Apparently two resistors are switched on one of the gear tooth sensors in the KOP.
R3 and R4 are the ones…see the pics…
So now we know what we have to do.
The bad board in our case is the one that was cut off the end, that is, the one with J1 and J2, the board with J4 and J5 is good.
But you should double check!
Did the resistor switch cause the Gear Tooth Sensor to go bad ? Or did the Gear Tooth Sensor have some other problem?
First off, how on earth did you figure that out!! And , THANK YOU!!!
Now, when I get to the school tomorrow, I’ll double check ours. It would be really good to know what the values are supposed to be. Removing and replacing those will be REAL FUN.
Is there a chance you can get cleaner pictures?
If not, I’ll get my camera out and see if I can get a couple of really sharp images when I get a chance.
One of our mentors took a magnifying glass and started looking at each of the part numbers. lol. thanks to him, we found the error. and so did team 1726
The part numbers for R3 and R4 are indeed swapped
on our gear tooth sensor that does not work. Without
a SMD rework station it will be quite a chore to swap
them to their correct positions. Perhaps a customized
soldering iron tip and a pair of tweezers will do it.
Eugene
Here is a quote from an email I recieved from my brother…he deals with small circuit boards with surface mount components on a regular basis, and successfully reworks them at home (with the help of his B&L stereo microscope)
"I can swap those in a few minutes, or better yet you can do it yourself.
The trick is the technique.
First, tools. You have to have a microscope or magnifier to see what you’re doing, a pair of very good but not too sharp tweezers (I use size 00), and a smallish but not teeny-tiny soldering iron tip. Some smallish solder wick and fine solder (.020" ideal, .031" acceptable) are needed as well.
The first resistor may be removed by placing the soldering iron tip across both ends of the resistor at the same time, so that it is heated up at both ends. Then lift it up with the tweezers, or it comes up on the iron and you pull it off the iron with the tweezers.
Now, use the solder wick to remove the solder from the PC board pad that is on the left if you’re right-handed, so that the end closest to the iron still has solder.
Do the same for the other part.
Now, with the correct resistor in the tweezers, hold it in place and heat up the solder blob while pressing the resistor into position. Then quickly solder the other end with the fine solder. Don’t linger with the iron; the other end will heat up and the part may stick to the iron again.
And an addendum…
"First, clean the iron tip by wiping it on a wet sponge.
Then, you need to put a blob of solder on the iron tip to make it want to heat up the resistor. Use more solder than you think is appropriate - the blob has to be at least as long as the resistor to heat both ends simultaneously.
This is tricky work! If you don’t have a lot of soldering experience, you will probably mess it up…and you need good eyesight and steady hands, and a clean, well lit area to work in.
If there is any type of college or industrial place nearby that deals with small circuit boards they might be willing to help, if you ask nicely…
It looks like your parents did a great job raising you and your brother. You both are excellent examples to follow. Tell your Bro, Thanks from all of us here on CD!!
We fixed ours with a hammer 
This is where having great mentors/sponsors is helpful. One of ours has a surface-mount soldering station with a stereoscopic microscope meant for this kind of work, and they were able to help us swap the resistors. We’d already ordered the Honeywell sensors by then, but it’s so much more compact to use the kit sensors.
So the board works properly after swapping the resistors?
Yes, it does.
That’s good to know! thanks for the report
With the help of a student with much better eyesight than I, we were able to swap the two resistors on the end board in the set. We verified that the resistors were properly aligned and soldered with a “dissection microscope” from the science lab. Man what a difference that thing made in being able to see what we had done.
I am hoping provide feedback on how well the circuit works now. Hopefully I can provide this information by tomorrow evening.
By the way, here is a hint or two that can really help make the swap go easier.
The gear tooth sensor it’s self is magnetized. When you get the resistors loose, they may get stuck on it. The hardest thing to do is re-solder R4. It is right next to the sensor. You will not be able to place the resistor on the board without holding it in place. I was able to make a very small wood chip into a holder, a toothpick would work well. Just put a notch in the tip and press it onto the resistor. That way you can hold the resistor in place with one hand, hold the solder in an other and the soldering iron in your third hand. :rolleyes: Yes, it takes two people.
Good luck!!
The other method for small chip items is to use two soldering irons. Ungar Princess my favorite but they are no longer sold. Good irons with a thermostactily controled heater at about 750f and small flat tips. Same process as far as good light and magnifiers, and solder wick to clean up. You use the two irons, one each side of the chip at once. I’ve had good luck getting parts removed with this method, but sometimes kill the removed part. Having a helper with the tweasers grab the part should help with the runining the part problem.
Looking at the PCB from left to right there is a sensor with J1 and J2, then the second board has J4 and J5. Which is the good one and which is the one with the parts in the wrong place?
Thanks.
George
The board with the parts in the wrong place is the end board. It’s the gear tooth sensor that only has 1 edge scribed. The other edge is smooth.