VEX Cortex Diode Busted - Questions

So one of the sides of a diode on my team’s Cortex has busted
(It’s labled D8, 2 ICs up from the bottom-right):

I think what happened is that an inexperienced member on my team plugged a 2-wire motor into a motor port 4 on the cortex without a Motor Controller 29, and ran it. Diode #8 on the Port 4 side is now busted, and Port 4 is now unusable.

The diode looks like a generic diode, with the code 502B written on the top.

*]If I replace this diode with an identical one, would this be considered a illegal ‘modification of electronics’ from the VEX Robotics Competition, or a legal repair?
*]If I am allowed to replace it, what specifications do these diodes have so that I may locate an identical one?

Of course I can simply use a Y-cable, or a Y-cable + Power Expander, but repairing the cortex to the best of abilities is almost always better.

This is an unofficial forum so this is an unofficial answer :slight_smile:

My best guess is that it’s a dual ESD suppression diode, probably something like this (probably not the exact part used, buyer beware !)


(part at digikey, $0.73)

When you get the part compare the voltage drop across it to the existing part before desoldering to make sure it’s the correct polarity.

I think this type of repair falls outside any of the competition rules, it’s unreasonable to be expected to write off a $250 cortex because repair is not allowed. The intent is to return the unit to original working condition.

You can try for an official answer but I have not had much luck with that type of request in the past.

Obviously you can send into VEX for repair, and would be my advice if pressed, that’s the safest way unless you have experience with rework.

Thank you; I posted this here to potentially get a link to a similar post (that I couldn’t find), but this was very useful information. I’ll try posting this for an official answer.

I believe the diode is connected to ground and prevents extra voltage applied to the Cortex chip. I think it is a zener diode that breaks down above the logic level voltage (5V?). Then the diode will blow instead of the cortex chip.

Just tying up loose ends here.

Our team also had a cortex with a dead motor port, port 9 in our case.

I replaced the ESD diode with the part above and everything is now good. I had checked the output pwm signal using a scope before I did this and voltage was low so there was no damage to the CPU driving that pin. The diode was also not physically damaged as the one in the picture from post #1 was, but still measured as a partial short with a meter.

As before, I do not recommend this repair unless you have experience with rework or have access to a parent/mentor that is willing to do it for you. The part only measures 2.8mm x 1.2mm so this is not a job for the RadioShack soldering iron.

It is very easy to damage the rest of your Cortex, the parts that were not broken, before…

The inexpensive Radio-Shack Soldering Iron…

I replace 0602 size fuses and resistors with a Radio-Shack Digital Soldering Station.

I meant avoid something like this, 40W $10 iron.

In the past, I have bought a few of those… But NEVER USE THEM on fine electronics…

I see the Radio-Shack Digital Soldering Station has gone up in price to $109.99 ( USD ) from $79.99 ( USD ) a couple years ago… And they have the Replacement Tips, at least On-Line…

My personal iron of choice at the moment is the Hakko FX888 with a very fine-point tip. Currently sells from SparkFun for US$100.

At about half that price is the Weller WLC100. It was my iron of choice before SMT, and I still keep it around for soldering larger items.

Either of these would be suitable for such repair work (with a good tip, of course).


  • Dean

I noticed that the Radio-Shack replacement Tips for the Radio-Shack Digital Soldering Station are Compatible with the Hakko Medium 900M, 900M-ESD, 907, 907-ESD, 933.

At least the Weller-WLC100-40 looks Temperature Adjustable…

My old Soldering Station is a Weller EC-2000, that needs the Plug in the Base fixed before it will work again.

Temperature Controlled and a Good Tip, are mandatory for Good Results…

A classic, used that one many times in the last century.

I don’t do this type of work that often but when I do this is what I use.

Welcome back Dean :slight_smile:

I am not familure with PACE, but that looks Industrial, and those Tweezer Tips look very nice… I could use that on those “blasted” 0603 Fuses and Resistors… One Shot Removal :wink:

Ah yes, your right… Dean out of Lurker Mode… :wink:

Welcome Back to the Gathering of the Vex Enthusiasts. Personally, I have missed your Input, but figured that you might be Posting where I am not Reading…

All the tips are quick release and replaceable, we have several different styles of tweezer style tips used for different packages (although to be honest, for most SMT parts beyond the 0603 and SOT-23 etc. types I have other more expert people do the work). This is not a cheap soldering station, somewhere around $2000.

At home I just have one of these, a Hakko 936.

It has been discontinued and replaced by the Hakko FX888 Dean mentioned.

I never really left, I was staying on top of the forums but just haven’t had much time to post. Hopefully I’ll have time to start being active again!


  • Dean