I'm copying a circuit from an old aftermarket Japanese automotive tachometer an not having any luck identifying a diode in the circuit. The circuit is a one shot multivibrator using two NPN transistors controled by an RC circuit. The diode (D1) is the on the trigger signal from the primary ignition circuit of the engine. I used a 1N4002 diode on the breadboard mock-up because I had one on hand and it appears to work fine. The markings on the mystery diode are "TT 2.F". What would be a good replacement diode?