PDP-12 Restoration at the RICM

[Brent Hilpert]

On 2015-Dec-24, at 1:58 PM, Jon Elson wrote:
> 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 flawlessly.
> 
> 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.

Similarly, I've replaced Ge with common Si in repairs of 60s-era discrete-component equipment and had no problems.