wrote in message
...

I posted a more detailed message in .design and haven't got a response
yet. But anyway, to keep this short and sweet. Is there any trick
that people use to test and diagnose DC tachometers? I have a 4
channel tach that is powered by 24VDC. Each channel has a VCC, Vout,
and gnd. It's supposed to be an open collector output, but when I
connect my external circuit all I get is a steady 24VDC on the Vout
with a lot of noise on top of it. Does this inherently mean
something? Are there any tricks I could attempt to find out what's
going on? Thanks.

Do you have a bypass capacitor (such as a 10nF mica) bypassing the power
pins on the chip?