[GreenKeys] M28 using 20ma

Duncan Brown duncanancy at earthlink.net
Sun Jul 28 12:14:44 EDT 2019 

The M14 & 15 TTYs were designed to replace Morse keys on telegraph land 
lines, so were designed for the same 60 ma, 120V supplies that the 
telegraphs used. You needed high voltage and substantial current to get 
through all the reactance of 10-20 miles of land lines.

The next generation (1950) of machines, the Teletype M28 and the 
Kleinschmidt machines where designed for both land lines and for use 
with modems and RTTY.  So they were designed for both 60 and 20 ma 
service. All the Kleinschmidt machines had a 20/60 ma switch. Most of 
the post WWII military RTTY gear would only operate on 20 ma. (The only 
one I can think of that used 60 ma was the AN/GRC-26 which started out 
using M14 & M15, but then changed over to Kleinschmidt machines.) The 
standard military wireline modem, the TH-5/TG (and later solid state 
TH-22/TG) only operated on 20ma.

So if  you are going to be operating over miles of wire, you should be 
using 60 ma. But if you are going over feet of wire to a VF or RF 
"modem", either current will work.

However, W9DDD makes a good point that 20 ma equipment designed for M33s 
might not be robust enough for a M28.

My 2 1/2 cents.

have fun,

Duncan
K2OEQ



On 7/28/2019 09:42, Ralph Irish wrote:
> When the topic of 20 ma vs. 60 ma came up, virtually everyone I can 
> remember recommended the 60 ma
> option.  One of them was Keith Petersen, who was the designer of the 
> famed TT/L-2 TU of the mid 60s.
> To also give some important credit, Keith used as the basis of his 
> design, the also well-known TT/L TU
> design by Irv Hoff and created a flock of modifications/improvements 
> to it.
>
> Keith lived just a few miles from me and was active in several area 
> radio clubs.   He and a few friends
> also put up the first 'two location' repeater in SE Michigan and it 
> was a huge success.  All of the other
> early repeaters used one location and had to deal with the usual 
> problems of de-sense with filters and
> the like.
>
> -   -   -   -
>
> One year, in anticipation of the Armed Forces Day annual RTTY message, 
> I asked him about this.  His
> recommendation was to wire things for 60 ma.   I set my equipment up 
> for 60 ma and never had any
> problems.
>
> At that time I had a Model 19 and was using the ESSCO TU which was 
> built from a series of kits from
> them.  The basic TU in that was almost component for component right 
> out of the RTTY Manual by
> CQ Magazine.  They have one item called, "The Transistorized W2JAV 
> TU".  The folks at ESSCO
> thought it was a great start for an all solid state TU and built their 
> design around it.  They had a Selector
> Magnet Driver board with some kind of  HV transistor to handle the 
> current and associated heat.  There
> was a power supply board also.  They offered the TU as a 850 shift for 
> 2125/2975 Hz and a 170 shift
> for 2125/2295 Hz.  But in the small cabinet they offered, one had to 
> choose one or the other.  There was
> no practical way to have both boards in there accessible by a switch.
>
> They also offered an AFSK board which was a plug-in item. I had the 
> entire package and had fair luck
> when putting it together and putting it on the air.
>
> I found a 'screen snapshot' of the cabinet.  I will attempt to attach 
> it to this e-mail.  Unusual cabinet
> shape!
>
> I have no 'scientific back-up' for using 60 ma as opposed to 20, but 
> more people I knew were using 60
> and having great luck on the air.
>
> For now,  73,
>
> Ralph
>
>
>
> ------------------------------------------------------------------------
> *From: *"Steve Garrison" <steve.n4tty at gmail.com>
> *To: *paul0926 at comcast.net
> *Cc: *greenkeys at mailman.qth.net
> *Sent: *Sunday, July 28, 2019 6:35:04 AM
> *Subject: *[GreenKeys] M28 using 20ma
>
> To change the machine from 60ma to 20ma, on my machines, I would make 
> a couple of wiring changes on the "A" terminal block in the LESU. 
>  That is where the wiring from the "C" block makes it's transition to 
> the connector that plugs into the typing unit.  If memory serves me, 
> and lots of times it fails me, that is annotated as the "R" plug on 
> the schematic of the wiring of a M28.  I could be wrong about the "R" 
> but I'm right on about the "A."  On all my machines so far anyway. 
>  Now if yours has been modified multiple times in the past, all of 
> this might not be of any use in your case.  It should however match 
> what you would find on a KSR or ASR schematic.
>
> I could possibly be prodded into sharing a home made schematic that 
> shows just this portion of the selector  magnet wiring from the "C" 
> block to the "A" block and then on to the "R" plug.
>
> This is how I have done several KSRs and I'm currently starting on an 
> ASR, so I know the Reperf is going to have to be fit into the picture.
>
> The thought just occurred to me, if you had the line relay, couldn't 
> you run the relay windings with the 20ma loop and use the internal 
> 60ma loop for the mags?  Providing of course your LESU has its own 
> 60ma supply.
>
>
> Steve G./N4TTY
>
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