The last production batch counted over 500 boards. Unfortunately, there have been some issues with the last production batch, which caused a significant number of boards to fail the tests.
Please note that the last production batch was the very first production batch of any "1541Us" ever, that was tested very thoroughly by an automated test system at the premises of the assembly company. It turns out that this "tester" is very stringent, as it measures and validates all kinds of parameters of the board. This caused some good boards to fail the tests, due to component value spread.
However, there were also cases of failure that were caused by problems in the printed circuit boards. For the last production batch, I allowed the manufacturer to choose and purchase these printed circuit boards themselves, and the result is disappointing. In some "spare" hours, I have started to investigate and fix the remaining boards. Some are easy to fix, and pass all tests after a simple touch up of the soldering or a replacement of a chip; others fail permanently.
It also seems that there are boards that fail after they were tested and sent. This is the worst that could happen, of course, as it involves unhappy users. Of course, all units come with full warranty, and I will always replace or repair a unit when it appears to be faulty.
In the mean time, I have started a new production batch in order to catch up with the open orders. With better agreements in writing, an early quality review and a quote from a higher quality PCB manufacturer, I am carefully confident that the next production batch will have a better yield and give me less sleepless hours.