Hi Bigal_scorpio,
The 1142 is the IR-LED and the 1990 is the phototransistor.
Take your multimeter and select diode test, measure across
the 1142 led, forward and reverse. Forward you should measure
about 1 volt, reverse should be open circuit.
Take a power supply and a current limiting resistor and give
the IR-LED about 10 mA, forward of course.
Take your multimeter and measure the 1990 side, forward and
reverse. Forward you should measure about 250 mV, reverse
again open circuit.
Any more questions before I put the damn thing back ?
on1aag.