The IIfx has a 1A fuse between the SWIM chip and the RFI filter, the SE and SE30 do not.

There is a 470 Ohm resister in line with the 5VDC line powering the ADB signal line, that could be blown or you burned a track, blown the RFI filter (Tokin D16-C), or the ADB transiever chip itself.

ADB bus signals are:

Pin 1 : ADB

2: n.c.

3: +5V

4: GND

So with a meter check to see if you have +5V (used to power ADB devices) between pins 3 and 4, and then check for ADB signal (will be a lower voltage because of the resistor ) between pins 1 and 4.

Referenced Chapter 8 Apple Desktop Bus page 293, Guide to Macintosh Family Hardware : Second Edition.