[GreenKeys] M28 ASR loop wiring

[Duncan Brown]

My background/experience has mainly been with Kleinschmidt units where 
the external loop connects directly to the selector magnets, with a 
switch or plug selection to choose between 20 & 60 ma. loop.  (no "Line 
Relay")
Up to now, all M28s I've worked with just ran when they were plugged in.


I am trying to correct the wiring of a M28 ASR that was rewired by "some 
ham", but I am getting confused by all the "flexibility" in the ASR design.

Presently, the unit is operational in both 'Line' & 'Test' modes, but 
when I plugged it into a 60 ma loop, I could only get a maximum of about 
50ma from the TU. Measuring the M28's Signal cable (connected to the 
ASR's terminals 5 & 15), I found that there was an internal 1K of 
resistance. Also, the Line Shunt relay was not shorting the line with 
the power off.  This was when I started to think that there had been 
some mods made to the wiring.

Upon further investigation, I have found that the external loop cable is 
connected _directly_ to the Selector Magnets (in series) through 
internal resistors of 400 + 600 ohms and the Auxiliary Reperf, bypassing 
the Line Shunt Relay and the polar Line relay.

In looking at the ASR's schematic (M28ASR W/LESU13), it looks like the 
Selector Magnets are normally wired in series with 3.55K, through the 
Line Relay contacts. This would give an Internal Loop current through 
the SMs of about 40 ma.
  1.  Is this correct, that the internal loop runs at 40ma and that is 
optimum for two SMs in series?

  2.  Is there any change/adjustment needed to the LESU for the external 
loop running 20 or 60 ma?  The external loop seems to run right through 
the line winding of the Line Relay. Can the Line relay take any input 
current? With in what range?  Or is there a change to the Line Relay 
bias current needed between 20 & 60 ma?

I could run this unit at 20-40 ma, but would rather get it restored to 
normal wiring (I haven had a chance to ring out all the changes yet).

Is there a good description somewhere of the options & changes needed 
for different loop currents of the M28?

Thanks,

Duncan
K2OEQ




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