USB host shield repair

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Many USB host shields required for Padawan/Shadow do not work properly and no data is transferred.

In 2023, Norbert Richartz found out through intensive research that only one chip was causing the problems.
It is therefore a hardware error and not a software error.

With a bit of skill you can replace the wrong chip with a correct one.
To do this, the old IC must be unsoldered and replaced with the correct new one.

The “defective” USB host shields use a 74HC125 IC, which must be replaced with a 74AHC125.
In the photo, the chip to be exchanged is outlined in violet.

The following instructions have now also been confirmed by others as a working solution to the problem.

Norberts instructions:

The shield has two quad-buffer/line-driver used as level converters (marked with green and violett rectangle on the picture).

Replace the violet marked IC with a 74AHC125

The original design (as described here https://chome.nerpa.tech/arduino_usb_host_shield_projects/) uses two different line drivers because of the different input voltage of 3.3V and 5V.
The green one is a 74HCT125, the violett one is a 74AHC125 (as also written on the PCB! HCT on top, AHC on the right).

Unfortunately most of the cheap chinese seller on eBay or AliExpress are selling boards, which are equipped with the same chip for both drivers, called 74HC125.

The 74HC125 is compatible with the 74HCT125 but not with the 74AHC125. So, if you own a working board with two 74HC125 it is by accident due to production tolerances.

How to solve the issue: 
I’m sorry to say, but you have to replace the violett marked 74HC125 by a 74AHC125 (e.g. the one from mouser. https://www.mouser.de/ProductDetail/771-AHC125D118).
I did that with 4 non-working boards, they all do fine now. If you like (but it is not necessary) you can also replace the 74HC125 by a 74HCT125 (https://www.mouser.de/ProductDetail/771-74HCT125D-T).
Then the shield is equipped exactly like the original design.

I hope some of you will be able to fix their boards now.

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