Unplug all connectors from the 628 and remove it from the rack.
Remove the top cover. Don’t forget the two allen head screws on the front panel.
Locate the ANALOG OUTPUT XLR connectors on the rear panel.
Depending upon the vintage of your 628, you’ll find one of two options:
A) 2 jumper sets of 2 each (J6, J7 and J10, J11) or,
B) 2 jumper sets of 3 each (J4, J5, J7 and J9, J10, J12).

If your 628 matches option A, then move both jumpers next to the output connector which you want to convert to microphone level rearward to the jumper pins closest to the XLR connector.

If your 628 matches option B, find the 3 jumpers nearest the XLR connector that you want to convert to microphone level. Move the left jumper (J4 or J10) so that it connects the two jumper pins furthest to the right. Move the right jumper (J7 or J12) so that it connects the two jumper pins furthest to the left. Now insure that the remaining jumper (J5 or J9) is in a rearward position that connects the two jumper pins nearest the XLR connector.

Replace the top cover and all retaining screws.
Reinstall the 628 in your rack.
Connect the power and audio cables