DECwriter Correspondent (LA12) mechanical issue...

[Josh Dersch]

Hi all --

Got myself a nice looking DECwriter Correspondent (AKA the LA12). It's 
having a bit of trouble moving the carriage assembly and I can't find a 
service manual for this thing.  (It's similar in some respects to the 
DECwriter IV, but the mechanical parts appear to be fairly different.)

The carriage assembly (print head, ribbon) is carried from one end of 
the platen to the other via a cable attached to a servo motor on one end 
and a pulley on the other; in the middle of this cable is a small metal 
ball -- this ball is (as far as I can tell) meant to sit in a small 
"cup" on the underside of the carriage and when the cable moves, the 
ball pulls the carriage along.  I'm assuming this was done this way 
since the Correspondent is meant to be portable, and allowing the 
carriage to break free of the pulley mechanism would probably reduce 
damage in the case of a sudden shock (like getting dropped.)

So far so good -- unfortunately the vast majority of the time, the ball 
leaves the carriage behind (especially on carriage return) and I can't 
quite figure out what's out of tolerance -- the carriage appears to move 
smoothly, nothing is bent out of shape or broken as far as I can tell, 
and there's plenty of tension in the cable.  There's also not much to 
adjust here so I'm kind of puzzled.

I've put up a few pictures (and a short video demonstrating the problem) 
with the print head/ribbon removed here:

http://yahozna.dyndns.org/scratch/la12

Anyone ever worked on one of these before?  Any ideas?

Thanks as always,
Josh