Sunday, May 3, 2009

Workbench fun - Repairing a broken USB device

My wife came to me after a day of teaching with a small crisis on her hands... or rather, in her hands. The RF wireless presentation remote she used every day (The Presentation Pilot Pro) was accidentally broken by a student. A helpful gesture of pushing in a chair happened to catch the USB receiver plugged into the back of her desktop, and snapped the plug right off, including a small piece of the circuit board. A new remote & receiver costs about $70, and to order just the receiver would still be $50, so any way to repair it and avoid that expense would be to our advantage. Of course this is my strange idea of fun, so I dove in and took it apart to survey the damage and see if it could be fixed.

The metal housing of the plug was actually soldered to the board for strength in two spots. One of those solder joints broke off the board, but the other joint held, taking a small piece of the circuit board with it when it was broken. As for the 4 connections for the USB, it wasn't looking good. I took several close-up photos of the damage since it was so small, and used my multimeter to try to find points I could solder to. Unfortunately the USB connections were solid, so when the plug was ripped off, it pulled off the metal contacts on the board and some of the substrate. This left me with nothing substantial to solder to, and I wasn't sure I could just put it back the way it was. I've done small repair work like this before, but this one wasn't so simple, so I decided to attack it from a different direction.

The largest chip on the board had a clearly written part number on it: "CY7C63723C" which after some Google searching turned out to be a "USB Combination Low-Speed USB and PS/2 Peripheral Controller." Jackpot. This was the USB interface chip, and in theory this is all I would need to connect the USB plug to. I found the spec sheet for the chip and found the four pins I needed, and double-checked the USB spec to make sure I connected it properly. To insulate and prevent any shorting, I covered the board by the connector with some electrical tape, and soldered the metal plug back in place. Using some small wires from Cat5E ethernet cable, I ran jumpers from the connector back to the USB chip. After some careful soldering, I was pretty thrilled to see that it worked when I plugged it into my PC! The device was recognized immediately, and I was able to move the mouse and use the buttons through the remote with no trouble.

With the receiver now working, I added some epoxy to the back of the board to strengthen the broken connection and fill in where the board was broken off. Lucky for us, the damage only affected the solder points for the plug and the structural part of the board for securing the plug. No traces were damaged, so I was able to bring it back to life with a little research and some careful fine-detail soldering. Now the receiver is working and has been re-assembled. The only signs of the repair are a couple wires visible through the hole for the indicator light.

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