View Full Version : Degradable FETS, is it possible?

2011.07.29, 01:45 AM

I'm looking at a strange problem with my MR02.
For the last 2 months I have been noticing that i'm becomming slower.
Not alot, but noticable. I constantly have the feeling that my car doesnt push any more.
I tried new batteries, new motor, new brushes, did a complete rebuild of the diff...

I use a PNT80 motor.
What I do notice that after I clean the motor with contact spray and put it back in it works great for about 2 minutes.
After that you see my lap times dropping for a couple laps and stabilize at about 0.3 0.4 sec of what I usually run.
First I tought myabe its the bushing oil leaking onto the commutator, so after cleaning I now am very carefull not to put to much on it.
That doesnt seem to help.

I tried 2 motors of my own, and one from another person who does great lap times with it. All the same.
Because I used the TRP com drops on my motors and was thinking that maybe this had affected the motor in some way.
I also tried a a kyosho stock, wich improves my lap time a little but it also doenst psuh as much as it should.

End speed on the straigth is normal.

Putting all this together my conclusion is that there might be something with the FETS.
I already took out the print, cleaned it but I cant see anything on the fets that looks suspicious.

So my question, is it possible that fets degrate? Are there people who basicly change their FETS out of precaution?
This is an MR02 2.4 almost 3 years of age.

Any other suggestions to as what the problem could be would be appreciated.

unearthed name
2011.07.29, 02:26 AM
uuum, have you check the battery terminals for rust? or maybe the terminals are loose?

2011.07.29, 06:28 AM
For less than $2 each, it's very affordable to have the fets replaced. Costs less than a new motor. Just make sure a qualified person does the work. You'll find the answer to your question for less than the cost of a new motor.

2011.07.29, 09:21 AM
Although it is possible, I think it is highly unlikely...

As already suggested check the battery tabs(sometimes just bending them out a bit does the trick ;)). this would be my first bet. Also check the wires/connection between the board and the chassis, and the board and the motor.

2011.07.29, 09:31 AM
i feel that fets do fade over time.... i find myself refetting my boards once a year.... I usually do it before big races... theres is current going throgh them which can cause them to break down and fade over time...

2011.07.29, 05:10 PM
If its "fading" as you say over a period
Maybe you have a cell (Battery) going bad
Try running a completely new set?

2011.07.29, 06:09 PM
I have a few cars that I feel have this fade... after replacing the fets, I dont feel too much difference. I changed the motor a few times, as well, and used a new set of cells. I need to check the batt connections to the terminals and resolder them... this is a car that I only used with 70t motors and was one of my early cars. I have good top end, but no punch...

Mike Keely
2011.07.29, 08:47 PM
Try a new pinion gear first. I changed fets & motors on one car and it didn't fix the problem. I felt stupid when I changed to a new pinion and it fixed the problem. The pinion slipped just enough that it was slow out of the turns but was fine on the straights.

2011.07.30, 02:20 AM
Thx for the replys so far.
I already did everything suggested here, except maybe the batterie links. I'll check that.
I was hoping someone would say yes change them, big difference but the comment of Emu put me down a bit :(
Anyway i'm "happy" to here i'm not the only one so it wasnt something i did.
I'll keep looking for it.

Any other suggestions are welcome.

2011.07.30, 07:31 AM
Dont want to be a downer, I feel that sometimes its more than the fets... I have older cars that have more usage, and have more punch... Do check your battery terminals as suggested here...

I think that it could be something about the fact that the car is only used with a 70t... so it almost allways has full throttle applied?

2011.07.30, 10:56 AM
The battery terminals are a good idea to check
If you do get any type of corrosion (Dirt) on them
(And we all know that no track dust will get in there
right?) that will increase resistance into the circuit
potentially slowing you down, alittle at a time until
the resistance is so great that the car will evenutally
not even run!
This is going to sound funny, but use a good pencil
erasure and really scrub the terminals
As for a pinion, Myself and Eugene already had this
discussion at the track one day, They do tend to slip
and maybe there is just enough slip in yours that you
are noticing the slower response?
Keep us informed please!

2011.07.31, 12:25 PM
Also check the crossover wire between the rear terminals... I've had a car slowly seem to die off a while back and found a short between the right rear contact plate and a small area of wire. The car would kill batts quick and get slower and slower each time. Lucky it didn't fry.

2011.11.03, 11:30 AM
I have same personal problem with 1 of my AWD.
happen after heavy use for practice, there's one time I gotta race. but night before D Day, it run so slow.
I change the batt, motor, still, the damn car sucks!
after tearing the chassy apart, check all moving parts and I mean all!
nothing's wrong.
just before I wanna change the FETs, I saw the motor terminal is sooo damn dirty (not shiny gold colour + additional soldering led that also darken)
fine sand them down a bit, make it shiny like gold and platinum shine, do the same to all plates on the chassy, and problem solved!
it run great and ready for next morning.