LED Display multiplexing voltages and currents

[LinkZ]

I've ordered this clock from aliexpress: http://www.aliexpress.com/item/0-56-Blue-LED-Meter-Electrical-Digital-Car-Motorcycle-Clock-Watch-DC-12V-24V-Time-Display/2019045353.html
I want to use his circuit to drive a much larger display made from several 3mm leds, so i will drive the 3mm leds with transistor configured as switch. I've desoldered the original display from the clock to figure out if is a common cathode or common anode. It's a common cathode. So I tried to measure the voltages from the microcontroller outputs (where the original display was soldered) of the clock but I get positive voltages only from certain pins and the others (where you expect to read the same voltages) are negative and/or just too low to lit a LED display. I need those voltages because I have to calculate the Vbe to drive the transistor configured as a switch. What did I missed?