[GreenKeys] 60 ma Loop Power Supply?

[Dave Hunter]

Hi Bill:

I have been able to resolve this issue. Last night, I went 
looking for an isolation transformer. Several years ago, I 
picked up a OneAC line conditioner at a yard sale, knowing 
it had an nice isolation transformer in it....

So, I dug that up, hit the junk box and dug up a bridge 
rectifier, electrolytic, and two 1.4 K 18 watt wirewound 
resistors. After removing the extraneous items from the 
OneAC box, I had room to assemble everything inside. Now, 
the resistance wasn't ideal, but it was the closest I could 
come with what I had.

End result, an isolated current limited supply that puts 
out about 150 volts, and connected to the teletype, the 
current measured by a DVM is 55 point something ma. Pretty 
lose to 60. I will play with the resistors later to get it 
closer to 60 ma. I would love to find a variable resistor 
of a large enough wattage  that would allow me to adjust it 
for either 60 or 20 ma. There is room in the box to put in 
a couple more to give me a 20 ma. output as well, should I 
ever finr an ASR33 machine locally. It makes a very nice, 
professional looking package.

I connected it this morning, and lo and behold, the machine 
now listens! I had a bit of trouble getting the printer to 
print, but discovered the tape key was stuck slightly down. 
I accidentally hit the send key and all of a sudden the 
printer sprang to life! 

There are a couple of problems with the print, for one, 25 
year old dried out ribbons which I am pretty sure were 
pretty well de-inked when it was put out of service - I 
have WD-40'd the ribbon to bring it somewhat back to life. 
The second problem is it seems to be printing a bot low on 
the platen, with the result the top portion of most of the 
letters prints best, the bottom halves of many letters not 
printing at all. But, as least the unit is now operating.

Thanks to all for your suggestions re: loop supply

Dave, VY2AC

On 5 Dec 2009 at 13:36, Bill Horne, W1AC wrote:

> Dave, 
> 
> There are several approaches you can take, depending on your
> tolerance for risk and the equipment you have on hand
> already.
> 
> 1. Use an existing Terminal Unit for the loop supply. Most
> TU's have
>    loop supplies built in, and will drive the 28's magnets
>    directly. 
> 
> 2. Use the supply built-in to some 28's. Many Model 28's
> come with a
>    loop supply already installed, and you can use that if
>    it's available. Check the shelves below the typing unit.
> 
> 3. Construct a "cheap and dirty" supply that connects
> directly to the
>    120 VAC line. I know this isn't politically correct, but
>    we've all done it: if you are _CERTAIN_ that the magnets
>    (and keyboard, etc.) are _NOT_ connected to the chassis,
>    _AND_ that their insulation won't fail in the near
>    future, you can simply rectify, filter, and limit the
>    line voltage for a power supply. 
> 
> 4. Construct a transformer-isolated supply. Any isolation
> transformer
>    will do, or (as someone else suggested) you can put a
>    couple of beefy 120/12 volt transformers back-to-back,
>    and get isolation that way. R=E/I, so 120 VDC/60 MA = 2K
>    Ohms. You'll need at least 7.2 
> watts
>    of heat disapation, so a 10 watt wirewound resister will
>    do fine. Remember to filter the output _and_ to use
>    diodes that can withstand a lot of peak-inverse-voltage,
>    since the magnets will generate a lot of kickback during
>    operation. 
> 
>    This is, by the way, the design used in most Teletype
>    loop current supplies: the higher voltage assures
>    "snappy" performance by minimizing reactive delays, and
>    you can plug in additional magnets (i.e., machines)
>    without worrying about changing the loop current. 
> 
> HTH.
> 
> 73,
> 
> Bill W1AC



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