15 May
2008
15 May
'08
11:07 a.m.
On Thursday 15 May 2008 13:47, Dave McGuire wrote:
On May 15, 2008, at 11:50 AM, Jeff Erwin wrote:
The _middle_ lead is not ground on the 79xx parts!
--
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space, ?a critter that can
be killed but can't be tamed. ?--Robert A. Heinlein, "The Puppet Masters"
-
Information is more dangerous than cannon to a society ruled by lies. --James
M Dakin
Case solved (I think). It seems to matter where
I ground the
probe. I
pulled all of the boards out of the system, no change. I then
grounded the
probe to the middle finger of the voltage regulator on the board
and the
sine wave went away.
The chassis may not be at digital ground.
Also watch out for the middle lead of three-terminal (78xx/79xx)
voltage regulators...it's not unheard of to bring them up above
ground a bit with a voltage divider to make them operate at different
voltages.