Reverse engineering vintage PCBs

[tony duell]

>
> What continuity tracer do you use?  Can you recommend a "good
> continuity tracer that is not fooled by diodes" that can be
> built/purchased for not a lot of money?

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