[SLL] OT--Hardware: New Build Won't go to POST

[johnbaxterlists at mac.com]

On Oct 14, 2007, at 11:48 AM, Andrew Sweger wrote:

> On Sun, 14 Oct 2007, Ana wrote:
>
>> How important is properly grounded house-power?
>
> It's important from a safety perspective. But the _normal_  
> operation of
> any electrical device should not depend on the presense of a  
> ground. If a
> device is leaking current to ground, something is wrong with it  
> (e.g., it
> would trip a GFI breaker). An improperly wired ground could cause  
> problems
> for the operator of a device if there's a potential difference between
> ground and neutral. But that's a separate world of hurt.
>

The Apple ][ switching power supply did require proper 3rd wire  
ground, and the machine would behave oddly in its absence.

At WTBS (the MIT student station before my successors sold the call  
sign to that sailor guy from Atlanta), we had two Ampex 310 reel to  
reel tape machines side by side.  One day, one of the engineers began  
complaining that he felt a tingle when touching the metal fronts of  
both drives at once (no one else did).  A little checking revealed  
that we had two ground busses running around the station.  One was a  
nice solid earth ground, the other was the "ground side" of the East  
Campus dormitory telephone system* (no one there remembered WHY that  
was).

The tingling started when the battery in that phone system was  
replaced--and we where then on the hot side (same polarity, but the  
ground had moved to the other side).  48 volts with enough amps is  
enough to tingle--the real surprise is that only one person felt it.

We fixed that ground, of course.  But not before building a relay  
circuit whose power was derived from the two grounds--with a careful  
drawing made for the record.

   --John

*An example of reuse:  the manual switch board started life as the  
first main MIT switchboard.  (The old guy who maintained it traced  
circuits by putting ring (90 volts at 20 cycles [they weren't hertz  
yet]) on the desired circuit, and then at boxes he would wet his left  
hand, grab the box (ground), wet his right hand and run fingers down  
the terminals until he found the ring.)