Hi,
to sum up I found following bugs with the pcb "XF551_B v6.0 (c) 2023 Dropcheck":
1) Power not working
2) SIO not working
3) FDC not working correctly
4) Not writing to disk
5) The dots of the 7-segment display are always on
6) A problem with the 8040/8050 jumper
7) OS crashing when using EEPROMs larger than 2764
Below please find the solutions to the problems mentioned above:
1) There is a missing connection from AC power input to the bridge rectifier circuit. Solder a bridge like shown in the picture below.

2) The transistors Q1, Q2 and Q3 are incorrectly orientated. Turn the transistors by 180 degrees.
3) The FDC suffers from incorrect connections. At U2 cut the traces of PINs 3,4,5,6,7 and 8 (see picture, I used a high precision 1.4mm drill) and rewire the Pins:
(The trace for PIN7 is to be found on the pcb's lower side)

FDC PIN 3 (A0) -> 8040 PIN 27
FDC PIN 4 (A1) -> 8040 PIN 28
FDC PIN 5 -> 8040 PIN 12
FDC PIN 6 -> 8040 PIN 13
FDC PIN 7 -> 8040 PIN 14
FDC PIN 8 -> 8040 PIN 15
4) Q4 in write gate protection circuit incorrectly orientated. Turn Q4 by 180 degrees.
5) Should be off on side 0 and on on side 1. Maybe I killed my transistor T1, needs to be rechecked once I have a spare transistor.
6) Jumper needs to be put in contrary to what is printed on the CPU.
7) Open address line at PIN26 causes problems, so solder a bridge to PIN27.
I did a quck boot using the XF551 mechanics and it works.
I ran an extensive check using a 3.5" 80 track HD drive in DD mode using the Bob Wooley OS with my 80T mod formatting the disk, writing the LBA sector number to each sector and verifying the LBA number reading the disk again. The test ran successful.
Have fun 😞
CU, Erhard
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