I have burnt 2 digital servos in a week. Noticed that after changing esc this problem arise. My question, how can a esc lead to a burn servo?

My setup:
Hobby king quik 45a esc pair to a 3300kv motor
Spring rc digital servo (some cheap china made servo)
Mx3x and rx 451

too much voltage to the servo

I doubt the voltage was too much. Most ESC's have a 5-6V BEC and thats the standard voltage for servos. I think the issue is more "some cheap china made servo"....

It is both possible that a HK ESC has a bad BEC that isn't properly regulated and not at all unheard of that two Chinese un-brand servos would go out in a week. Take your pick...

I cant say the servos are at fault, mainly i toasted 2 of the same servos within a week. Someone else who is running the same servos has no issue at all. So i suspect the culprit might be the esc. I have tried switching between the sanwa and the stock rtr kyosho radios and end up burning both servos. So now it comes down to the esc. So was wondering how can the esc have an effect on this? servo states that it can take up to 7.4v. Pardon my noobness, but im lost.

Also when i mate the esc to a normal analog servo it works fine without issues.

maybe your endpoints are not set right. when you put the servo in listen for noise. if you turn hard right it should not be locked up against your arm making noise. back it off until just a piece of paper can get in-between. then check the other side. then let it center and make sure it not making any noise. if you cant hear very good like myself i use my finger to feel for vibration if. if you feel vibration you binding some where. This could be cause you changed the radio all back to 100% on everything like your have to do to calibrate the esc. after calibration is done you still have to go back and check your end points. if at any time the servo is making a louder than normal sound it is over working the motor inside it and will toast it very quick. if you don't know what louder than normal sound or vibration is plug the servo in turn car on with the servo horn of the servo then listen or fell the servo. if you hook it up and buzz or vibrate like you cell phone on vibe then you got you setting wrong or a bind somewhere. also does you car have bearing on the steering bell post might want to check them too they could be seized up.

Checked the epa and it's all fine. Even reset the setting on my tx.

Sorry chap. Seems to me like you've been stung with bad servos. It's very likely the servos simply failed. If they are quite new id contact HK and ask for a refund then try to buy a known brand decent servo. I'm sure your troubles will then be over.

Ahhh just an update guys, I switch out the esc with another esc and it seems the problem ia gone.... Now I'm wondering how can a esc have such an effect on the servo

Really strange. I've never known such a phenomena with an esc damaging servos! Your running on 2s and the servos are ok with 2s output so if the bec wasn't doing a good job of regulating the voltage the max spikes would be pack voltage. Saying that if it's happening at a full lipo pack it would be 8.4v and that could be enough to fry the servo but I doubt it. If it was really an issue it could affect the reciever as well. All very strange!

Yup strange huh? But the so called faulty esc when paired with a analog servo has no issues.... Anybody has experience this before?

Yes that is why i think it something wrong else where. unless the rx is rated at an higher voltage then that would be why the rx does not fry out. tyour rx are now 6 , 7.4 and 8.4. servo max out at 7.4 at the moment but im sure they can and will handle more for awhile. i have ran 6 volts analog servos at 6.6 volts for a long time before i switched to high volts. it got to be something though. what are the spec of this cheap servo and what is it in. it could just be you buddy drive better not saying that is the case so dont take an offense to it. grab some savox for 70 to 80 they work great i had some in the tmaxx for like 3 years now for steering duty.

+1 grab some savox servos they are probably the next best thing for the price point. Just be careful savox servos can suffer brownout issues if the esc BEC is quite weak - you may need to add a glitch buster cap to the Rx.

Just what i am thinking im going to get me a savox the next time. Still, nobody has a definite answer to why or how a esc can lead to a burn servo?
I was thinking because of the bes. Faulty bec? As its not coping up...