On 15 May 2008 at 12:00, cctalk-request at
classiccmp.org wrote:
Date: Thu, 15 May 2008 11:48:42 -0400
From: Allison
case ground may not be circuit ground... make sure the
the probe ground
is actually circuit ground.
Many machines made the case RF ground and seperate from teh DC ground
using capactiors (true for altair, NS* horizon, CCS, Compupro that I
have).
That's my guess also. That 70v 60Hz signal probably doesn't
represent anything more than stray AC pickup by the case. Find the
real signal ground.
A couple of weeks ago, I was working on a friend's old Roland analog
synthesizer and discovered that there were *two* grounds on the thing-
-digital and analog. And they weren't the same. Neither one
corresponded to case ground or even the sleeve on the audio output
jacks. Fortunately, both grounds had labeled TPs on the PCBs.
Cheers,
Chuck