I'm struggling with a digital circuit that seems to be defying my ability to troubleshoot and would appreciate any suggestions. I received a pcb and parts list and followed the assembly instructions; I've done lots of kits before and I inspected my solder joints pretty carefully. The gif attached shows the circuit, which feeds a pair of logic gates that control a counter. The IC is an 8561 fast comparator; a negative pulse at the jumper input is supposed to result in the output going high, which does not happen. The input device is a photomultiplier (pmt) which detects small pulses of light. I've looked at the pmt output on a scope and it is in fact a negative voltage pulse. Initially, the input pins are both low. When I apply a light pulse to the pmt, the input goes high. The output of the comparator (according to my logic probe) does not change. What's going on between the input and the comparator that should make the negative pulse send the comparator high? Thanks.