powering up older machines - is it safe?

2 Jun 2008

...
   The main worry
is not a capacitor Failing. The main worry IMHO is that if 
 there's a fault in the power supply regulation circuitry then that could 
 make the +5V line jump to a high enough voltage to wipe out just about 
 every chip in the machine. Expansive and difficult to put right.  
 I don't disagree with the concern, but I'd be interested in hearing
 estimates on the probability of this taking place.  Has anyone had it
 happen to them? 
Yes, sort-of....

I've never had it happen in a classic computer, but years ago I was 
making a homebrew SC/MP machine. I'd obtained a surplus PSU which I'd 
tested and worked fine, and then stuck on the shelf. I was foolish enough 
to get it out a couple of mnths later and attempt to use it to power my 
homebrew board. Alas something had failed in the PSU (I forget what, it 
might just have been a dry joint...) the +5V output was at about 9V. This 
wiped out the SC/MP, a couple of 2114s, and at least one TTL chip 
(amazingly much of the TTL eurvived). 

I was a schoolboy at the time, and it took a lot of weeks of saving 
pocket money to replace that SC/MP. I guess that's why I'm now (over-) 
cautious...

-tony

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powering up older machines - is it safe?