I've had that one and a Hobbico Mini Tach. The Hanger 9 DVT was the best tach I've had and it would still vary a couple of hundred RPM all by itself. It was destroyed in a car crash. I built my first frequency counter from a Heathkit in the 70's and it was a lot more stable than these things. Maybe I'm expecting too much. Fromeco is supposed to be coming out with a revamped TNT tach for $95.00. If I hear from the first time users that they are as stable as the original TNT that is going to be my next buy.

Try pulling off the circular piece from around the light dependent resistor in the front and painting the inside black. I used a marker and it helped somewhat.

John