powering up older machines - is it safe?

[John Robertson]

Tony Duell wrote:
>> 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.
>   
I install 6VDC Transorbs on older TTL logic boards to protect the very 
expensive circuitry from a bad supply. The one I use is the 1N5908 - 
has  a great surge reaction time, and if there is just too much current 
the little bugger will short out and take down the 5V supply...perhaps 
an inline fuse would be a good idea in case of runaway Vcc!

John :-#)#

PS. I recommend checking capacitors with an ohm-meter/ESR combination to 
check for shorted parts.

> 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
>
>
>   


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