[cctalk] Re: Replacements for DEC 3639

[Paul Koning via cctalk]

> On Oct 27, 2025, at 6:26 AM, Christian Corti via cctalk 
> <cctalk@classiccmp.org> wrote:
> 
> It seems that nowadays you can't get *any* replacement for failing DEC 3639 
> aka 2N3639 transistors. All parts are obsolete and unobtanium, e.g. 2N3640, 
> PN3640, MMBT3640 and so on.
> 
> So, what can be used instead? The most important electrical parameter is the 
> storage time. It needs to be *very* low, around 20-30 ns.
> 
> Does it mean that a failing PDP-8 will stay a dead PDP-8 from now on?
> 
> Christian

Have you tried with an ordinary switching transistor that's readily available?  
I'm surprised at that storage time, why would it need to be that low for a 
machine that runs at just a couple of MHz?

        paull