The TAB was quite easy to adjust. Once I located the clutch.
To see the mechanism and linkages I opened the back. On the Ambassador it is easy -- it just unlatches by pulling out on the top sides.
I also removed the cover from the back of the carriage rail. This cover does not need removed. I first tried a few clicks more on the mainspring; no good. Rather than getting the mainspring too tight I decided to see why the TAB would not work without pressing the carriage release or pushing the carriage to the left.
I looked under the carriage and could see how the carriage was released from the escapement, but I could not see why the same bar that moved for CR and TAB did not let the carriage move when TAB was pressed. In order to see what happens I removed the platen and paper guide tray. Both are quite easy to do. This allows a clear view of the escapement, and I could see the TAB clutch and it was loose, but not allowing the carriage to move.
There is no open area to adjust this; it must be done from the back looking in through the frame to the centrifugal clutch of the tabulator. On the ears of the clutch are 4 spring mounting slots.
The springs on mine (there are 2 halves to the centrifugal weights of the clutch) were both in the middle slot. I used a spring hook fed through the open frame to reset the springs each in one slot closer to the clutch pad (from the back of the typewriter from where I was working would be moving each one one space to the right). I tested the tab. It worked.
The full length movement is a bit slow. I left it slow as I do not like slamming tab actions. Moving the spring 2 spaces to the right most slot would make the TAB even faster, but also it kind of slams.
If you never worked on a typewriter before you may not want to make this adjustment. If your tabulator does not work you have nothing to loose unless you loose the springs. The typewriter looks like having to replace either of these springs or the clutch is a major operation.
I do not have a borescope so I could not get any images of the springs or how I adjusted them. It is impossible to get any image without a borescope.