[GreenKeys] Model 28 KSR wall mount model

[Duncan Brown]

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"
>   
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