<span style="font-size: smaller;">ORIGINAL: JPMacG I have now had 4 out of 4 HS225MGs fail. My advice is don't use them on a critical control surface.
Throttle is OK. I don't know for sure that my failures were related to voltage, but I was using a 5 cell NiMH pack in each case</span>
Unusual to have so many stop working if not age related, the following might assist with diagnosis/repair:
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Servo - Burnout. Cause & Prevention
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Servo - FAQ : Metal Gear Servos - DIY Repair & Extend MG Servo Gear Life
More information available under sub sections
"Glitches & Jitter in Receiver, Servo & ESC - Causes and Cures"
"Servo - Alterations, Calculators, Databases, Leads, Repairs, Convert to an ESC or winch & FAQ."
below
"Radio Systems, Accessories, Alterations and FAQ" at
Alan's Hobby, Model & RC FAQ Web Links
Alan T.