[GreenKeys] M28 printer troubleshooting - type box positioning

[Dale Hughes]

Hello Folk,

My name is Dale, I live in Canberra, Australia, and I'm new to this group.
I've recently acquired a Teletype Model 28 receive only machine that was
apparently used at a NASA tracking station near here. It hasn't been used
for probably 40 years, but it's in good condition and basically works after
cleaning and lubrication.

I'm running it on a 75 baud 60 mA current loop and at 110 VAC 60 Hz through
a sine-wave inverter because power here is 50 Hz. It prints RYRY...
more-or-less perfectly with only the very occasional X instead of a Y.
However does have a problem with the type box positioning and it
consistently misplaces characters based on the previous character. For
example (char input on left, prints on right):

MMMNMHMspace MMMNMMMspace  (space is the space bar char)   here the H is
printed as an M
VVVCVPVI                VVVCVVVI                       here the P is
printed as a V
BBBDBZBE               BBBDBBBE                     here the Z is printed
as a B
GGGRGLGlf              GGGRGGGlf     (lf is the line feed char) here the L
is printed as a G

Looking at the character placement in the type box, the problem occurs for
all rows and the problem lines up with the two columns
of characters either side of the centre line in the type box which are the
two columns corresponding to bits 4=space & 5=mark:
H             T
Y              Z
P              L
Q             W

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
if bit 5 goes from space to mark. The state of bits 1,2 and 3 don't seem to
matter. The end result of all of this is that the incoming message
is corrupted and the machine is not very usable.

I've checked the code bars on both sides of the machine and they are
correct for all characters, so I think the selector mechanism is working
properly. The problem seems to be in the type box positioning mechanism,
however I'm not familiar with the M28 machine and don't want to just pull
it apart
blindly looking for a problem because I'll never be able to put it all
back-to-together again!

I'm hoping someone in the group can point me in the direction of the likely
problem and how to fix it! I can send a photo of the printout showing the
results of the QUICK BROWN FOX... message if that is helpful.

Many thanks and sorry for the long email.

Regards

Dale VK1DSH
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