A poltergeist in my machine?

[Jon Elson]

On 09/07/2015 05:09 PM, Noel Chiappa wrote:
>   And when I put my 'scope lead on the second input... the memory
> suddenly starts working!
>
> Well, I could see that - the added resistance or capacitance or whatever of
> the probe might have had some effect on a circuit that was right on the edge.
> But here's where the ghost enters the machine.
>
> I pull the 'scope probe ..... and the memory keeps working!
>
> I can power cycle the machine, leave it off for 15 minutes, power it back on
> - and the memory still works fine!
>
>
> Does anyone have _any_ idea WTF is going on here?!?!
>
>
I've seen this plenty before.  Here are a couple possible 
causes.

1.  There was a conductive hair on the board, and you 
knocked it off with the probe.

2.  There was a bad solder joint and the pressure of the 
probe broke through the oxide.  it will fail again within a 
day or a couple weeks if this is the cause.  This is my 
highest probability guess. Check that pin and the power and 
ground pins of that chip for ANY signs of funky soldering.

3.  The chip has a bad internal wire bond, and the pressure 
on the lead made it work.  Probably will fail again if this 
is the case.

I've seen some REALLY crazy stuff.  The wildest one was a 
short inside a 2-layer board in a tape controller.  it just 
went catatonic one day.  The failure was the master_reset/ 
line was shorted to ground.  I finally tracked it to a 1" 
length of circuit trace, with no ground really close to it 
on either side!  I cut the trace at both ends and put a 
piece of wire-wrap wire to tie them together.  I peeled the 
shorted track off the board, and there was NO TRACE 
whatsoever of what could have been causing the short.  I had 
expected there was a little pit in the epoxy laminate that 
got plated, but no such thing!

Jon