[GreenKeys] Model 28 KSR wall mount model

[Jim Cooper]

(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"
 
> 
> 
> On 9/8/2024 9:42 PM, Jim Cooper wrote:
> > On 8 Sep 2024 at 18:58, David S wrote:
> >
> >> tip and ring 0
> >> tip and sleeve 0
> >> ring and sleeve 0
> > short circuit between all sections?
> > that should not be ...
> > are you measuring on the very low resistance scale?
> >   
> >> Are the selector magnets located near the
> >> "range finder" knob?  I see selector magnets
> >> referenced in the manual located there maybe
> > yes ... they are very obvious when you see them !!
> > wire wound on cores, usually two sections.
> >> Sleeve and chassis was the only place I got
> >> any resistance readings just reading on the trs
> >> plug part
> > that's even stranger, since the sleeve is generally
> > ground ... and presumably the chassis should be also.
> >
> >> I put detailed pictures and notes in - Digging
> >> into the Teletype Model 28 Wall Mount -
> >> Stickelman.net
> >> <http://stickelman.net/index.php/2024/09/07/dig
> >> ging-into-the-teletyp e-model-28-wall-mount/> -
> >> if there is something important I missed
> >> getting photos of, please let me know
> > Excellent photography !
> >
> > "Called a range finder in the manual, I thought this set words per
> minute, but
> > there is a gear that does that somewhere in the machine"
> >
> > speed is set by the small nylon gear on the motor shaft
> > and the large gear with which it meshes.  You should see
> > a part number stamped on the large gear, from which you
> > can look up in a table in the manual to see what speed machine
> > you have.
> >
> > The range finder is used to select what time during the data bit
> > it tested to see if it is a mark or space ... ie:  for 45 baud
> (60wpm)
> > the data bits are 22msec each ... remember Time Constant in
> > an R+L circuit ...  the selector magnet has a large inductance,
> > thus the current will increase from space (0 ma) to mark (60 ma)
> > with a bit of delay ... let's say that delay is 2 msec ... then
> the
> > lowest setting of the range finder that won't cause garble would
> > be  2/22  (first two msec out of the 22) ... = 0.09 ...  or 9 on
> the
> > scale of 0 to 100 on the rangefinder.  If it takes longer than
> 2msec
> > for the current to reach full 'mark current' then the setting o
> > the range finder will be a larger number.
> >
> > To set the range finder (after the machine is working) we send
> > RYRYRY on the local look and move the range finder to the lowest
> > setting that does not garble (note the number); then move it to
> > the highest number before it starts to garble (note the number).
> > Subtract the smaller number from the larger number and
> > divide by two --- set the range finder to that dial number.
> >
> >> Turning the motor was smooth and easy, there
> >> were several mechanisms that all moved about as
> >> I turned and no weird noises.  That part
> >> -seems- ok maybe,
> > sounds like you are in good shape on that ...
> >
> >> but the interior of the motor does not move.
> > not sure what you mean by that ... if you are
> > turning by hand and the main shaft is turning
> > and things are happening then the motor is
> > good ... remember, the OUTER part of the
> > motor (but inside the metal case) is a FIXED
> > winding.  The armature of the motor is inside
> > the fixed winding and has the shaft coming
> > out both ends -- one end for the fan, one end
> > for the little gear.
> >
> >> The manuals mention a clutch that perhaps needs
> >> tripped?
> > when you have marking loop current the printer
> > should be 'idle' and not printing anything ... thus
> > the clutch disengages all the character selection
> > mechanism ... when the loop current goes to zero
> > (space) the clutch lets the selection stuff start
> > working (during the 'start pulse') and everything
> > happens for the next series of pulses until the
> > 'stop pulse' (mark) signals the end of the character
> > code.   BUT you were turning the motor with NO
> > loop current, so that clutch will let all the mechanics
> > function all the time.
> >
> >   > I am taking it slow and easy, ill dig into
> >> where the power comes in (box c) and trace out
> >> the tip/ring/sleeve connections - they go to
> >> positions 5 and 15 (and chassis) so I will
> >> hunt those down.
> > M28 wall mount wiring diagrams are at
> > https://www.navy-radio.com/manuals/tty/573-116-400-iss2-6902.pdf
> >
> >> Thank you so much for all your help, it is
> >> very much appreciated