[GreenKeys] Tape Reader Power Resistor

[Troy Peterson]

I did measure 100vdc across one of the capacitors. It still seemed to me 
that those wires would need to be carrying at least 500ma to 1amp to 
make that resistor reasonable but even 500ma seemed high for that wire 
to me... I guess it probably could handle it... I am probably vastly 
underestimating it's power handling capability...  I use 28 awg wire for 
most digital stuff I do because it's common, cheap, and reliable and I 
have lots... but it's certainly larger than needed... This wire in the 
TTY is probably about 30awg and maybe the power rating for that resistor 
isn't as high as I'm estimating... it's probably actually well under 
50w... I'm sure I could find the schematic in my binder of pages. As you 
it's probably not super precise as I assume it's just to get the voltage 
in range for the solenoid coil (my guess) and it was probably a 10 or 
20% tolerance part.

I did speak to Paul before about something else and he was not willing 
to ship to the UK, but a small resistor might be a different story.

Cheers,
Troy

On 09/06/2022 13:24, Ethan Blanton wrote:
> 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