Blue Bolb Out Of Focus

This is hard for me to visualize what you are describing (attaching pictures is much better), but there is two major things that can cause blue picture problems. 1) the blue picture tube itself, and 2) the convergence output stage for the blue (see bottom section).

The picture tube is no longer available (blue CRT UE21743) for the 57F500, but the 57F500A is still available:

Manufacturer    Hitachi Part Number  UE22866 Description  DP23 57 PRT ASSY (BLUE)   List  $273.61

With a little effort you can pull the speaker grille straight off to expose the six screws on the bottom of the bezel/screen assembly. remove these screws and the bezel lifts up to come off and expose the CRTs and mirror. You can look into the blue and see if the blob is present to confirm the tube is bad or dust on the lens assembly.

CONVERGENCE: If your problem has to do with the convergence circuit then when you are trying to align the blue to the other colors then you will find in some areas of the screen the blue doesn't move smoothly and may stick or jump or fold over. Then a technician needs to replace the output parts and realign. this is usually a $350-$450 repair bill. Some people do replace these two output ICs on their own:

Manufacturer   AE Select Part Number STK392-110 Description IC   List $14.95

in stock here: http://www.andrewselectronics.com/welcome.aspx

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TV Dan