Funny Tek 2445 scope problem

[Jeff Erwin]

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
>> 
>