This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.
The module you are talking about is L2 "D-15C". This is an 8-pin thing containing four inductors (coils). Three inductors are used. They are in series with the ADB Desktop connector. Evidently, one of the inductors burned out. This would explain the smoke and the loss of ADB functioning. The three inductors run ... pin 1 to pin 8 pin 2 to pin 7 pin 3 to pin 6 You can use an Ohm meter to detect which one is open. (Example: the correct reading from pin 2 to pin 7 would, probably, be less than an Ohm.) Since the pin2-pin7 inductor connects to +5V on the pin7 side, it is the best candidate for a burn out should pin2 some how have been shorted to ground. Note: Jon Christopher reported that when his L2 module bombed it was due to a short in a spliced-on KB cable. It turned out that the resulting burn out fused some of the inductors together inside the module. So, if you detect any break after such a burn out, it is probably best to just remove the module and replace all three inductors. Replacing the inductors should be fairly easy, although it will probably be necessary to remove the motherboard. The value of the inductors is not critical, so 15-20 turns of small wire-- like wirewrap wire-- wrapped on a pencil or screwdriver shaft will make a small coil you can use. Make three coils. After removing the damaged L2 module, use an Ohm meter to check for a short to ground at pins 1, 2, and 3. (If, as in the case of a short in a spliced-on KB cable, you know where the short is/was, you can skip this check.) Eliminate the short before continuing. Solder your home-brew coils in place (pin 1 to pin 8 for the first coil, etc.), put everything together, and your GS should be as good as new. By: Rubywand