[GreenKeys] New to me Model 28 and needing assistance

[Jim Cooper]

On 7 Oct 2021 at 14:20, SK Campbell wrote:

> Again, before I heavily modify this
> unit back to semi-stock operation
> for amateur radio use, I wanted to
> ensure it is mechanically sound and
> functionally operational. Since I do
> not want to damage any components, I
> am seeking any and all guidance and
> information on moving forward with
> least amount of consternation, on
> possible interface connections,
> testing and troubleshooting. 

Steve ...  I'm going to use some text that I recently sent to 
Mike regarding debugging his M28 printer, but I'll add some 
stuff in front for your situation.   This is just MY 
suggestiong, remembering what I can from my days at 
Mackay Radio and ITT Worldcom M28 Leased Lines 
department.  (a long, long time ago!)  

Here's my first suggestion :  

since you describe quite a complex wiring arrangement on 
your unit, my suggestion is that you trace the printer 
selector magnets back to the first terminal block they go to 
...  then remove, AND LABEL, any wires on each of the two 
TB screws -- except the wires to the sel. magnet. (Now you 
have JUST the sel. magnet isolated).  

Now do the same thing from the kbd contact back to the 
first TB it's wires go to, and do the same thing -- remove, 
AND LABEL, any other wires going to those two TB screws.  
(Now you have JUST the keyboard contacts isolated).   
Next, put an ohmmeter across the two TB screws going to 
the keyboard contacts, and with motor OFF [in fact, 
everything disconnected from power!] press the R key on 
the keyboard; now turn the motor in the proper direction 
(backwards could jam and break something) and watch the 
keyboard 'distribute' the start, 5-bits, and stop and you 
should see the ohmmeter showing open and closed contacts.  
If that does not work, figure out why before going any 
further!  If the contacts appear to be keying ok, put away 
the ohmmeter.  

Next, get out the ohmmeter again (!) and put it across the 
two TB screws that go to the selector magnet; measure the 
resistance of the selector magnet coils and be sure the 
value is 'reasonable'...  I don't remember the exact ohms; 
it's shown in the parts manual.  If you see more than zero 
ohms, and less than about 100 ohms they are probably ok. If 
not, figure out why before going any further.  If ok, put 
away the ohmmeter (again!).  

Now, connect a jumper wire from one of the keyboard 
screws to one of the selector magnet TB Terminal Block 
screws (ie: put them in series).  

If you have a loop supply already, let's use that; if not, set 
up a 120vDC, 20mA power supply and loop (I suspect the 
M28 sel. magnet coils are set up for 20mA loop). Remember, 
you need a bit fairly high-power adjustable (either slider or 
wire-wound) resistor in the loop.  Turn OFF the loop power 
supply and be sure it discharges. Get out the ohmmeter 
again, but now set it to the 100mA DC range; put it across 
the 'loop' wires (one end of the resistor and the other wire 
from the pwr supply) and adjust the resistor for 20mA in 
the loop. Turn off the power supply and let it discharge 
through the loop. Disconnect one side of the milliameter and 
connect it to the unused keyboard TB screw; then connect 
the other side of the loop to the unused screw from the 
selector magnets, through the milliammeter.  Turn on the 
loop supply; if the milliameter doesn't read positive, turn 
off the loop supply and let it discharge - then switch the 
milliammeter leads and start the loop supply again.  

IF your keyboard contacts are closed (like they should be at 
idle) and the selector magnets are good and the wires from 
the Terminal Block screws are good, you should see LOOP 
CURRENT on the milliameter; re-adjust it for 20mA.  

Again, the printer motor should be OFF and you should have 
figured out which way it normally rotates.  

With the 20mA flowing in the local loop, press the Y key on 
the keyboard and then slowly rotate the motor in the 
proper direction ...  as you rotate, the kbd should generate 
the proper mark and space sequence for the letter you 
pressed; as you do that, the local loop will be keyed, the 
selector magnet will respond; the code selection bars will 
respond to the selector magnet moving; and that bit should 
set up properly on the code selection bar of that bit.  When 
all five code bits have been generated by the kbd, the code 
selection bars should be in the proper positions -- if they 
are, then they are working ok; then you will have to continue 
manually rotating the motor and watch the typebox 
alignment and see if it is responding correctly.  This should 
all work in 'slow motion' as you manually rotate the motor.  

A code bar or other item missing a spring should be spotted 
right away as it will not respond as it should.  A code bar or 
other lever that is 'stuck' for one reason or another (gunk, 
obstructed, no spring, etc.) should be obvious, and you can 
do your mechanical magic to figure out where and why.  

If the code for Y set up correctly on all the code bars and 
levers, do the same thing by pressing the R key on the 
keyboard and slowly rotate the motor while watching the 
code bars set up.  In this case, ALL FIVE CODE BARS 
should go to the opposite position (that's why RY is the 
universal testing combination for Baudot code).  

If all of the above checks out, everything sets up to the 
proper combinations, nothing jams, etc. then you can turn on 
the motor.  With the loop still connected, the printer should 
'rest' in MARK mode (quiet and nothing much happening).  
Again, press the Y keyboard key and the printer should 
print a Y on the paper!  Press the R key and the printer 
should print an R on the paper!  If so, then type the 
universal test message; THE QUICK BROWN FOX JUMPED 
OVER THE LAZY DOGS BACK 1234567890 TIMES  

You will notice that sentence has every letter of the 
alphabet in it...   and the numbers.  

If the test sentence prints perfectly, go for a nice dinner!  

At this point, you have eliminated all the garbage wiring in 
the unknown setup you have, and you have confirmed that 
the two MAJOR components are working properly -- the 
keyboard and the printer.  

You could now connect your terminal unit into the loop and 
start receiving RTTY traffic if you want to.  All the rest is 
now just deciding which of the various gadgets and stuff in 
the cabinet you want to include, and checking the wiring 
from each gadget to it's TB screws; testing the gadget if 
there is a way; and then adding it to the loop (if that's 
where it goes) by connections on the TB. Do ONE GADGET 
at a time -- don't try to add a bunch of them at once, 
because if things don't work you won't know where the 
problem might be.  

THAT's my suggestion.   

Jim  W2JC