When they serviced the machine, did they include stitch samples when the machine was returned to you? If so, then it would appear the machine was working properly at the time. Verify that you are threading the machine correctly--not skipping any thread guides along the way, inserting the bobbin turning the wrong direction. Verify everything you are doing with the manual.
Take it back to the technician(s) who worked on it and request that he or she address the problem. Telling you to buy a new machine is not sufficient answer. They charged you a service call and did not repair it. If there are parts or more repairs required, they should be able to tell you what it would take and cost to accomplish having a working machine. From that point, you should be able to make your own judgment whether fixing it is economical or buying new is the better answer.
There are some things you can try but without knowing more of what the machine is doing, recommendations are difficult.
For one: remove the top thread from the machine and ALWAYS RAISE the presser foot when threading the top thread.
Install a brand new needle and verify that it is installed facing the correct direction.
Set the top tension to the midway point--this can always be tweaked.
If it is actually a tension issue, the following web sites may be helpful. Check how your top and bobbin tensions appear. If the bobbin thread still appears loose, the bobbin tension may need tightening. (The bobbin tension should have been properly set with the part was replaced.) You can try tightening the bobbin tension by turning the screw. CAUTION: mark the beginning point of the screw BEFORE making any adjustments so you can return it to the original point if things don't work. AND, turn only 1/8 to 1/4 of a turn at a time and recheck the stitch quality between turns.
Understanding Thread Tension Threads