[GreenKeys] Modem w/20 ma interface

[Ralph Irish]

Jim, Goeff et al

The old, faithful, Motorola (and probably others) MJE-340 has been used in many
60 ma loop drivers for years.  I will be surprised if they are not available
any longer.  It is a medium to high voltage, NPN bi-polar transistor that will
handle up to an amp, under ideal conditions.  Heat sinking is strongly advised.
Something four to five times the size of the transistor, with a little of that
white, silicon heat transfer compound.

If someone can locate a schematic for an ST-6 TU, I think that it uses the MJE-340
in the loop driver.  If not, whatever it does use will be a suitable substitute.
(I don't know the first thing about getting links on here to specific spots that
might have this circuit.  Sorry.)

There are probably others in the very familiar TO-3 case which would do the job
equally well.  Can't think of a number off hand.  Again, heat sinking recommended
or required.

Good luck,

Ralph - W8ROI


- - - -

On Monday, 15 December 2014 at 20:45:30 CST, Jim Haynes wrote:


Message: 2
Date: Mon, 15 Dec 2014 20:45:30 -0600 (CST)
From: Jim Haynes <jhhaynes at earthlink.net>
To: Geoff <vk2tfg at ozemail.com.au>
Cc: greenkeys at mailman.qth.net
Subject: Re: [GreenKeys] Modem with 20mA current loop interface
Message-ID: <alpine.LFD.2.11.1412152035180.6111 at localhost>
Content-Type: TEXT/PLAIN; charset=US-ASCII; format=flowed

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.

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