It seems that the AD converter is defective. You have to change the TDA8703.
Please read the instructions how to proceed: velleman.be/be/en/support/warranty/
There may be soldering bridge between two pins of the AD converter. Or there is bad contact (soldering) in one of the output data signals from the AD converter.
Check all the data signals (soldering, possible soldering bridges) from the AD converter to the RAM IC5 and to IC6.
It seems that everything else is OK but the AD converter…
Closer look to the screenshot images, shows quite clearly that the AD converter TDA8703 is defective.
It seems to be working OK only when the trace is at the upper section of the screen.
Hi,
A TDA8703 does not cost very much (9 Euro’s) and is (at least here in Ostend) easy to obtain in an ordinary “electronic components” shop.
So, it should be easy to test if the TDA8703 is defective or not (if yes: your problem is solved, if no: you have a spare TDA).