I've been doing more digging in M28 schematics & WD's and have answered some of my questions.

1. Due to blurry images, I was mistaken about the internal loop current. There is resistance of about 4.0K with 120VDC which gives an internal loop current of 30ma through the series-connected selector magnets. (That's closer to 20ma we normally think of with series connection. )

2. I get the impression that M28s were normally set up for 60 ma external loop.  If you want to change to 20 ma, there are changes in the LESU that must be made to decrease the bias current of the Line Relay.

When I get back to the Museum on Tuesday, there are a couple more strange connections I need to check out, but I think now that it should be relatively easy to get this unit back to normal.

73,

Duncan
K2OEQ

On 10-Mar-24 13:35, Duncan Brown wrote:

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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