RE: ds821 dx7 servo defect
This sounds alot like the Hitec Karbonite gear failures from a few years ago. All new stronger plastic gear material followed by output shafts breaking off and planes crashing. Hitec provided longer screws reminded everyone not to use loctite on the screws (you don't need it on plastic and it ruins it anyway) and suggested not using the servos on planes greater than 12lbs. Haven't heard nearly as many complaints after that. Maybe Hitec sold JR the Karbonite formula for a good price but not all the fixes to make it work. The DS811 (I have one that works great) was standard nylon gears and the DS821 uses "New, longer-lasting plastic gear material" and is listed as a composite. You might be able to retro fit the DS811 gears into the DS821 if you trust nylon more that the new stuff.