This is my Strip Down and continous rotation mod to the Futuba S3004.  Right now PM Hobby sells these here in Calgary for about $18.

Specs as listed at robotshop:

Specifications :
• Single ball bearing on output shaft
• Precise tight fit throughout the geartrain
• Being standard size and lightweight, it fits many applications.
• Three pole motor
• Speed: 0.19 sec/60o at 6V
• Torque: 4.1 Kg-cm (57oz-in) at 6V
• Standard "J"-type connector with ~5" of wire
• Nylon gears

Start by removing the horn, flipping the servo over and removing the bottom 4 screws.

Have a quick look at the gears layout and note locations for later.  A snapshot with a digital camera always helps if you have a short memory   I usually place the gears right above where I am working in the proper order.

Have a look where the output shaft (the one with the bearing) was.  You'll see a shaft sticking through the hole with 2 flat sides.  This is the feedback potentiometer.  Unlike the S148, this servo's output gear is completely molded so there is no plate to remove.  Instead we must decide on a different plan of attack.  If you have the room, you could simply desolder the potentiometer and mount it outside the case for easy trim adjust.

Since I prefer to leave the case looking stock, I'll cut the potentiometer's shaft so that it will no longer turn with the output gear.  Carefully extract the board to gain access.  This is a bit tricky as the potentiometer is soldered last onto the board, and is held into the case with 2 clips molded into the shell.

Once the shaft is snipped, reassemble the board into the shell.  It took a bit for me to finess the pot properly back inside.. I had to push it into place from the bottom with a screwdriver so that it would clip back in.  I also filed a bit of a slot in it so that I could adjust the trim.  Remember to also cut the stop bar on the output shaft.

Quick test with my servo tester - all is good.  90 stops the motor completely, and forward/reverse are both working perfectly.  All in All Took about 1/2 hour including taking pictures.  I think future mods should take ~ 20 minutes.  I also think I can skip removing the board and just cut the potentiometer shaft flush with the casing.  will be a touch harder to set the pot, but will drastically cut down mod time.