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Interesting discussion by someone thinking of procuring a PDP-11/34, though having difficulty finding suitable space for it: https://www.reddit.com/r/retrobattlestations/comments/dztvci/minimum_viable_system_for_pdp11/

A comment that caught my eye:

Keeping it in the garage will destroy it with the humidity and condensation as the seasons change. I've worked on machinery over the years that's been kept in garages and sheds, corrosion - and these are basic machines, it will be much less kind to electronics.

That seems surprisingly pessimistic! I don't have experience with mainframes or minicomputers, but I've been working with personal computers of various kinds for going on four decades, and I have never seen hardware rendered inoperable just by being in a location that is not climate controlled.

Is there something about the technology with which the older minicomputers were built, that makes them more susceptible to moisture damage?

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    Is there anything that makes you think new computers wouldn't be vulnerable to being left in a damp garage for several years? Also, the quote's "will destroy" is probably over dramatic and an over-generalisation. Some places's climates will affect "tech" far more than others. – TripeHound Nov 27 '19 at 10:09
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    Wire-wrapped backplane? In general, I'd guess: older computers have much more actual wiring than modern devices, wires are metal, and many metals corrode in damp air. – another-dave Nov 27 '19 at 12:47
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    If the ICs are socketed (typical for 70s kit) then the socket contacts can corrode and the sockets are sometimes a bit susceptible to corrosion underneath depending on the amount of moisture and various air-borne materials. I think "will destroy" is a bit of an exaggeration although normal operation might be compromised (until cleaned). Due to the relatively large number of boards, there are more connectors which can be somewhat of an issue. – Peter Smith Nov 27 '19 at 13:45
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    I don't know about mainframes or minicomputers, but with older personal computers, the parameter ram batteries (especially the 3.6v Lithium 1/2 AA ones) will literally explode when subjected to the repeated heat cycles of an unairconditioned garage. This will completely destroy not only the traces on a motherboard but the ICs themselves. – Glen Yates Nov 27 '19 at 17:30
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    Wire-wrap is pretty climate tolerant. So much so that in an industrial settings with corrosive gases, the wire-wrapped planes isn't that affected (if they are correctly done.) – Stefan Skoglund Nov 28 '19 at 13:51
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If you watch enough YouTube videos of repairing old micros — and even ones that aren't that old (486s didn't come out that long ago, did they?) — you'll see a lot that have suffered some sort of damage due to storage over the years. Some damage, such as corrosion due to battery leakage or cracked circuit boards, is not due to climate. However some, such as oxidized contacts or corroded circuit board traces, is.

Old mainframes and minicomputers have a lot of points of failure compared to micros. Their construction often involves a lot of socketed components, plugs and connectors, and wire-wrapped connections. Each of these is an unprotected point where corrosion can occur. Repeated heating and cooling cycles can unseat socketed components and connectors, or weaken iffy solder joints. Humidity can corrode exposed wire, pins, and traces. Many old printed circuit boards do not have protective solder mask layers, which leaves them exposed as well. And there are a lot of boards, discrete components, and wired connections, which means a lot more pieces that can go bad.

Contrast this to the construction of most micros: more modern circuit boards, often (but not always, depending on age) with a protective solder mask layer. Many components are directly soldered to the board, and there are often only one or two boards. Almost all the connections are internal to the circuit boards. There are fewer discrete components, because there are more complex integrated circuits. There are far fewer exposed connections that can flake out, or components to be damaged.

Also, don't discount age as a factor. Depending on the computer, the quality of construction of some of the components may simply not have been as good because no one yet knew how to make them better, so ambient conditions may damage them too.

Saying "will destroy" may be something of an exaggeration depending on climate. A moderate or even hot, fairly dry climate will likely be reasonably kind to electronics if they're sheltered. A cool and damp one without too much temperature variation may not be a death sentence. In my opinion, a warning like that is still a good idea to someone who doesn't know the ins and outs of old electronics, since it will prevent disappointment and wasted money.

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    Of course old computers were built with lead containing solder rather than lead free solder which makes their soldered joints less susceptible to fractures under cycling temperatures – houninym Nov 28 '19 at 15:38
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    @houninym unless they're not old enough to have lead but are old enough for it to be illegal, so they are in that nasty period where they don't have a new process but are now using lead-free solders, which are basically guaranteed to fail even in perfect condition. – Nelson Nov 29 '19 at 5:16
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    I don't think what I wrote precluded that at all... old computers were built with lead solder, at some period in time manufacturing switched away from lead solder and they became more subject to brittle failure of the solder joints. That's independent of legal constraints on lead... it was not mandatory to use lead solder, just everyone used to because that was the standard. At some point it became mandatory not to use lead but there was nothing to stop manufacturing switching any time before that date. – houninym Nov 29 '19 at 10:19
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    @houninym - the lead free directive permits old equipment that used Leaded solder (SnPb) to be repaired with leaded solder; new build equipment requires lead free. SnPb was (and remains) popular because it is eutectic with a liquidus / solidus point of 183C (far lower than any lead free solder). Reflow occurs at about 40C lower than lead free solders. It is still legal to use SnPb in a number of applications even within the EU. – Peter Smith Nov 29 '19 at 15:39
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Is there something about the technology with which the older minicomputers were built, that makes them more susceptible to moisture damage?

No, it's about any human made - in fact even any pysical item at all.

I've been working with personal computers of various kinds for going on four decades, and I have never seen hardware rendered inoperable just by being in a location that is not climate controlled.

The issue isn't about being climate controlled or not, but what conditions it experiances over each day and years. Stuff in a shed in SoCal will stay much longer without damage than in a nice garage in the North-West.

That's why you find way more worthwhile project material when searching for an old car in Nevada vs. Oregon. And the same is true for old computers and anything else.

Bottom line: What people tell you about their experience when storing computers in garages or alike, depends mostly on their values for $GARAGE and $KLIMA which need to be mentioned to make the story worthwhile.

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Someone can correct me if I am wrong, but I think larger components/contacts create larger shear forces due to the different rates of thermal expansion in different materials.

For instance, a tiny solder joint will have a much smaller difference between the expansion of the solder and of the plastic.

  • This is right, but it doesn't really apply to the old technologies used for assembling ancient computers, though. Because they used essentially through-hole components with long legs, and the legs will easily absorb these expansion differences. What you say is much more relevant for newer, surface-mounted components, especially BGA chips, which are large and have no legs at all (but plenty of pins). – dim lost faith in SE Nov 28 '19 at 11:34
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The specifications for most computers include (or used to) prohibition of 'condensing moisture' conditions. The 'condensing moisture' means dew. On a ship, below waterline, if there's no air conditioning, you'd violate that condition.

The reason is that the electronic parts of a computer have multiple different small metal parts. Adjacent dissimilar metals (copper traces, tin/lead/copper/silver/antomony solder, silver/gold/nickel/zinc plating, steel/brass screws, etc.) are subject to electrolytic corrosion if they get wet, and are SMALL parts, so it only takes a small bit of corrosion to ruin them.

Most working and living spaces don't get the moisture, or temperature swings, that produce that dew (unless it's on a windowpane) but in my garage, it certainly CAN happen. It wouldn't be good for the electronic parts of a computer, or even the mechanical ones.

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