X-Y axes right angle adjustment?

[BurnsCrack]

Hi guys,

Could someone help me with setting X and Y axis to the right angle? I have been using my i3 Pro B for a long time and for majority of printed parts it seemed fine. However, recently I've noticed that X and Y axes are not set to exactly 90° to each other. There's small deviation of may be 0.5-1°

This results in the fact that for example a long box and a lid don't match each other if printed in different orientations (along X axis and along Y axis respectively). Also, because of that round objects are printed in elliptic shape. Visually it's impossible to recognize, it looks round, but when measured has deviation of ~1mm in diameter. That for example makes printing any gears impossible cause they won't work as part of one well tuned mechanism

The motor steps are tuned fine - narrow sticks printed along X and Y axes have exact length, so that tuning is not a problem. All symptoms say that X and Y axes are not aligned to 90° angle.https://mobdro.bio/ https://kodi.bio/

However I can't find where exactly I could assemble it incorrectly or which nuts/bolts are responsible for that.

So the main question is: How to tune angle between X and Y axes and what is the best way to measure it?