This is a very basic cheap electric toothbrush, a twin pack of new heads costs more than the whole unit complete with batteries.They seem to have a relativity short life,I fully expect the next evolution to be a fully sealed disposable item. How ever with a little TLC the power unit can be made to out last several heads.
Step 1: Basic Maintenance
The regular problem seems to be the drain hole and then the top section of the motor unit blocks up with toothpaste residue.
Just taking a tooth pick to the drain hole and if necessary using one to remove the build up from around the output shaft. Then flushing it all through with water every week or so should help extend the life quite a bit.
If things have got to the point that the motor won't spin before you start doing this even with new batteries manually turning the motor shaft with a pair of long nosed pliers (eye brow tweezers make a good substitute) can be enough to free the motor up and restore it to life. How ever this means things have progressed beyond just being gunged up, whilst this will work and do so a couple more times the next step will be needed at some point.
Step 2: Dismantle
Previous incarnations of the handle for this brand of tooth brush haven't been dismantleable, with out destroying the handle, due to a change in design, (smaller motor & AAA batteries instead of AA) the motor switch and battery terminals are now all mounted in one easily removable carrier.
Grasp one of the 2 locator clips with a pair of long nosed pliers pull it slightly inwards to disengage it from the clip and withdraw the carrier assembly. As can be seen the top of the seal is coated with toothpaste residue to remove the seal requires removing the drive gear, this is simply achieved by gripping the base of the drive gear with the pliers and pulling it off. As can be seen in the last picture the seal isn't perfect water has penetrated around the shaft and cause a build up of corrosion
Step 3: Clean and Lubricate
Start by thoroughly cleaning any remaining toothpaste residue from the plastic body and leave it to dry whilst you turn your attention to the motor.
The rubber seal is probably adequate for damp or even occasional immersion in water, what it can't stand it seems is prolonged standing water. Over a period of months, if not cleaned regularly per step 2, toothpaste residue clogs the drain hole and then the seal is left trying to hold back a water for 24 hours at a time.
Remove the sludge from the end of the motor and the shaft bearing, a piece of rag or facial tissue is all that should be needed, spray WD40 or similar into the motor hold the shaft and spin the motor around by hand rock the shaft up and down as well to help free it off leave it to stand and before refitting the seal clean off any WD40 (or what ever your choice of water displacing and freeing oil was, and note WD40 is not a lubricating oil) that has drained back out of the motor.
Thoroughly clean and dry the seal, then fill the cup on the motor side with clean Vaseline. This is acting as both a lubricant for the shaft in the bearing and as a secondary seal to help prevent further water ingress into the motor. (It wasn't developed to do either and may well cause the seal to degrade but this isn't a long term repair just a life extending one)
Step 4: Test, Reasemble and Test
Switch the motor on put the batteries in the holder and touch the terminals of the motor pack onto the batteries the motor should run freely at full speed. If it fails to do so try checking the switch contacts are clean and new batteries, if it still fails to run there is no point in continuing. you could try stripping the motor completely I did one of my first intructables on this topic
Assuming the motor runs refit the seal, and replace the drive sprocket pushing it firmly into place. It is then simply a case of sliding the motor assembly back into the case making sure the switch is aligned with the correct side of the case and pushing firmly until the lugs click into their slots in the case. Make sure the switch is off and refit the batteries and brush head and check everything works.
If you keep the drain hole clean the motor should last you a good few months more.