Sorry to state the obvious, but it depends on the computer. When I used to work on military computers they were rated down to -55C I think. The parts which were only connected in a lab were rated down to 0C/32F. My old Germanium transistor machine turns itself off below 10C/50F because thats what Mullard / GEC (the British one), the transistor manufacturers specified. Now they are getting on for 50 years old I don't power it up below 13C/55F. Normally we shut down the restoration project in November but today, with the aid of a couple of fan heaters, we were working on it. Increased levels of solar radiation has a few advantages, though its due to peak in 2012 I'm told. I imagine with valves, the potential problem would be the glass cracking. Cold is certainly more of a problem than overheating, generally if an operator can stand the heat so can the computer. I've heard tales of old ICT 1301s being operated by people stripped down to their pants (underpants for those to the left of the pond) when the air conditioning failed because otherwise the payroll run would not be done, and of course they themselves would not be paid as well as taking flak from all the other workers. At work back in the 80s I had an Apple ][ euro-plus or maybe it was an Apple ///, anyway it was fitted with an external 'ICE' winchester drive. ICE being the brand. As we moved into winter it took longer and longer to start up in the morning depending on the temperature. We used to laugh at how appropriate the name was. Of course it eventually failed completely but at least we got a lot of warning and had a fairly full backup. Roger Holmes On 9 Dec 2009, at 06:56, cctalk-request at classiccmp.org wrote: