The 66031 was the märklin DELTA decoder offered from 1997 to 2002. Even though in 2001 märklin has still been building delta locos (e.g. the class 151 from the 29645 set) with older 6603 delta decoders that have a 701.17 chip. Here I present the schematic of my 66031 delta decoder.
The schematic is also available as PDF-File.
The component placement plan:
Some components are labelled X?, these are parts with reserved space on the PCB but which are not actually placed on the PCB. In the placement plan these are the red boxes. Thanks to Andreas Kohout, I got information on these components. On his decoder the IC1 is a 701.21A (mine is a 'B'), and the replacement number of the whole board is 602 004. The missing components are as follows:
|X1||resistor: 150 k|
|X2||resistor: 150 k|
|X3||resistor: 27 k|
|X4||resistor: 56 k|
|X6||zener diode: 3.9 V|
|X7||zener diode: 3.9 V|
|X8||resistor: 10 k|
|X9||resistor: 100 k|
|X10||dual P-channel MOSFET: 9948, e. g. Fairchild NDS9948|
|X11||same electrolythic capacitor as C2: 10 uF, 10 or 16 V|
If the MOSFET X10 is placed, the Solderbridge (to the right of X9) on the PCB is left open, of course. The purpose of the additional circuitry seems to be to apply positive voltage to the black and orange cables only if the supply voltage (across C1) is high enough. In this case I would expect the threshold at 2.5 V (X10 drain-source voltage, also across X9) + 3.9 V (X6) + 0.25 V (across X8), which makes a total of about 6.6 V of minimum supply voltage. However, I wonder about the reason for this protection circuitry. Maybe IC1 is faulty at low supply voltages and outputs erratic pulses on the motor or light pins, and to avoid damage, positive voltage output is cut-off.
Another schematic of a 66031, which slightly differs from mine above, can be found on the website of Dr. Michael König, see there in the 3rd paragraph.