Thank you all for your thoughts! Helped me further, but i am not sure.
As suggested by grumpydoc, i looked at the DACPEN-LT and DACPWR-LT, and they never go low.
I did the same measurements on an identical DAC on the A8 board, and this shows a very different pattern:
- On the Datalines on the A8, there are bursts visible when i turn the knobs.
- On the A3 chip, there are no bursts visible at all, only some sort of carrier wave.
-On the START pin of the A8 chip (is EN on the A3 dac), the LOW signals are visible also
- On the A3 chip, the signal is Always HIGH.
All the inputs for the 74LS138 contain data, but i dont have a logic analiser to determine if the outputs are set low or not
I took the multiplexing chip 74LS138 off the board, and put it on a breadboard.
Then i checked all outputs and inputs, and checked it against the truth table in the specsheet.
All outputs checked out, except that output Y2 ( the one going to the DAC, DACPEN-LT) does not drop to 0.12V, as all others, but dropped to 1.23V
To be sure, i will order a new 74LS138, and in the mean time i asked QQE06-40 for the board. That would be very helpful indeed.
Nice to have some extra brains