10 Jun
2008
10 Jun
'08
4:14 p.m.
Its called a ground loop. This can be dangerous if there is nothing to
limit the ground
loop current.
Is the Imsai and the scope plugged into the same properly grounded outlet?
----- Original Message -----
From: "Jeff Erwin" <jefferwin at gmail.com>
To: "Dave McGuire" <mcguire at neurotica.com>; "General Discussion:
On-Topic
Posts Only" <cctech at classiccmp.org>
Sent: Thursday, May 15, 2008 11:50 AM
Subject: Re: Funny Tek 2445 scope problem
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.
Very odd.
On 5/15/08 8:18 AM, "Dave McGuire" <mcguire at neurotica.com> wrote:
On May 15, 2008, at 10:20 AM, Jeff Erwin wrote:
I purchased a used Tek 2445 scope which works
well on both CH1 and
CH2. Both
10x probes check out as well. I am working on getting an rs232
interface
working, if I scope the TX signal from my terminal I get a clean
and synced
image on the scope on both channels. All is good.
I am rebuiding an IMSAI 8080. The unit I have is working well, 48K
of RAM
all tested, I can enter programs through the front panel and run them.
Getting it to talk to my terminal program is the task at hand.
Now comes the mystery. If I scope ANY bus line, chip lead on any
board, I
get a 60Hz sine wave that measures about 70 volts. Now, in my
logical head
I realize this can't be real, the IMSAI would not only not work but
would be
pouring smoke from every nook and cranny, bringing my wife running
with
claims of 'I told you so...'.
So, what the hell? I have checked ground, I have checked the bus
lines, I
have checked the scope. I do have the active terminator card in
the last
slot and as I mentoned, the IMSAI works fine otherwise.
Where am I picking up this 60hz sine wave?
That sounds to me like a very nasty grounding problem. Be careful!
-Dave