Disconnect all cables (power supply, keyer ..) from the transceiver and place it on a plane and clean surface.
Place the transceiver upside down, with the frontpanel showing into your direction. Remove the eight screws of the lower part of the chassis.
Remove the lower part of the chassis.
Locate the blue coaxial connector (with grey coaxial cable) in the upper right corner of the board and pull it out.
Now turn the transceiver around, with the backside of the transceiver pointing towards to you and remove the 4 screws of the chassis.
After you have removed the screws, turn the transceiver on it's right side (as seen from the backside).
After pulling the two parts of the radio ca. 2cm apart, you can fold over the right side as shown in the picture. Take care that the coaxial-cable that you removed earlier doesn't get stuck anywhere.
After you have 'unfolded' the transceiver, remove the yellow tape which is fixing the bunch of cables running across the shielding ..
Then remove the 4 screws of the shielding ...
.. and remove it carefully without damaging any wires.
You now see the LOCAL UNIT which contains the reference oscillator in the box shown.
After opening the box you will see a the XTAL, a few resistors and capacitors and the trimmer TC1002.
Now simply turn around the trimmer a few times. Remember in which position it was before you started turning it and turn it to the same position in the end!
That's it. Now reasseble the box containing the reference osc, then replace the shielding with 4 screws.
'Fold' the two parts of the transceiver together again, be careful not to damage any wires. Don't forget to put the coaxial cable which you disconnected earlier in place!
Mount the upper part of the chassis with 4 screws.
Turn the transceiver around and put the coaxial cable back into it's original position....
Mount the last part of the chassis. Be carefull not to damage the white ribbon cable.
That's it! Enjoy your FT-890.