Hi All, the problem was solved: A defective 2N3904 (Q3 on KDSP2 board) was showing a huge leakage current and this way was loading down the AUXBUS. Just didn't believe this plain old bipolar could be the reason. Witch a new 2N3904 AUXBUS stays at 5.6 V. Thanks for help. VY 73 de Marcus, DL1EKC