[GreenKeys] M28 using 20ma

[Ralph Irish]

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 



----- Original Message -----

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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