From: Winnipeg, MB
The Germans evidently were using printed circuit boards (pressed cardboard) before the war and used them in some of their electronics during the war ... dunno about the US or Commonwealth, though.
I would think that transistors and semiconductor technology would also be easier to set up than 'by the end of the war' ... patents for transistor like devices go back to the 1920s but were too advanced for the semiconductor technology available ... transistors as we know them date to 1947 ... so, with a wartime effort triggered by a Final Countdown like scenario, I would expect experimental models within a year, field prototypes within 1 1/2 to 2 years, and the real deal, something like limited mass production within 2-3 at most.
That's because the modern engineers who come back with the ships know what to do, know the dead ends, and that will save a hell of a lot of development time, since they were fairly close IRL anyway.
The research could probably advance pretty quickly but I am not so sure the exotic rare-earth elements needed for manufacture of transistors and semiconductors were being produced in any kind of quantity back then. My understanding is that things like selenium and germanium come from far away places like southeast Asia and Indonesia. At best, they are in Africa or South America but have not been discovered yet by mineral prospectors. Global trade never really took off until the post-war freedom of the seas and air travel made it possible to develop resources worldwide.
No matter how bad a situation is, you can always make it worse. - Chris Hadfield : An Astronaut's Guide To Life On Earth