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The sound module of 34664

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photo of 34664

From Ben Haest I have got the following information on the sound module in the Belgian double traction locos of series 202 / 203. Based on my webpage on the 602749 sound module he has analysed the sound module of this loco and has made some recordings. He has generously given me his results for publication.

Technical Details

This double loco is equipped with a delta decoder and thus does not offer any remote controllable functions. The decoder employs a 701.17 chip. For details see my decoder page, where you also find links on how access function outputs.

The sound module is connected to the decoder via two power supply connections (orange = positive and violet = negative pole), and to the decoder's headlight (gray and yellow) outputs. The reason for the connection to the headlights becomes clear quickly. As you can see from Dr. König's schematics of the 6603 delta decoders with 701.17 chip (again, you find the link on my decoder page) the light outputs are driven from the decoder chip's motor outputs. I. e. the output is a pulsed voltage, and so the light intensity is related to the motor speed. The sound module needs these pulses to derive the speed information. Circuit details will be explained later.

The Schematic

Here is a schematic of the 34664's sound module:
schematic of 34664 sound, 99kB

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

I, Siegfried Grob, have modified Ben's original schematic in two details: I have drawn a connection between IC2's pin 17 (t5) and GND. I assume this from my analysis of the American F7 sound module, which has the same sound IC and requires just this connection. Secondly I doubt the value of the resistor between IC2 pin 23 (Iosc) and GND. I'd expect something around 180k. My modifications are made in green color in the schematic.

The function of the circuit should be quite clear and is very similar to the one of the 602749. 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 V, which is the case if T1's emitter output is 3.9 Volt. IC1 is a threshold switch. If the supply voltage across C5 is not too high, the voltage divider R8, R9 delivers a voltage below 1.3 V to pin 6, so the output pin 7 goes low, which activates the audio amplifier via the shutdown pin 1 of IC3. This also turns off the transistor that previously tied the input pin 3 to ground. Then, if the stabilised supply voltage across C6 is sufficiently high, the voltage divider R11, R12 can deliver a voltage above 1.3 V to pin 3, so the output pin 1 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 7 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 (here with 8 Ohm impedance). 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.

With the current configuration of trigger inputs, t1 to t8, the IC continuously plays a diesel sound. The input "Iosc", pin 23, is a modulating input that controls the sampling rate (i.e. playback rate). The idle control current is set by the 1,7 kOhm resistor, but this current can be increased by the transistor parallel to the resistor. This increases the sampling rate and the diesel sound is played at a higher pitch.

The input pulses on either the yellow or gray input pin are regenerated by the BC856 PNP transistor. These pulses are averaged by the following resistor network and the 1,0 uF electrolythic capacitor. The voltage across this capacitor varies from zero to approximately 6,8 V, in relation to the motor speed. Through the 390 kOhm resistor flows a current proportional to the capacitor voltage into the right NPN transistor marked by (*). This transistor together with the left one, also marked by (*), is a circuit known as current mirror. By these means, the current through the 390 kOhm resistor is reproduced (mirrored) as current into the left transistor's collector, and thus makes the added control current into the control input "Iosc".

Audio Bits

The following table lists a few sounds stored in this particular IC, i.e. the sound IC labeled MKL 502 603 V1.0. Sound examples are available as WAV-files from Ben's recordings.

The sound of the series 202/203 diesel loco is activated by t5=low.

Input Sound WAV märklin model märklin number
t5=low diesel motor t5a.wav
202/203 34664
t5=low, t8=low steam chuffs t58.wav rotary snow plow ?
t6=low diesel horn t6.wav ?  

Some comments from Ben on the sounds of this module:

When you lower the resistor at t2 (pin 14), from 47k to a few kOhm, the frequency of the diesel engine running "idle" increases.

Triggering t6 (pin 18) starts the horn. It is a beautiful sound. However ... after the horn, the complete sequence of the diesel engine starts again. Suppose you run the loc at a certain speed, and you trigger the horn, then the diesel engine starts again from zero to idle to running speed. I could not find a way to skip the start up sequence. Also the frequency of the horn changes with the engine speed. Of course this effect can be eliminated easily but the trick is that the horn is not a constant noise. Instead, it is a real horn with a fade-out after which the engine sound starts again automatically. Listen here: t5-t6.wav (178 kB).

So the trick to eliminate the engine speed temporarily while the horn blows does not work as one can not exactly control the change from the horn to the engine sound.

When you pull down t8 (pin 20) before you restart the sound module selects the sound of a steam loc. At idle the sound is very nice. However, when you increase the speed of the loc and the frequency increases accordingly, the sounds becomes un-natural again. It does not sound like a real steam loc running at higher speed. Listen here: t58full.wav (348 kB).

When you trigger t6 and before the sequence of the horn is finished you pull down t8 the also the steam engine sound will start. With the steam engine sound I could not find any other sounds and once the steam engine is activated, you can not switch to the diesel engine sound again. You have to reset the whole module (interrupt the power supply and start again).

Lowering the value of the resistor at t2 (pin 14) does not have any effect on the steam engine sound.

Conclusion: the module will do well for the diesel engine sound but the use of the horn is not interesting until I can find a way to continue the engine sound immediately after the sequence of the horn (without the engine start-up sequence). If any one finds a way to solve the problem I will be most interested!

Links and References