Some lessons from the School of Hard Knocks: 1. Disassemble and assemble over a large flat surface. There are some small pieces that are easy to drop and lose. A table saw makes a very poor workbench for this project, even more so if there is an inch of sawdust under it.
2. When you remove the screws from the servo body, lift the body up in the direction the screws came out. This will leave the gears in the bottom part where they won't fall out. Take careful note where they go - I suggest you take a photo for use when re-assembling. 3. Use a very low wattage soldering iron - 15 W is ideal. Use a conical point tip if possible. Modern servos use very fine copper tracks that are easy to lift away from the board if you cook them. They also use surface-mount devices and everything is very tightly packed on the circuit board. It's easy to melt an adjacent joint on a pad or track and ruin the whole thing. You probably won't damage the semiconductors before you damage the physical layout. 4. Use a desoldering vacuum pump rather than copper braid (or worse, spare copper wire). See #2 for the reasons. You can buy a bulb or spring-loaded pump for just a few dollars at Radio Shack and many other places. As soon as you melt the solder joint, remove the iron and immediately suck away the liquid solder. Don't be timid with the suction. If you get the hang of it, you'll clean the joint on the first attempt. Once you desolder the motor terminals, the board will probably lift out the case very easily. A small dab of Vaseline (poor man's flux) on the joint will help the iron melt it quickly. 5. I suggest you cut the pot wires rather than trying to desolder each from the board. Again see #2 for the reasons. If you want to replace the pot, it's easy to solder each cut wire back together. If you are replacing the pot with resistors, it's no big deal to solder them to the wires. I found it very difficult to get the pot wires out and the new leads in without melting adjacent joints and I was worried I had disconnected some surface-mount pins. Don't forget to seal everything with hot glue, silicone, or electrical tape to prevent shorts. 6. If you remove the motor from the case, mark both in some way so you get it back in the same orientation. 7. When re-soldering the motor terminals, apply the iron where the terminal meets the board just long enough to melt solder over the joint. If it cools with a shiny surface, it's good. If the surface is dull, apply some Vaseline or flux and heat it just enough to make it flow. 8. Use fine rosin-cored solder for electronics work, not the thicker stuff you get in the electrical section of the hardware store, and especially not the really thick bars you use for leadlighting or heavy plumbing. Shake excess solder off the iron frequently. You want to melt new solder over the hot joint - don't carry molten solder around on the tip. New solder contains a flux agent which makes it flow and stick. 9. 5k pots seems to be the standard, but it wouldn't hurt to measure across the outside terminals just to confirm. The values of the resistors you use aren't especially important as they simply form a voltage divider - lower values will consume a little more power, so err on the upper side if you like. If you have a handful of equivalent resistors with a wide tolerance, try to find a pair with closely-matched values, otherwise the neutral position will be biased in one direction and you'll have to trim your transmitter to compensate. 10. If the gear train needs the gear that was sitting on the pot, and the pot provides mechanical support for that gear, you'll need to butcher the pot to allow it to rotate completely, as shown in one of the links above. If the gear isn't needed to drive the output shaft, you can remove it and leave the pot in place - center it and fix its shaft with some hot glue. (Since resistors cost literally a few pennies, I would do this only if I had no resistors.) 11. Allow about 10 times longer than you expect this process to take. There's nothing worse than running out of time when you have a hot soldering iron and a bench full of tiny pieces. If all goes well, use the spare time to celebrate your prowess. --~--~---------~--~----~------------~-------~--~----~ You are currently subscribed to the "R/C Tank Combat" group. To post a message, send email to [email protected] To unsubscribe, send email to [email protected] Visit the group at http://groups.google.com/group/rctankcombat -~----------~----~----~----~------~----~------~--~---
