[GreenKeys] Modem with 20mA current loop interface

[Jim Haynes]

On Tue, 16 Dec 2014, Geoff wrote:
> 	I have several model 32s that are on my ""to do" list. I have the circuit
> diagram for the select magnet driver but it does not give the details of the
> semiconductors.. The transistors in particular will be important for anyone
> (like myself) wanting to reproduce the driver circuit.
>
> 	Is anyone able to supply the details of these transistors? Even the original
> would be enough to start the process.
>
I don't have the details, but there is nothing exotic about them. 
Teletype used house-numbered transistors that were pretty close to
the popular ones of the day.  At first it was 2N404 for PNP and 2N35
for NPN.  Later on the general purpose transistors were coded P-22
and N-33 for PNP and NPN respectively, and they were more or less based
on those old ones.  And the power transistors similarly were pretty
common items.  In the selector magnet driver you can probably substitute
ordinary silicon transistors that can handle the voltage and current.

In logic circuits it is a little different because the earlier transistors
had higher base-emitter reverse breakdown voltages than the later silicon
ones.  I remember Fairchild was advertising you could just substitute
their 2N3638- series of transistors for the germanium transistors you were
commonly using.  But we found that was not generally possible in Teletype
circuits because of the base-emitter breakdown.