[GreenKeys] 60 ma Loop Power Supply?

Sat Dec 5 20:56:39 EST 2009

As some folks do, I circumvent the serial connection altogether on the PC. I
simply use the audio out on the PC and route in my (too many) ST-6's which
also supply loop current.  Then I run MMTTY.exe to send secret Iraq war
communiqués and my kids think I am a secret agent!  

But seriously.  MMTTY already converts ascii to baudot. I can cut and past
any document into it and away the 28ASR goes. I can even punch tape that
way. I have picked up a lot of ST-6s' over the last few years. I bought a
beautiful one in NH hamfest this past fall. Mint shape, all cards, just $5.
So for folks looking for loop supplies, these ST'6's can be great and also
have a TU in them hihi. Don't know why but they keep following me home!

73 Eugene W2HX

-----Original Message-----
From: greenkeys-bounces at mailman.qth.net
[mailto:greenkeys-bounces at mailman.qth.net] On Behalf Of Bill Horne
Sent: Saturday, December 05, 2009 7:37 PM
To: greenkeys at mailman.qth.net
Subject: Re: [GreenKeys] 60 ma Loop Power Supply?

Dave Hunter wrote:
> 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.
>   

Glad I could help. Of course, now you're faced with the problem of how 
to get your machine to copy from a computer: it's OK to run "brag tapes" 
when there are visitors in the shack, but nothing impressives the kids 
at Christmas more than seeing your machine print out some "super 
seecrit" details on ship positions or weather in Kandahar.

Step one is to set up an interface between your PC and the current loop.

   1. At the start of the computer revolution, loop-to-RS-232 interfaces
      were common, since Model 33 Teletypes were the terminal du jour
      for PDP-8 and other low-end computers. You'll see them on Ebay all
      the time, in the $50 range.
   2. If you choose to roll your own, the technical work has already
      been done for you, since the a member of the Morse Telegraph Club
      long ago designed an interface that connects RS-232 devices (e.g.,
      modems) to local Morse sounder circuits. Since Morse sounders are
      solenoids just like your machine's selector magnets, there's no
      need to reinvent that particular wheel. I'll provide details on
      request.

Once you have a circuit in place to convert your RS-232 serial signal to 
a current loop, you then need to figure out how to get your PC to 
produce Baudot output. Fortunately, there's an easy workaround here as 
well: assuming you can re-time your serial port to 45 baud, you need 
only send ASCII characters which are equivalent to the Baudot letters 
you really want. The Baudot sequence "RY", for example, is simply "u+" 
in ASCII: the "high order" bits, which are transmitted last, are all "1" 
in the equivalent ASCII sequence, so that the Baudot teletype sees them 
as a stop pulse followed by an inter-character gap.

If you _can't_ get your serial port to go that slow, then the plot 
thickens: you'll need to use ASCII characters which will translate 
multiple bits into a single bit position of the Baudot code. For 
example, you could clock out an entire ten-bit ASCII character in 22 ms 
by setting the UART serial clock to 454 bps, and that means that every 
ASCII character you sent would occupy _one_ Baudot bit position at 45 
baud. Assuming your serial port can be set to _not_ send parity, you 
could then send ten ASCII characters in place of a single Baudot 
character, i.e., the ASCII sequence 
NUL-DEL-NUL-DEL-NUL-DEL-DEL-DEL-DEL-DEL would be equivalent to a Baudot 
"Y" if sent at 454 baud. I know that's confusing, but it does work.

Good luck with it. Maybe we'll meet on the air sometime.

73,

Bill W1AC

-- 
E. William Horne
William Warren Consulting
Computer & Network Installations, Security, and Service
http://william-warren.com
781-784-7287

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