[cctalk] Diablo Model 40 Series - Disturbed head positioning

Hello everyone I need your help to identify an issue on my Diablo Model 40 Series. I don't know where to look, it's so vast ! Here's the problem: When RUN is activated, the drive begins its spin up and simultaneously deploys the heads (normal) but instead of stabilizing them, the Head Positioner receives a burst of reverse/forward micro signals. The heads "vibrate", this creates an audible frequency "BRRRRRRRRRRRRRRRRRR", and it is infinite, the heads are never loaded and the drive never reaches READY. At first I thought that perhaps the track zero sensor was defective or something of the same order but when I disengage RUN mode, the drive unloads the heads and they should be in a fixed position, here they continue to reverse/forward but more slowly than in RUN mode. Because the heads continues to mess around even in unload mode, this a priori excludes alignment problems. Here is a video of that issue: https://youtu.be/HzzxLnSdEOg Other information, if I cut the power while the drive is in RUN mode, it does not do an emergency retraction of the heads, related problem? I was hoping for a power supply problem but all the voltages and even on the main board cage seem ok (with a multimeter). If one of you had already encountered this problem of lack of head stabilization and continuous reverse/forward on this type of drive? Thanks ! Dominique