Reverse engineering vintage PCBs

Amazingly the diode test/beep range of a fairly cheap DMM works well (better in a lot of cases than the one on a more expensive DMM). The range applies a contant current through the probes and displays the voltage between them. The beeper is triggered by an analogue comparator set to less than the forward voltage of a silicon diode. This has an advantage of the scheme used on more expensive meters which compare the digital output of the ADC with some threshold value, meaning the beeper only triggers after an ADC conversion. This is a right pain in that it takes long enough that you can't stroke the probe along a line of pins and hope to find the one that is connected. And of course the treshold is low enough that a diode will not trigger the beep. -tony