Being frustrated with encountering segments or digits that stayed partly on even when they are supposed to be off, I turned my attention to the driver transistors I was using.

I realised that these were grab bag transistors bought decades ago when I was a newbie. Being seconds (cough, rejects) they are actually poorer than pulls from circuit boards that have been verified. I had graded them by gain using a component tester. But in my newbie days I had been using them in battery driven circuits, usually not more than 6 V. Now my circuits involve higher voltages, The component tester I had used only goes up to 9 V and there is no indication of leakage or worse C-E breakdown.

I whipped up the circuit above. I dialed up 12 V on the bench supply and connected a voltmeter across the C-E terminals. Originally I had a 100 k resistor between B and E but I decided to omit that, the better to detect leaks as any C-B leakage current would get amplified. I decided that any measurement below 12 V indicated a dodgy transistor. For example a voltage of 11.99 V corresponds to a leakage current of 1 µA.

For PNP transistors, I just reversed the power supply. Result is I culled about 15 - 20 % of the grab bag stock. Now I can use the OOS (Old Old Stock) with greater confidence.