Look, the chances that any regulators failed while the machine was
sitting unused are VERY LOW.
For someone who takes the time to ask what their first step is,
suggesting that they begin by removing
components from the boards boarders on the absurd. The risk of causing
dammage in this process is
probably much higher than the chance that there is any problem in the
machine.
Unless something is eating the etches off your board as they sat, a
formerly running Altair really should
not have any open grounds on its voltage regulators. So in this
specific case, yes, your paranoid!
I'm be far more concerned with the idea of someone new to computer
restoration is being advised to start
removing parts from boards on the astronomically unlikely chance that a
7805 ground has opened.
The practical reality remains that most probably, the Altair will power
up and run just fine after a good cleaning.
New defective components are one thing, but we can assume that the
Altair was working at one point, so unless something has gone wrong, it
should still be working today.
As for resistance checks, I have to strongly disagree Tony. If you
verify that there are no shorts caused by broken
decoupling caps, loose parts, and so on, then the chances are nothing
rude will happen when you flick the switch, and this is the only point
behind such tests. Resistance checks have absolutley nothing to do with
testing the 7905 regulators functionality whatsoever, and that is not
why such tests are a good idea before applying power.
The regulators generally dont fail while sitting around. Getting a bad
part into a new project is a very different situation than trying to
return a formerly working machine to service. No practical set of
testing, other than dynamic (functional) testing can detect every
possible failed component. So the question should be, what is the
PRACTICAL level of testing that should be applied PRIOR to powering up
the Altair.
In all the replies to the original post, now many suggested ANY testing
at all?
My point being, its easy enough to check the power supply filter caps
BEFORE attemtping any snake-oil rejunivation methods, but this was not
mentioned, and you seem to agree this is probably not needed here. So
why does the list as a whole sit by while reccomendations of pulling
parts off of boards float by, when this is one of the VERY LAST things a
beginner in vintage restoration should attempt?
I agree, many vintage chips are hard to replace. So don't remove them
unless its absolutley mandatory to do so, after you have isolated a real
failure.
The idea of a restoration should be to do as little to the machine as
possible in order to return it to its original operating condition. In
the case of a S-100 box like an Altair, very little is really needed
before you can feel quite safe about flipping the power switch, given
that the machine used to run just fine.
The bottom line is just this simple, "if it ain't broke, don't fix it".
Quite different from the advice this Altair owner got from the list.
Tony Duell wrote:
>>But realistially, how often do these old caps really fail? Lets limit
>>this to the main filter caps in an Altair or
>>Imsai class S-100 box. Should these caps really be 'reformed' be
>>default when a machine is returned to service?
>>
>>Probably not.
>>
>
>Agreed. I've worked on a lot of old computers and of course I've had
>capacitors fail. I've had just about all components fail. But I've never
>seen a computer where all the capacitors have failed due to old age. I've
>just had random failures.
>
>>If the caps pass this sort of test, there is little justification for
>>removing parts and risking dammage. A few resistance checks is all
>>thats needed before you can feel quite safe before you flip the power
>>switch for the first time.
>>
>
>Sorry, but no!.
>
>'Cold' resistance checks will not spot a 3 terminal regulator where the
>common pin has gone open-circuit or some other internal failure has
>occured. Such a failure could result in the full unregulated input
>voltage appearing at the output. Which will kill chips all over the machine.
>
<snip!>
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Received on Fri Sep 27 2002 - 16:59:00 BST