A4000TX rev 1.1 PC Floppy Drive Fix
There is a flaw in the rev 1.1 motherboard design when using a PC floppy drive (J300 jumper set to "PC" position). Thanks dorken for reporting the issue.
When the J300 jumper is set to "PC" position, U356 pin 1 receives an incorrect voltage which may cause the chip not to function correctly. U356 is responsible for transmitting Amiga/PC specific floppy drive signals between the Amiga/PC floppy drive and the motherboard.
The issue may occur only when the J300 is set to "PC". When using Amiga floppy drives (J300 set to "AMI"), the chip receives a correct voltage and this fix is not needed. I have used my motherboard with a PC floppy drive without the fix and haven't noticed a problem but your mileage may vary and it's recommended to do the fix if you are using the motherboard with a PC floppy drive.
The issue can be fixed either by replacing one component on the motherboard (Method #1) or by rerouting the U356 pin 1 to another trace (Method #2).
Method #1: Replacing a component
The issue can be fixed by replacing the Q301 (2N3904) with a mosfet such as FDN357N. Mouser stocks FDN357N
here. No other modification is needed.
Method #2: Rerouting a trace
The issue can be fixed by cutting the trace that goes to U356 pin 1 and connecting the U356 pin 1 to another trace with a jumper wire. No other modification is needed.
This can done either on the top side (Method #2A) or on the bottom side (Method #2B). Do only one of these modifications.
Method #2A: Doing the fix on the top side
Cut the trace that goes to U356 pin 1 (marked with X in the picture). Solder a jumper wire between U356 pin 1 and R365
upper pad as show in the picture.
Method #2B: Doing the fix on the bottom side
Cut the trace (marked with X in the picture). Solder a jumper wire between the cut trace and J300 middle pin as shown in the pictures.
Sorry for the inconvenience.