VEX motor 393 not reversing

VEX V5 Technical Support UNOFFICIAL Tech Support
1

Hi, I have been having issues with my 393 motors on port one and ten not working in reverse. It is not limited to those motors either, as I have tried multiple motors with no different results. I have recalibrated the joysticks, and they do put out a negative signal when pushed down. Please take a look at my code attached below. Thank you, your help is greatly appreciated.

task main()
{
while(1 == 1)
{
// right motor //
motor[port1] = vexRT[Ch2];
// left motor //
motor[port10] = vexRT[Ch3]*-1;
//first motor for lift//
motor[port6] = vexRT[Btn6U]*126;
motor[port6] = -vexRT[Btn6D]*126;
// second motor for lift //
motor[port7] = vexRT[Btn6U]*126;
motor[port7] = -vexRT[Btn6D]*126;
// chain motor//
motor[port9] = vexRT[Btn5U]126;
motor[port9] = vexRT[Btn5D]-126;
}

}

2

When you move the joystick forwards and backwards, and the value changes to a negative, does the motor not move at all or at the same speed but forwards?

Is this all of your code? There might be something else hiding that is setting your motors and overriding this piece of code.

3

Damaged H-Bridge in the cortex probably caused by a shorted wire or motor.

4

It doesn’t move at all, and yes that is all of my code.

5

Thanks for the advice, I’ll check it out.

6

In that case James is most likely correct that it is a physical problem in the Cortex, as the code is fine.

7

You can try changing the polarity of the motor to see if it will spin the other direction, but not the original direction.

If this is true, The motor is most likely fine, but it is a motor controller/h bridge problem

8

Thanks, I think it may be a damaged h bridge. Would I have to go through vex to repair this or can I do it myself?

9

Thanks for all the help, I think that’s what I’m dealing with.

10

You could do it yourself, but it might be tricky and I don’t know where you’d find an identical replacement.

Contact VEX, they’ll be able to help you figure out if it’s still under warranty and if they can fix it. (if the warranty is over I think they will still fix it, but you’ll have to pay for it)

11

My advice is also to contact VEX, however, depending on how damaged the components are it may not be possible to replace them in which case the whole board would need to be swapped.

Last year I did help another team do the repair themselves, or rather I put them in touch with a rework house that I had used who charged them a token fee to do it. There are two different types of FET used for the H-bridge, they also vary depending on age of the cortex although they are all more or less compatible. Here is some information I forwarded to another student last year, if you know of someone who works in an electronics related field they may be able to help you, I personally would probably not try and replace them myself, again my advice is to have VEX do the work.

Motor ports 1 & 10 each use two parts to make the H-Bridge. A dual N Channel Mosfet and a Dual P channel Mosfet. One part is on each side of the circuit board, one or both of them (but most likely just one) may be damaged.

The part on the underneath side of the board is an FDS4935BZ. The part on the upper side can be one of two alternatives, cortex version A3 and earlier used an AO4818B, cortex version A4 used an IRF8313, they look pin compatible and performance is almost the same.

Open up your cortex to try and see which parts are damaged, try replacing those, this may or may not work depending how hot they became and if they caused damage to the board.

You can order from digikey, here are links.

Primary side - N Channel Mosfet

IRF8313

IRF8313PBF-ND
http://www.digikey.com/product-detai...PBF-ND/2095840
$0.89 each

AO4818B

785-1059-1-ND
http://www.digikey.com/product-detai…9-1-ND/1856001
$0.71 each

Secondary side - P Channel Mosfet

FDS4935B

FDS4935BZCT-ND
http://www.digikey.com/product-detai…ZCT-ND/1154660
$0.92 each

12

If it is a damaged H Bridge, I believe that VEX would make you pay full price. Also, I would wait until after Worlds Product Announcements in the unlikely event of a new cortex or something of the sort. Unlikely, but hey.

Also, thanks again for that. H bridges are pretty easy to replace. As you can see from the above links, the chips themselves are pretty cheap. If you open up the cortex housing, you will generally see a clearly broken chip.

13

Thanks everyone for all the help, I will definitely try and contact vex and see if they can repair it. If the won’t I’ll look into fixing it myself. Again thanks for the help, I appreciate it and I will reply with any updates on the status.