might be worthwhile explaining the importance of these parts mike,with so many inports comming in.
You're right....
The MOV (Metal Oxide Varistor) acts like a zener diode in that when a voltage is sensed that equals or is higher than its rating it quickly short circuits and forces the mains fuse to blow (as long as it isn't a nail!).
Sometimes if it is just a transient spike in the mains it will not permanently short but just do so momentarily to protect the machine and trust that the fuse will do its job. Other times the MOV will short circuit and must be replaced but be happy that it has very likely protected your machine from much more serious damage.
The Varistor is placed in series with one side of the mains and when cold (power off state) it exhibits a high resistance and so slows the inrush of current when the machine is first switched on. This initially limits the current flow basically to offer a "soft start" to the machine and allow things like the main filter capacitors time to charge to capacity gently without stressing them. As this part warms up the resistance drops to quite a low value, typically a couple of ohms and so the full mains potential is applied to the machine after the initial start.
Thermistors, as they age, lose the ability to switch down to very low resistances and can typically only go down to around 10~20 ohms. This can cause random resets and can be a tricky fault to find.
Machines wired for 110V (USA and Japan) will have the incorrect value MOV installed that will usually have a breakdown voltage of around 150~170V. If you plug this machine into 230V the MOV will instantly conduct and you will be greeted with a shower of sparks and then silence.
It is my recomendation that if you are going to the trouble to open up the machine and gain access to the MOV you may as well change both parts at the same time.
