Funny Tek 2445 scope problem

[Allison]

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
>
>