[GreenKeys] M28ASR Wiring

[Steve Garrison]

Seasons Greetings my fellow Greenkeyers!

 

Let's hope I can make this short and to the point, but most importantly
understandable.

 

I have looked though untold different wiring diagrams for M28ASRs and have
found one thing in common on every one I have come across!

That is the 4 wires from the selector magnets of the LP always windup in the
same place on terminal block "A" in the LESU, regardless of the LESU that is
present.  Those are screw terminals 1, 2, 3, & 4 on the A-Block.  With the
LESU turned over so the terminal block is accessible, terminal 1 would be
the closet to you if you are positioned at the front of the cabinet.    The
4 wires from the selector magnets are coming in from the left side of the
A-Block and there is additional wiring on the right side of the block.
There is a strap between terminal 1 & 2, which shorts the 2 wires from the
selector magnets together leaving  terminals 3 & 4 to be the terminals that
supply the loop current.  This arrangement has the coils of the selector
magnets in series which would indicate that they were expecting to be driven
by a 20ma loop current.  I see no notes anywhere about changing this
arrangement on the A-Block for 60ma operation, where the selector magnets
would be in a parallel arrangements.  I have seen notes about changing the
wiring directly on the selector magnets for the reperfs if changing from
20ma to 60ma, but nothing about the LP.

 

Is this because in a standard setup the line relay is driven by an outside
loop, which could be 60ma, and the contacts of the line relay control the
loop current supplied by the rectifier assembly usually included on an LESU?
And that loop current is always 20ma?

 

The reason for my question is that I modified, by moving a few wires around,
a 28KSR and its LESU to bypass the line relay and rectifier and drive the
magnets directly from terminals 3 & 4 on the A-Block.   And ever since then
I've had my loop current set to 60ma with my selector magnets in series.

 

So far nothing has seemed to go wrong, no smoke or even the smell of
something wrong, and the LP seems to work just fine.

 

Going forward, I'm helping a fellow Greenkeyer, via the phone and emails try
to clean up the horrible mess of wiring left over from decade of technicians
messing with the original wiring in his M28ASR.  I would like to use the
same approach I did with my M28KSR, which maybe I now need to move a few
more wires on the A-Block to make it right for 60ma operation, but wanted to
get a consensus from the group about what I have discovered.

 

Well it wasn't short, but I hope it was understandable!

 

Steve G./N4TTY

 

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.qth.net/pipermail/greenkeys/attachments/20201224/1c409fd9/attachment.html>