How can I check to see if my logic probe is broken? I'm working on a broken Firepower board, and the only readings I get are either high, pulsing high, and nothing. The "low" light never lights up. I haven't used this thing in a while, so I'm not sure if something's wrong with it or if nothing on this broken board is sending a "low" pulse. I figured grounding pin 40 of the CPU while holding the logic probe to that pin might show a low signal, but the high light just goes off while it's grounded and then comes back on when the ground is taken away.

It's an Elenco LP900, one of these guys:

http://www.amazon.com/Elenco-LP900-625-Logic-Pulser/dp/B0002E7X5O/ref=sr_1_2

Should I just chuck it and get a new probe? If so, do I need one that pulses, or can I go for the cheaper one?

That switch is for the pulser, to alternate between 400 and .5 pulses per second.

There's nothing I can touch to get the Low light to come on, so I think this thing is junked. Oh well. Guess I'll get a new one.

Turns out the power source on the board was too low for it to get an accurate reading - about 3.9v. Once I connected the board to a better bench supply (giving about 4.5v) the thing started showing "low" again. Too late, though, since I already ordered a replacement from Amazon. BOOOOO.

Already tried that. Didn't work. DOUBLE BOOOO.