Vintage Synth Repair

sunlight is much worse than cold, but beware of damp, especially if
the instrument has any exposed metalwork (like if the paint has
started to come off a control panel for instance) as it will quickly
rust. also likely to suffer from damp are the foam insulation
materials like you would find under the control panel of a rogue, or
pretty much any synth with slider controls. 
occasionally, a build up of condensation on a board will cause
problems when you move a synth out of cold into a warmer environment;
let it acclimatise.
other than that.... have you any instruments or other studio equipment
with moving parts? I have had problems with 1/4" decks where the
capstan refuses to turn because the oil in the bearings got too cold.
this can lead to power supply &/or servo problems down the line. so
always try turning any such part by hand before powering the device
after storage.
if you own a mellotron or a hammond, the same things apply. my m400
has to be kept in a reasonably warm place or it seizes up.
hth-
duncan.