In response to the MMD posting of Mr. Skip Downing, I know of no manual
available today for the T-98 Welte Mignon (green) system.  Some years
ago Frank Adams reprinted some charts showing the expression units and
related valve chests as well as the installation manual for T-98 pianos.
Other than that there is precious little that has been reprinted.

The tracker bar scale is listed in several publications.  My best
suggestion is to try to find a friend with a post 1920 T-100 [red Welte]
piano and examine the layout of the components.  The expression units
will be similar except that the valve units controlling the expression
will be designed for the T-98 scale.  The layout will be very similar.
The roll drive, tempo and rewind are very similar as well.  Pre-1920
T-100 red paper Welte instruments are sufficiently different as to not
help much and are likely to be more confusing.

Rebuilding a Welte-Mignon T-98 (green) piano without first hand
knowledge of the system will be quite a challenge, especially with
the lead tubing removed and parts perhaps missing.

Tom Jansen offers a nice selection of T-98 recuts as well as the
most critical T-98 Skala (test roll).  The expression units cannot
be calibrated without a proper test roll.  Tom's web site is
http://www.maesto.com/ for further information.  I should add that
I have a number of Tom Jansen's recut T-100 rolls and am very happy
with them.  I expect his T-98 rolls are at least as good.

I have been talking to a friend about publishing a service manual
for the Welte-Mignon but this is still a long way off at this point.

I wish Mr. Downing well in his endeavor.

Mark Reinhart
Charles Town, West Virginia, USA