28 Sep
2002
28 Sep
'02
4:10 a.m.
OK, so if you're all worried about dead voltage regulators, why not just
put a crowbar on a board when you first power it up, and make sure that it
doesn't fire? Doing this one board at a time seems reasonable. If the
crowbar doesn't fire, and you can measure 5V+- output from each regulator
as it's powered up, you should be just fine to do a test without the
crowbars.
Of course, since it's a linear supply, you could (and probably should)
test it first under no load and make sure you get a good +8V or so output,
with good filtering. Then, you aught to be able to try powering the
boards up one-at-a-time with crowbars on them, to make sure everything is
OK.
-- Pat
On Sat, 28 Sep 2002, Bob Shannon wrote:
This is getting absurd!
Tony Duell wrote:
I think your failing to address the relative risks here. You may have
good reasons for using and reccomending
your particular approach to bringing up a vintage machine, but please
review the advice the list as a whole
has given to this Altair ower:
Main filter cap restoration was reccomended without bothering to test if
such a risky technique is even needed.
This is simply BAD advice, period. Zero safety tests were reccomended,
in fact, its not clear from the old messages just what should be done
prior t0 applying power for the first time. Disassembly was reccomended
to the point of removing chips from boards, despite the lack of
documentation.
Do you think the list gave this Altair owner good advice?
I don't.
Really? Here we must agree to disagree. Old sockets suck, and Altairs
generally don't use them for this
reason. If I recall correctly, you had to pay extra for sockets (or was
that the Imsai??).
Sure, and pigs sometimes fly, but the FAA is not too concerned over
this. Parts can fail for apparently
unknown reasons, but is this likley enough that it should determin how
an old Altair is brought back to life?
Once again, we may have to agree to disagree.
The reason why is that the original post was seeking advice on how to
safely return an Altair to operation.
As you have already agreed, the chances are that ZERO chips need to be
changed.
How about starting with small steps, and learning new skills as needed?
Ahh, so perhaps the most efficient course of action is to take
precautions agains messy failures, ones that
would burn up etches, etc. Clearly burning up rare chips would also
fall under this concept as well, so we
have to go back to the relative risks envolved.
IMO, having a person new to vintage restoration start by removing chips
from boards is FAR more risky
than applying power after a few basic tests.
The Altair uses +8 unregulated. If this were applies to TTL parts, some
~might~ fail, but you are not
going to destroy all of them as you suggest.
So once again we are back to the question of how likley is it that such
a regulator failure exists in this machine?
In your own words, "most likely is _is_ still working". So how then is
it any SAFER to reccomend that the
original poster start pulling chips off boards?
Frankly, I'd risk any chip in the machine to not mess up PCB etches, as
many parts (probably) are not in sockets.
The advice and discussion have gotten far too abstract, and really are
not giving the original poster good advice on how to proceed and what to
check for.
S-100 vintage decoupling caps generally don't fail I agree, but they are
by far the most commonly dammaged
components I find on S-100 boards, due to mechanical dammage. They get
chiped and broken, often shorting.
Other times they break one lead, and short something nearby.
The risk here is a burnt etch. Not a huge risk, but given that this
risk can be eliminated in seconds with a meter, its
not an unwise thing to test, is it?
Hmm, I'm going to take a board full of old TTL devices, and run them at
8 volts. I'll let you know when
they fail.
Don't hold your breath though.
Yes, it can happen. And if it did happen, there is a chance, not a
certainty, that a device might fail.
But you have already agreeded that this is most probably NOT going to
happen in this case.
Given that this is most probably NOT going to happen, the process of
removing parts from a machine
is far more likley to CAUSE problems than it is to prevent them.
Perhaps the best test for the regulators is to leave them alone, given
that they probably are working
just fine.
What that has to do with it is simply this, the list gave the original
poster some BAD ADVICE.
Your overstating the risks, and the potential for dammage.
Then why was the original poster reccomended to 'reform' the caps?
Once again, good for you Tony. We really are impressed.
How many chips has the poster changed?
Mainly, is there a really good reason to remove them? This last point
may be a matter of opinion for
people with experiance, but thats not who the lists replys to the poster
should be written for now is it?
Risk versus risk. Which risk is greater? The low probability that a
regulator has failed, AND that it will
dammage other components, or the risk of breaking a pin or the hermetic
seal on some vintage chip?
Preventitive maintainance is not a repair, an oil change on your car is
not a repair. Your grasping at straws here.
And this has ZERO to do with an Altair.
Apparently one of your 7805's Tony. If you want to make this personal,
take it off-list.
--
"This fucking university has shown time and time again that it is
completely fucking incompetent when it comes to employing technology"
-- Anonymous
http://dilbert.com/comics/dilbert/archive/images/dilbert2040637020924.gif
Look, the
chances that any regulators failed while the machine was
sitting unused are VERY LOW.
But the chances of you having a complete set of spare ICs is even lower.
I have a pretty well-stocked junk box (even if I say so myself) and I
wouldn't have all the chips in stock...
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.
The risk of causing damage by removing _socketed_ parts is extremely low.
In fact I would argue you _must_ remove and reseat all socketed chips to
cure bad connections anyway. I wouldn't suggest desoldering chips unless
there was no alternative _and_ they were very rare _and_ you had a lot of
practice in desoldering ICs. FWIW, I _have_ once desoldered a rare custom
chip in order to rnsure a PSU wasn't going to wreck it.
Good for you. Has the Altair owner done this? Do they have the
equipment needed for antistatic protection?
To be honest, if you can't remove socketed ICs without damage then you
need to spend a bit of time practicing before starting work on an Altair
(or anything else moderately rare).
Hmmm, did anyone tell the Altair owner this Tony?
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
Hmmm... ICs (including regulators) can fail internally, even when just
sitting around. I have seen this happen.
Also, if any of the regulators are in TO3 cans, the ground is made via
the mounting screws. If these are loose/have oxidised then the ground may
well be open. Sure, you should check them for tightness. Sure you should
remove them and clean them. But you won't catch me then powering up the
machine with said regulator powering anything but a dummy load. Only when
I know it's OK will I add some chips.
Let me get this clear, you would REMOVE soldered-in TO-3 regulators
rather than simply check that
their case was grounded with an ohm meter?
Thats quite some restoration style you have there. Ever restored a
machine larger than a microcomputer?
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
Why? You're going to have to learn to remove ICs sooner or later.
Including removing soldered-in ICs without damaging chip or board.
7805
ground has opened.
The practical reality remains that most probably, the Altair will power
up and run just fine after a good cleaning.
Agreed. But if it doesn't then you might have a real mess on your hands.
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.
Most likely it _is_ still working. But the damage caused if a regulator
has failed is sufficiently great to make it worth checking.
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
Any reasonable PSU (including every 3 terminal regulator I've worked
with) has current limiting. A shorted decoupling capacitor is not going
to do much damage. Abyway, ceramic capacitors don't short that often.
To turn your argument back on yourself, I'd be more worried about the
open-circuit voltage of the ohmmeter doing some damage (if nothing is
shorted). Do you know the open-circuit voltage of your meter? Does a
newcomer?
Its been a long time since meters were made that would apply enough
current (thats what matters, not the
voltage) to dammage devices.
Testing the power to ground resistance, even with something like a
Simson 260, is not going to cause dammage.
Once again it seems the argument is grasping at straws rather than
focusing on the original posters needs.
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
The point is, though that a regulator failure will do a lot of damage.
Sure you can do resistance checks as well. You can also check for shorts
between bus lines, you can check 101 other things. Most of them will not
have the effect that a 5V line rising to 8V will have.
why such
tests are a good idea before applying power.
The regulators generally dont fail while sitting around. Getting a bad
Maybe not, but it sure can happen....
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
Of course not. Which is why you have to test the regulator under working
conditions. Namely by giving it a suitable input voltage and load and
seeing what the output voltage is. This can only be done with the PSU
powered up, of course.
But equally, you choose a load which will not be damaged and/or expensive
to repair/replace if things go wrong. A 6V car bulb rather than a PCB of
unobtainable silicon...
Aren't you over-reacting to a risk that you admit is unlikely?
An extender board with a fuze is a much better idea if you insist on
such tests.
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?
What's that got to do with it? Many people apply power to things without
doing tests first. I've seen the results in some cases (not just
computers). I've also had to sort out the resulting mess. When you
consider that a single loose screw on a regulator mounting could wipe out
the entire PCB-fuil of ICs, you take a little more care.
My point
being, its easy enough to check the power supply filter caps
FWIW, shorts in good-quality modern electrolytics are rare. A failed
capacitor will not damage any of the chips. If it shorts it might take
out the rectifier. It will blow the fuse, but unless some idiot has put a
nail in the fuseholder, it's not going to burn out the transformer. The
risk from failed smoothing capacitor is minimal.
The only real risk is mechanical damage due to the darn thing exploding.
Modern capacitors are, of course, designed to vent safely, so this is a
non-issue too. But yes, I would check them for shorts. I'd check the
rectifier diodes for shorts too. I'd stick a megger on the transformer.
Venting is a non-issue? Its really nasty stuff for your health and for
the health of the machine, but I agree
its not a major risk here. Thats part of why I object to the advice the
poster was given by the list.
Is it any wonder it can take me a good few weeks before powering up a new
toy. Is it also any wonder that major disasters at first power-on are
unheard-of here?
If it takes you a few weeks to power up an old S-100 box, you either
have too little time, or your going
in way over your head.
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?
Why? Take anti-static precautions (which you should do before even
opening the case on a machine like this), and pull the chips out
carefully. You _will not damage them_. I've removed thousands of ICs from
sockets and never damaged a single one....
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.
It might be too late then....
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
Of course. Which means keeping as many of the original parts as possible
and not risking them unnecessarily.
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".
That is quite the most rediculous proverb I have ever heard. Several good
books on engineering that I've read point out this is the best way to
ruin most electrical or mechanical devices. I suppose you wait for a
bearing to seize up before oiling it? I suppose you wait for a PSU to
drift far enough out of regulation to do major damage before bothering to
check and fix it?
Quite
different from the advice this Altair owner got from the list.
Just out of curiousity, who died and made you the god of classic computing?
-tony
