View Full Version : Fried FET's?

2006.12.09, 05:00 PM
I built a new motor for my Overland with the following:
-Gold BB Can
-Xmod Gen-1 Stage 1 Arm
-Xmod Endbell
-Xmod Brushes

It ran pretty slow when I was driving it (I figured it was low batteries, they havent been used in a while). I charged them up and when I went to drive again it stopped working. Did I fry the FET's? I took it apart and looked at them, and they seemed okay. I can't tell if there's a melted plastic smell because I had a flaming Mini-LST servo after a snow run today....


2006.12.10, 03:11 AM
Are you getting steering responses now ? or nothing at all....

Hook the motor up directly to a power supply, even 1/2 cells, does it run ? if not, it could be the brushes....

Have any of the legs of the FET's become un-soldered ? belive it or not, on an Iwaver I've had a fet slide off the board where it's got so hot it melted the solder, but it didn't blow!

If the motor runs from the power supply ok, are the motor leads making good contact with the plates that you screw them on to ?

Can you go in one direction and not the other, eg forward and reverse ? It's actually quite rare that you fry both sets, as going forwards you don't use the rear ones, and vice versa...

Just a quick checklist, write back with results :)

2006.12.10, 11:28 AM
It stopped running forward, then a few minutes later it stopped running in reverse.

I havent gotten a good look at the FET's themselves, it was pretty dark when I was looking at them.

Steering still works.

I'll check the other stuff and report back later.

2006.12.10, 11:47 AM
Okay, just did the other checks.

The FET's are still soldered in place, and they look fine.

The motor ran on one AAA battery.

The contacts are pretty dirty, I lit up a loose LED I had sitting around with the motor contacts on the board, and the little things that srew onto the board aren't going in all they way, so I might try making a spacer to press them tigher after I clean them up.

2006.12.10, 12:22 PM
I've gotten it to run in reverse, but it cuts on and off.

I think the forward FET is blown, but which one is it, and can I replace it with an Xmod Gen-1 FET?

2006.12.11, 12:51 AM
No... don't use those Xmod fets on a Z. Try to get a set of 3010s here at the shop. I would still try to find out exactly why the fets fried in the first place or you're just going to run into the same problem again. That motor combo sounds very strange to me... I think you would've gotten more performance out of a stock motor, those Xmod armatures are weak.

I wouldn't test a board like that... sounds risky just hooking an LED to it.

Sometimes a fet can burn up and you will not see the tiny blister on the fet, if I get a scenario like this I usually just swap out both fets just to be sure I'll fix the problem on the first go round.

If you run some solder onto the holes where the motor leads screw mount onto the board, you can usually re-tap the threads on the board, making a tighter fit, but you have to be careful not to strip it out again by over-tightening. (never do work like this with the board installed in the chassis). Or... get the holes below the board repaired, I've used glue in these, heated them with a lighter, then gave them a pinch with some pliers to get the threads tighter, I sometimes do this with the screw installed part-way... to kinda re-form the plastic around the screw threads. (it's careful work... you have to really watch not to over-do it with the heat, try to get it mushy.. not melted)

There's another active thread on here now with Bye Bye's fet replacement video, always a great start to watch his vid... takes alot of the mystery out of it:


Good luck.