View Full Version : damaged ethernet port

08-30-2001, 06:41 AM
I recently got a G3 yosemite from a friend who went online during a lightening storm, and fried the ethernet port. The rest of the machine works fine, and I put in an ethernet card, but lose a valuable PCI slot. I called my local repair folks who said I would need to replace the entire logic board. I'm wondering if there is a less drastic repair, and if not, is it the kind of thing that could be found used. I've assembled a few PC's in the past, know my way around a soldering iron, and can't afford the $500 they say it will cost. Any info would help.

08-30-2001, 07:10 PM
Well¨÷ Don't know since I don't have a yosemite board to study. In most cases there's some sort of isolation between the port and the driver component. What I would do is study the logicboard for visual clues indicating a blown component. If unable to find visual clues, I would pull out the multimeter and start checking components.

The most suspect devices would be labeled "F" for fuse or "L" for inductor (loop). All of these should show close to zero ohm on a resistance check. If either of these type of components show an open circuit (infinite resistance) they're blown. Also note that in-circuit component readings are going to show effects from neighboring components and likely would not show a full open circuit. If these don't show near zero ohms, remove the component and test it off the board.

Next, I'd examine the "R" components (resistors). These should show resistance comparable to the number on the component (Get your magnifying glasses out!) Note that a resistor marked as "0" is actually just a jumper. These are frequently used as fuses since the will also open given current exceeding the component's designated wattage. If any resistor shows an open circuit they are also blown.

There are also "resistor packs." These are a set on resistors in a single component (Usually labeled "U" Note that "U" is used for any integrated circuit and is not necessarily a resistor pack.) Depending on the type, the resistors might be individual along side each other or more commonly sharing a single common end. The only way to find out is to look up the component at someplace like Digikey. Again, logic dictates that none of the resistors in the pack should show a resistance higher than the component's designation.

Then there are the components labeled "C". These are capacitors. They are very difficult to test with a simple multimeter. Carefully examine the surface mounted variety of these for hairline cracks indicating that they blew apart as a result of the power surge. Those that are in "cans" are electrolytic and will usually show bloating (raised off of the logicboard due to the rubber seal under the component being pushed out.) when blown.

On ethernet circuits there is usually an isolation transformer. It would also be labeled with a "U." You can identify it as a black box with a shiny top (epoxy filled). It would be a 16 pin component with pins 4 (counted from the white dot) and 13 (opposite 4) missing. This may be a surface mount component making it much harder to replace. The nice thing about it, and the reason I mention it last, is that it can't really be tested, but would very likely be the cause of the problem.

Finding the faulty component(s) is only part of the solution. Next you need to determine the original value of the component and replace it with an exact match. I keep a junk box of old, obsolete or damaged boards for spare part in just this sort of situation. The isolator component is very common on most ethernet boards, but could differ depending on the part number. If you find one with the same part number, it will replace the old one.

08-31-2001, 04:13 PM
wow, thanks for the details! I will inspect as much as i can on my own, and I'm printing out you directions to take to a co-worker who is an audio electronics repairman. He has much more sophisticated test equipment. I really appreciate this level of instruction, thanks for the advice!