my Data General Nova 4/X, disk woes continue

[Tony Duell]

> 
> Tony wrote:
> > I am never in favour of shotgun replacements. It's the brother of
> > board-swapping in that you don't know what the fault was, you don't know
> > you've found it, so you can't know it's fixed.
> 
> I'm not a fan of shotgun replacements.  Board-swapping can be a useful
> diagnostic measure, in that if you swap a board out and the machine
> starts working, you have most likely isolated the fault to that board,

With the emphasiss on 'likely'...

I have actually seen a case like that (somewhat simplified) : 

2 modules, A and B. Module A outputs a signal to B. A was marginal, but 
withing spec on timing, B was just out of spec. System failed. Replacing 
_A_ with one that was less marginal got the system to work. So you spend 
a long time working on the old  module A, which in fact is working within 
spec, missing the fact that B is the problem.

> making it easier to focus your efforts to track down the actual failed
> component.
> 
> I've found this technique to be especially useful when the symptom is
> sporadic flakyness.

And that's the one time I'd never use it. Intermittant faults are a pain 
to trace because you don't know you've fixed them (Murphey's law will 
ensure tha the machine works fro a bit after you've done something to it 
:-)). With such faults you _must_ find the real cause and put it right. 
Swap modules, you might think you've traced the fault to a particular 
module, only for it to come back and bite you later on because it was 
somewhere totally different.

-tony