15 May
2008
15 May
'08
7:20 a.m.
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?
Thanks in advance! I love the fact that there are others out there that
remember these great old machines and appreciate them for what they
represent!
Jeff Erwin
15 May
15 May
8:18 a.m.
On May 15, 2008, at 10:20 AM, Jeff Erwin wrote:
--
Dave McGuire
Port Charlotte, FL
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
8:50 a.m.
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
10:47 a.m.
On May 15, 2008, at 11:50 AM, Jeff Erwin wrote:
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.
-Dave
--
Dave McGuire
Port Charlotte, FL
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. 
3:18 p.m.
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.
The _middle_ lead is not ground on the 79xx parts! 
10 Jun
10 Jun
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
15 May
15 May
8:48 a.m.
Jeff,
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).
Allison
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?
Thanks in advance! I love the fact that there are others out there that
remember these great old machines and appreciate them for what they
represent!
Jeff Erwin
5813
days inactive
5839
days old
7 comments
6 participants
participants (6)
-
ajp166＠verizon.net -
ard＠p850ug1.demon.co.uk -
bshannon＠tiac.net -
jefferwin＠gmail.com -
mcguire＠neurotica.com -
rtellason＠verizon.net