David,

Some M28s have a "feature" that shorts the signal line when the unit is unpowered.  So terminals 5 & 15 are shorted together when the power is off.  This is done to take the unit out of the current  loop that might have other machines connected into it.  Turn the power on & test continuity at terminals 5 & 15 and on the plug again.


On the selector magnet, have you identified the armature? This is a lever hanging down under the selector magnet.  With the power on, push  up gently on the armature and the machine should stop "running open".  Move the armature up & down slightly and you should get some random printing and the type box will move.

Some M28s have a polar relay or "selector magnet driver" module between  terminals 5 & 15 and the selector magnet winding. You need to find the schematic for your ESU (Electrical Service Unit) or an overall schematic of the unit to see how the unit is wired.

With a schematic & WD, you should be able to figure out how the unit was wired originally.  But beware, it may have had it's wiring modified at some point, which will add to the complexities! (I went through this with a M28ASR last year.)

Good luck & have fun,

Duncan Brown, K2OEQ
USASA    31J30

Antique Wireless Association
 Communication Technologies Museum
   Asst. Curator, Commercial Equipment
(also chief TTY op & repairman)

i-Telex: 212503

www.antiquewireless.org

AWA Youtube channel: https://www.youtube.com/channel/UCX55peBhzeX1qps_VYXdLBA

Virtual AWA Museum TTY Tour:  https://www.youtube.com/watch?v=TpB6jM90VV8




On 9/13/2024 4:28 PM, Jim Cooper wrote:
(I am copying this to the GreenKeys list since 
some of the info might be of interest to other 
Model 28 owners)

On 13 Sep 2024 at 14:23, David S wrote:


Apologies, I meant no reading, as in
air, (no continuity reads as 0L on my
multimeter) although I think maybe I
am measuring in the wrong spots.  I
am putting leads on the tip and ring
(as an example) which are next to
each other but I would expect these
to be electrically separate. 
Apologies if I am misunderstanding,
its taking a bit to understand this
machine.  I did try a variety of
scales. 

ok...  when checking just 'continuity' (whether
one point is connected to another point) we generally 
say "it's OPEN" if there is no connection and 
"it's SHORTED" if they are connected.  

I would expect none of the three connections on the 
PLUG to be connected to each other, EXCEPT if going 
through something like the selector magnet coil. 

Check the CONTINUITY (no resistance) between 
pin 5 of the teerminal strip and the three contacts 
on the plug;  one of the plug contacts ought to be 
connected to pin 5. 

Do the same, all three on the plug, to pin 15 on the 
terminal strip.  I'd expect one of them to be connected. 
 

I did find these go to terminal
board locations 5 and 15.  Wiring
diagram just gives wire jacket color
and not term or destination though (I
am sure I can find the destination,
just not deciphering it yet).  Ill
trace these farther back and see
what they are going to.  I assume
ultimately they will go to they
selector magnets, but they don't now
(no continuity), I assume because
they will have to switch on based on
something and are just off when
idle/unpowered. 

ok, make not of which (tip, ring or sleeve) 
goes to pin 5 and which goes to pin 15... 
that's the start of your own 'wiring diagram' 
 

I did find the two selector magnets

fantastic !!   now you need to find the electrical path 
from the TRS plug to the selector magnets... 
 

The speed has me stumped, the large
gear has 195996 stamped on it, but I
cannot find that part anywhere in
any of the gear parts lists I have
found, about 5 lists. 

I just looked on line and did not find any 
reference to that number.  It's a quite high 
part number, so my guess is you might have 
a 75 baud / 100wpm unit. 
 

I did clean it, lube it, and inspect
wiring and felt confident enough to
plug it in and it spins, but it
keeps striking the same empty
character over and over and the
keyboard is unresponsive. So good
news is that the motor works, but
based on videos I have seen it
should be that the motor spins in the
background but the type striker
should be idle?  

Only idle if you have the 60mA going through 
the selector magnet coils.  Which I presume at 
this point you do not. 

What you are seeing is called "running open" ...


Keyboard unlock key has no  effect,
none of the keys have any give and
the only key depressed is the LTRS
key, which I believe is the "un
shift" function, opposite of FIGS? 

If the LTRS key is depressed and does 
not pop back up, it would seem that it is 
stuck... which is not good. 

My recollection is the KBD LOCK is a 
momentary press function, which releases 
all the rest of the keys to be used. 
But ... you say the LTRS key is stuck in the 
depressed condition...  that needs to be 
cleared before the kbd will work. 


The carriage does not advance either.

The carriage spacing is suppressed when 
"running open" 
 

I am not sure the term for this
condition where it runs wild like
this with dead keyboard. 

running open ... 

it MIGHT be that "running open" locks 
the keyboard ... that would make sense, 
because there's no point in typing on the 


keyboard if the machine is not "in the loop"