powering up older machines - is it safe?

[Tony Duell]

> Anyway, a friend asked me to try and sell a DEC Robin for him (he tells 
> me it's a DEC VT180 with a separate drive unit that he tells me was only 
> available to DEC employees), and my question is this:
> 
> Is it safe to try and plug this machine in and try to power it up?  I've 

I tend to err very much on the side of caution in this (and have had, 
shall we say, 'differences of opinion' with other list members about this 
in the past). I never just plug in an unknown machine to 'see what happens'.

> seen various discussions about old capacitors dying, etc but I'm not 

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.

Now, of course, this circuit could fail at any time, and most good power 
supplies have what's called a 'crowbar circuit' to protect against this. 
But I still feel that wehn you come to a machine that's not been used for 
some time, it's safest to disconenct the PSU from the rest of the 
machine, connect the former to a dummy load (a set of suitably-rated 
light bulbs) and test the pwoer supply separately. In some cases I've 
even made up test boxes/harnesses to test commonly-used (for me) PSUs 
quickly and easily.

The problem (for you) is that doing this, on an unknown machine, takes 
some experience. You have to know how to disconnect the PSU from the 
logic (but leave the mains input connected), you have to work out 
suitable dummy loads and connect them to the right wires/pins on the PSU. 
Somebody elase here can probably talk you through waht to do, but it's 
still a bit of work.

-tony