear. 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. IC1 is a threshold switch. If voltage divider R8, R9 delivers a voltage below 1.3 V to pin 6, the output pin 7 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 1.3 V to pin 3, the output pin 1 goes low, which lets T2 close the path from emitter to collector to deliver supply to the soundchip, IC2. The feedback via R13 introduces some hysteresis to make it a schmitt trigger.

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. Compared to single-ended amplifiers this increases the power by a factor of 4 (that is 6 dB), and eliminates any caps at the output. 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. Two of these trigger inputs, t2 and t3, can start either a "default" sound, or an "alternative" sound. The selection is determined by the "alt"-input, pin 10. Here, this pin is tied low via a zero-Ohm resistor. The pin is internally pulled high, so if the zero-Ohm resistor is removed, the IC switches to the alternative sounds. The other trigger inputs always play the same sound, independent from the "alt"-input. As you see, five of the seven inputs are not used here, they are externally pulled high via 47k resistors, R3 to R7. If these resistors were missing, the circuit would still work the same way. All sounds, including those not used in this loco, are listed in a table below with listening examples. If you want to quickly check what sounds are available on your own sound module, 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 pin of IC2 or equivalently the solder pad of the appropriate pull-up resistor which are marked by yellow spots on the placeplan. 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 8 and 9, 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

The following table lists all sounds stored in this particular IC, i.e. the sound IC labeled MKL 502 601 V1.2. Sounds are sorted by the input pin that trigger them. Sound examples are available as WAV-files, from my own recordings, and from märklin's German website.

The sounds of the class 216 diesel loco are t2, activated by function F2, and t3, activated by function F3, with alt=low. The F7 horn on t2 with alt=high plays a short or a long horn, alternatingly with every triggering.

All "my WAV" recordings were made from the 602749 circuit of my BR216. The conclusions which other sound-enhanced models use this IC are drawn from studying märklin's product descriptions including sound examples (the "märklin WAV" in the table) on their website and comparing and matching them with my own recordings .

The mode in the upper table says how the function is activated. Generally, when you press a function button on your control unit, you activate the function output of the decoder; pressing again, you deactivate the function output. Those sounds with the one-shot mode only play once when the function has been activated. Deactivating the function does not play anything. Sounds with continuous mode play as long as the function is kept activated. Deactivating the function lets the sound come to an end.

The mode of sound number 7, the diesel motor, is never ending. This means, activating the function lets the diesel motor start and come to idle running mode. Deactivating the function leaves the diesel sound activated! Once again activating the function interrupts the diesel motor and immediately starts with the motor starting sequence again. The only way to change between an idle and a running diesel is changing the value of the oscillator resistor to pin 6 by switching a second resistor in parallel via a transistor, to effectively raise the sample rate and thus the tone / pitch of the diesel. For details on this modification, see the chapter below on "turning on the diesel motor".

What happens when this diesel motor is running and you trigger one of the horns or bells? Well, the diesel motor is interrupted to silence and the IC switches to playing back the horn. Once the horn is finished, in some cases the diesel plays again. Precisely, the diesel is reactivated after most diesel horns and bells (t1, t2a, t3a, t3b, t6), but remains silent after the short F7 diesel horn and the steam sounds (t2b, t4, t5)! The silence after steam sounds can be broken by a diesel horn which reactivates the diesel motor at the end.

Options for Improvement

What does this mean for the hobbyist familiar with a soldering iron?

• Since there is one function output (F1) of the decoder available and not yet used, it is an easy task to wire this function output to another sound input. This is done by connecting a soundchip trigger input to the decoder function output, as with the existing two trigger inputs: see input t2 via resistor R1 and smoothing cap C1; or input t3 via R2 and C2. You simply connect the decoder function output via a 3.3k resistor in series to the according soundchip trigger pin, or equivalently to the solder pad of the pull-up resistor, R3 - R7, marked by yellow spots on the placeplan above. If you want to be really perfect you can insert a 100n smoothing capacitor which replaces the pull-up resistor, or leave the resistor and solder the cap in parallel, i.e. on top of the resistor. Usually it also works without the cap, as it does in my case.
• You can have American F7 horns instead of the German V160 horns. Simply remove the zero-Ohm resistor (marked "000") connected to IC2, pin 10. In the placeplan above, this is component 0 between C2 and C3.
• You can experiment with the tone by changing the value of oscillator resistor R21.
• You could also make a "ghost waggon", with a 60960 function decoder and this sound module and couple it directly behind your loco. This provides sound effects to trains with small locos that do not have enough space to build in any sound. The sound module has sound effects for old steam locos as well as modern diesel locos, and horns of electric locos would not sound very different.
• I do NOT recommend to play any sounds continuously over a period of time. The reason is that I do not consider the cooling means of transistor T1 to be adequate for continuous operation of the audio amplifier IC3. T1 might die from overheating! The thermal stress of T1 can be reduced by inserting a properly rated resistor (i.e. at maximum current draw the resistor has a voltage drop of about 7 V, I recommend a resistor of 47 Ohm, 1Watt) or a zener diode (about 7 V) in series to the orange power supply wire. For details see the chapter below on "turning on the diesel motor".

Please have a look at these schematics, illustrating the wiring between decoder, motor, and sound module before, and after the modification:

• original wiring before modification, Screen resolution (853 × 542 pixel, 13 kB); Printer resolution (300 dpi, 42 kB);
• variant (1): new circuit with discrete components and wiring after modification, Screen resolution (853 × 542 pixel, 20 kB); Printer resolution (300 dpi, 62 kB)
• variant (2): new circuit with PIC microprocessor and wiring after modification, Screen resolution (853 × 542 pixel, 19 kB); Printer resolution (300 dpi, 60 kB)

From the "original wiring before modification" you can see that decoder and sound are connected by four wires: f2 (brown/green), f3 (brown/yellow), Vcc (orange), and Gnd (violet). A second orange wire from the decoder goes to the front headlight, a second orange wire from the sound goes to the rear headlight.

The get a running diesel sound the resistor on the sound board determining the sampling (playback) rate needs to be varied. This resistor is R21, connected to pin 6 of IC2. By connecting a light dependent resistor (LDR) in parallel to it, the total resistance decreases with increasing amount of light on the LDR. The light is made from a light bulb connected to the decoder's motor output. So higher motor voltage makes faster sound, because of the brighter light bulb which reduces the LDR's resistance. My thanks go to RoB who has suggested this principal circuit.

Description of variant (1)

The detailed modifications are as follows. Please refer also to above schematic "variant (1): new circuit with discrete components and wiring after modification":

• make a lamp/LDR optocoupler. For example, take a rubber shrink hose, put LDR and lamp inside so that the lamp shines on the LDR surface, shrink the hose with heat, and close the open ends light-tight, e. g. with coloured rubber silicone, plasticine, or putty.
  Alternatively, make a box from cardboard: Assume your LDR has an area of 5 × 5 mm, fold a 30 × 20 mm piece of painted black cardboard into a tube of 30 mm length and 5 × 5 mm square cross section. Put the LDR in one end and close this end with a cardboard cover that has two small holes through which the LDR wires are fed. For the other end, take a precision IC socket, cut out a piece with two socket pins and file this to 5 × 5 mm area. This serves as socket for the light bulb at the other end and should fit exactly into the tube opening. This allows exchange of the bulb if necessary.
  Generally make sure that your optocoupler construction is light-tight. The LDR is very sensitive even to a very small amount of light! These optocoupler components are referenced as L91 and R97.
• connect one end of the LDR to pin 6 of IC2, or better to the end of resistor R21. The LDR's other end solder to resistor R95. After R95 follows transistor T91. Attention: Keep the wiring of the LDR from IC pin to transistor as short and compact as possible. The IC is very sensitive to induced interference (e.g. from motor wiring)!
• connect the lamp L91 to the motor output together with resistors R97 and potentiometer P91.
• connect the decoder f1 output via a 3.3k resistor to IC2, pin 21, or better to the appropriate end of R3 (yellow spot on picture above). This will allow triggering the diesel with f1.
• I recommend to insert a 47 Ohm (or lower) / 1 Watt resistor between decoder's Vcc output and sound's Vcc input (orange). This will reduce heat dissipation of the sound board's voltage regulating transistor T1, and prevent its damage in continuous operation. If you do this, however, you have to rewire the orange cable from the sound to the rear headlight, and supply the headlight directly from the decoder, not from the sound board.
• complete the remaining circuitry. The solder pads on the decoder where the cables were soldered to previously might serve as the pads on which to solder e. g. components D91, D92, R92, R94.

You can now start the diesel with function f1. As usual, with f2 and f3 the two horns can be activated, but the challenge now is to keep f2 or f3 activated only as long as the horn is playing, but turn it off at the moment when the diesel comes back.

The circuit works as follows: The brightness of lamp L91 varies depending on the motor voltage. The brighter L91, the smaller the resistance of R97. To limit its minimum resistance, R95 is placed in series. T91 usually closes the path to ground voltage, since the voltage divider R92, R93, R94 makes enough voltage at the transistor base. R97 and R95 make a "bypass" parallel to resistor R21. A lower value of R95 makes a higher maximum playback rate, but if R95 is too low, the sound chip IC2 will hang up! With P91 the slope of the playback rate can be adjusted. This means, if diesel sound increases to steep, turn up P91, if the diesel sound merely rises with higher speed, turn down P1.

Problem is that the horns, triggered by f2 or f3, also change their playback rate accordingly and sound unnaturally high. To prevent this, activating a function output ties the upper end of R93 to low voltage via diode D91, or D92 respectively. Thus T91 lacks sufficient base voltage, the LDR bypass is turned off, and the horns are played back at natural rate.

Here is a photo showing the decoder side after diesel sound conversion. Note that almost all additional components are soldered to the right edge of the decoder board.

Here is a photo showing the sound board side after diesel sound conversion. Note the optocoupler construction, tied down with a red cable knot.

Here is an audio sample after a successful conversion. You'll hear the two horns, the diesel being started, the motor starting to rotate up to full speed and proportionally the diesel sound accelerating, reduction of the motor speed, playing horns, and returning back to standstill with idle diesel sound. 602749di.mp3 (251 kB, 63 seconds, 32 kb/s, mono).

Description of variant (2)

This variant (2) differs from (1) in that some discrete components are removed, and a microcontroller, a microchip PIC 12C508, is used. The use of a microcontroller allows further features:

• the diesel can be turned off with f1
• the LDR is automatically turned off and on while horns are playing
• fewer components required

This variant has been suggested, programmed, and tested by Graham Guthrie. You should be familiar with PICs and have a PIC programmer. If not, then better use variant (1). For the modifications please refer also to above schematic "variant (2): new circuit with PIC microprocessor and wiring after modification":

• programme a PIC 12C508 with this binary code: 602749di.hex.
• make a lamp/LDR optocoupler as described previously.
• build the circuitry and connect everything as illustrated in the schematic above.

You can now start and turn off the diesel with function f1. The turn off will be rather abrupt. As usual, with f2 and f3 the two horns can be activated. Here you can forget about the previously described challenge of precise f2 or f3 timing, this is done automatically by the PIC.

The circuit works as described for variant (1). What is new is the fact that the PIC is monitoring the f1, f2, and f3 inputs. It can then control the LDR bypass through an I/O pin, which is used here in an open collector configuration. The PIC takes its supply from the emitter of T1 to operate from the permanent low supply voltage. To turn off the diesel sound, input pin 3 of IC1 is pulled low, which will then turn off T2, thus cutting power to the sound IC.

Spare Parts

You might have guessed why this page is titled 602749 - it is the spare part number of the sound board. Here are some more part numbers: