I did not watch the video, but from your description alone I can tell this is the D/A converter. Very likely the highest bit is not working. This can eigther be the CPU pin itsself, the buffer IC7, or the R2R network. So I try probing the CPU pins first, the hightest bit (pin 17) should change when playing very high or low notes.
> I have a question only maybe you can answer about the 101 CPUs, as I'm having a very difficult time troubleshooting a strange problem on a friend's unit, and i’m considering upgrading it to your 1-oh1. I first wanted to ask if you think the problem i’m experiencing is related to the CPU.
> Basically, If I play down the keys the tuning will jump up (once i reach 2.6V reading on the CV out) and it starts playing down again. I made a video of it: -----------
> The area around the D/A converter isn’t pretty, looked like it got burned by a wrong power supply. However I replaced or tested nearly everything, especially in the D/A area. The unit can never be calibrated with VR-2 adjustments to make CV out reach 0V (as per instructed in the service manual at step 1A). The lowest CV out I can reach is 2.6V.
> I have my own 101, that i tested against and swapped many parts with (as I'm not super-troubleshooter), including the keyboard except the CPU. I have a 7301-2K, the problematic unit has a 7301-3A.
> So my questions for you are: Have you ever seen anything like this before, and do you think the CPU could be the problem, and do you by chance have any original 101’s I could buy from you? I live in Berlin too.
If it does, probe behind the buffer and then behind VR3. If the signal is there, check for continuity from R70.
If you can not measure any pin change on the CPU, it is probably defective ( and should of course be replaced with a SH-1oh1