Hadn't used the piezo on my JP6 for a long time and today I noticed that there was no sound coming from the A string. Any idea what could be the problem and how I can solve this? Many thanks.
It may be the case, that the very thin piezo cable connected to the saddle for A string is damaged or malfunctioning. You can try contacting EBMM customer service =)
I had the same thing on my JP6 when it was new some years ago. Had to send it back home to EBMM for a replacement saddle and it's been perfect ever since.
Call customer service and see what they tell you.