[GreenKeys] Tape Reader Power Resistor

[Ethan Blanton]

Troy Peterson via GreenKeys wrote:
> Does anyone know where I can find a replacement? I did a
> cursory search of my usual (UK) component suppliers and 850 ohm
> doesn't even seem to be a common value, let alone available in high
> power. I'm not sure what the power rating of this resistor is, or
> really even what kind of resistor it is...

According to the wiring diagram, that 183081 printed on it is the
Teletype part number.  I would ask Paul at RTTY Electronics
(mr_rtty at pacbell.net, he's on this list) if he has that part number,
since you say you'd like things to remain original.

Keep in mind tha thet wires don't have to carry a large current if
they are carrying a large voltage!  Many current loop devices in
Teletype circuits may drop a small number of mA over a large voltage,
which is a significant amount of power.  I don't actually know what
the valid range of voltages for a 20 mA current loop is (I'd be
interested to know, in fact), but for a 60 mA current loop it's quite
likely to drop 60 mA at 120 V or more, which is over 7 watts -- but
can be easily carried on 30 gauge wire (or even smaller!).

I doubt the precise resistance is critical here, although a different
resistance may entail (for example) adjustments to the current source.
It looks like you need to chase down the schematic for the board
that's on (the wiring diagram I checked just shows the resistor wired
to "Board Assembly Reader Power Pack", part 183087) and see what it's
doing.  Wire wound resistors are often relatively precise, so I could
be wrong.

Ethan