Re: 10th KFC225 failure in 6 years

Peter Clark <Invalidi@[EMAIL PROTECTED]
> wrote

>On Sat, 03 May 2008 19:54:51 +0100, Peter <nospam@[EMAIL PROTECTED]
>
>wrote:
>
>>
>>Peter Clark <Invalidi@[EMAIL PROTECTED]
> wrote
>>
>>>Don't they have to reset the offsets once the repaired servo is
>>>installed?
>>
>>No.
>>
>>The servo has *internal* trims for stuff like zero and scale.
>>
>>Just as well :)
>
>K.  I know we reset the nulls when replacing them in the KAP140, but
>the only other stuff I have any wrench time behind is the
>Cessna/ARC/Sperry/whatever 800B, S-Tec 1500, 55X, and the GFC700.
>Anyway.

I think the KAP140 servos are completely different. I recall seeing
their schematics a while ago - a completely analog design with some
beefy power transistors.

The KFC225 servos (KS270C/271C/272C) use a switching amplifier which
has a duff current limit design: a linear circuit which causes massive
dissipation in the switching MOSFETs (which are SMT, no heatsinking)
the moment the current limit mode is entered.

As to what causes this in the first place, I don't know but the motor
(specifically its brushes) is the only thing being driven.