[GreenKeys] M28 printer troubleshooting - type box positioning

[Dale Hughes]

Hi Nicolas,

Many thanks for your thoughtful reply and I have checked those parts of the
machine, all the springs are in place. I have checked the operation of the
selector mechanism a number of times and it seems fine. Sending the various
bits, either as a single bit, or combinations of bits, generates the
correct mark-space positioning of the code bars. I was puzzled for a while
about the common bar, but it became clear that there are in fact two
'common' arrangements. The first one is bits-1 and 2 which is the common
code bar that comes from the selector mechanism and this is used for the
vertical positioning of the type box. There is also another 'common' for
bits-4 and 5 which are part of the horizontal positioning of the type
box. Bit-3
selects which side of the neutral alley is used as you have pointed out.

In the meantime I have found there is a spring missing on the bell crank
that links the code bar for bit-4 to part of the horizontal
shift mechanism. It seems that the missing spring causes the
horizontal shift to miss a column of characters depending upon the
direction of travel which is what I was finding when sending the character
pairs. In that part of the machine there are three small springs which
restore the 'plates' (I'm not sure what else to call them...) for bit-4,
bit-5 and the combined bit-4-5 combinations which determines which column
of characters in the type box is used. Bit-3 selects which side of the
neutral alley is used as you have pointed out. It's a very clever system
for sure. Nick England sent me a link to a useful document (you may have
already seen it):

https://www.navy-radio.com/manuals/tty/jh/ttyman3/TeletypeManuals/model28/Instructional/28_Printer.pdf

and pages 21 and 22 show the relevant functions for the type box
positioning which helped me to locate where the spring is missing. It's a
tiny little spring and I hope I can get some to replace the set of three...
They are Teletype part number 45027 which I found from the assembly
diagrams in one of the Teletype manuals. Of course there may still be other
problems once the springs are replaced and I'll have to deal with that
situation if it arises...

I hope your restoration work is going well! Thanks again for your
suggestions.

Regards

Dale



On Tue, 14 Oct 2025 at 02:07, John Doe <jdoe10449 at gmail.com> wrote:

> Hi Dale, how ya goin'?  Greetings from Canada.
>
> I'm new to the group too and I've been restoring an M28 ASR for the last 6
> months or so. The mechanism is still fresh in my mind and I'd like to take
> a crack at helping you with your machine.
>
> Good work testing and gathering the data to share. I can't claim to be an
> expert but something didn't sit right with me in your message. Your quote
> here:
>
> *"The problem seems to be with bit 4 going from mark to space if bit 5 is mark. There is no problem with bit 4 = space (...) The state of bits 1,2 and 3 don't seem to matter."*
>
>
> It's my understanding that the only difference between H-T, Y-Z, P-L, Q-W,
> is Bit-3. Please see the attached "Dales_Machine_1.PNG" for a map. Both
> columns are fixed by bits 4 & 5 - the left column (HYPQ) if bit 3 is on or
> the right (TZLW) if bit 3 is off - and the rows are selected by the 4
> possible combinations of bits 1 & 2.
>
> If you and the others in the group can agree that Bit-3 is the problem,
> let's start with checking the selector mechanism (I know you checked it
> already). See attached "Dales_Machine_2.PNG". Let's hope it's just a lack
> of lubrication or a missing spring. Repeated information: Check that all
> springs are present on the push levers and intermediate arms and verify
> that movement of the mechanism is snappy and free of sticking.
>
> Amendment: You may need to check the springs on the selecting levers that
> ride the cam.  I failed to highlight them in the attachment because you
> will probably need to remove the typing unit to be able to see them;
> they're underneath.  It's not a difficult job - just 4 screws to remove and
> the typing unit should pull straight up.  Get a sharp 10" long flat head
> screwdriver - preferably hollow ground and press down hard on the selector
> side to prevent the driver from slipping.
>
> Hope this helps.
>
> Nicholas Gromak
>
>
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