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:
>
> >>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...
> >
>
> 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.
>
> >>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.
> >
> 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??).
>
> >
> >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.
> >
>
> 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.
>
> >
> >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.
> >
>
> 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?
>
> >>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.
> >
>
> 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.
>
> >>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.
> >
> 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.
>
> >>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.
> >
> 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?
>
> >
> >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.
> >
> 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.
>
> >>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....
> >
> 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.
>
> >>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.
> >
> Perhaps the best test for the regulators is to leave them alone, given
> that they probably are working
> just fine.
>
> >
> >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.
> >
> 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.
>
> >>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.
> >
> Then why was the original poster reccomended to 'reform' the caps?
>
> >
> >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....
> >
> 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?
>
> >>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....
> >
>
> 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?
>
> >>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?
> >
> 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.
>
> >> 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
> >
> 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
Received on Sat Sep 28 2002 - 11:10:00 BST
