Normally when ship faces problem with either picture moving from side to side or found acquire echoes not possible, it was either cause by ARPA pcb or by rotary encoder . Replacing Rotary encoder GTX-A188 can be a disaster if technician not trained by maker to do the job.  
Step 1 : Isolate power from the supply inside the MK7 transceiver on the left.
Step 2 : Removed the 4 screws holding the cage.
Step 3 : Removed the connector from rotary encoder.
Step 4 : Removed the 2 x + screws PH1 from both sides of encoder.
Step 5 : Removed all connectors between cage and housing.
Step 6 : Slide the cage out of the housing.
Step 7 : Slide the rotary encoder down by unscrew the two small hex screw holding the rotating joint.
Step 8 : Insert the new rotary encoder back. Do the steps in reverse order.