This interesting source, obtained in a 2001 catalog, can vary the output voltage between -16 and +16 V, thus allowing the reversal of the rotation of a motor, in addition to speed control. Q1 can be a BD135 and Q2 a BD136 for currents up to 500 mA orTIP31 for Q1 and Q2 a TIP32 for currents up to 2A, mounted on heatsinks. The transformer has 2 A secondary current and R3 is a 2.2 k ohm wire potentiometer. The diodes can be 1N5402 or even 1N5404 which are more common. Originally the circuit was designed to control the movement of trains in miniature railways.
