Shorted SSCv4 with bad CAT5 Cable

[masterelectrician2112]

Hello all,

I was not thinking clearly and used an untested cat5 cable that I built a few years ago to plug in a SSCv4 to an active hub.  I am not exactly sure what was shorted, but the SSC released the magic smoke and looks like this now (see pics).  It looks like the 5v regulator needs to be replaced.  The RS485 chip and 120ohm resistor look charred, but I don't know if they are still good, especially the chip.  So the question is, should I be able to just replace the 5v regulator and go or do you think more is wrong?  I don't know enough about the layout of the circuit to determine what shorted or what is damaged.  I would appreciate all the guidance I can get.  Thanks in advance!

[tbone321]

I can tell just looking at the damage that your "untested" cable has placed 12V on the data inputs.  This fried the 485 causing it to short out, dumping a full 12V on the 5V side of the SSC.  This caused the 5V regulator to cook trying to drop the voltage.  So at a minimum, the regulator and the 485 are toast and need to be replaced.  The resistor may be ok.  When you remove the 5V regulator, unsolder 1 leg of that resistor and measure it to see if it is still alive and within spec.  Also give the board a good clean and make sure that the traces are still ok.  The PIC may be ok.  Even though it is on the 3V side of the board, it may have gotten a hit on its input when the 485 cooked.  The MOSFET is capable of withstanding 12V so it should be ok.  I would remove the PIC before testing the board with the new 5V reg and 485 and make sure that the 3.3V regulator is still functioning and putting out the correct voltage.  If it was damaged it could dump 5V on the PIC and fry it as well.  You could always put the PIC into another working SSC to see if it survived the hit. 

[n1ist]

It's easy to test the resistor.  Just remove the '485 and unplug the RJ45 cable, and measure across the resistor.  You should read about 120 ohms.  If the cable wasn't plugged in long, there's a good chance it's OK. 

/mike

[masterelectrician2112]

Thanks to both of you for your helpful responses.  I have ordered replacement parts and spares and will see what needs to be replaced.  Thanks again.