PDP-12 Restoration at the RICM

Jon Elson elson at pico-systems.com
Thu Dec 24 15:58:41 CST 2015

On 12/24/2015 01:22 PM, Jay Jaeger wrote:
> On 12/24/2015 9:59 AM, Michael Thompson wrote:
>> We have been able to fix all types of broken flip-chips. Sourcing the
>> components is sometimes a challenge. The Germanium transistors for the TU20
>> on the PDP-9 were hard to find.
> I remember replacing a germanium (at least I think it was) transistor on
> an SMS card with an ordinary silicon transistor in a 7094-II floating
> point unit back around 1974.  Luckily, that worked fine, though for a
> museum I imagine one would prefer to use the "real thing".  ;)
Yes, I overhauled an old HP digital frequency synthesizer 
that was all built out of PNP Germanium transistors (no 
ICs.)  I substituted the first one with a VHF Silicon 
transistor and did some tests on the bias, etc. and was 
pleased to find it was a total drop-in replacement.  I 
replaced over 10 of them in that unit, and they all worked 

I can imagine some difficult circuits where you couldn't get 
away with this, maybe a magnetic read amp or a timing 
circuit or something, but I think in most cases a Silicon 
transistor will work well.


More information about the cctalk mailing list