hello
these is crystal diode (germanium) check the image probably all 4 with same measurement.. (i presume )
If you have multimeter , desolder all 4 select diode modality and make the measurement ..if 1 or more is with different value replace.

thank you a lot for that information, orx65. you give me a hope. ;)

is it really necessary to desolder the diodes first before measuring them?

i did the measurement already - you can see the values on the picture i got from all the diodes (more or less same values).

do all diodes have to show the exact same value? or is it not a problem when one diode shows 0,619 and other one 0,625?

best regards

hi
yes for correct measurement is necessary desolder them as any other different electronic component..unfortunately i don't have schematic of this synth i know that a few difference can to be a normal tollerance anyway the correct value for germanium diode is 0,5 - 0,8 v ..do you want to be sure ? replace all 4 ..example link
http://www.ebay.it/itm/AA117-Diodo-a...sAAOSwEZdaCY1T

let me know , good luck

Here's a photo of the 2nd board - in that board comes the signal from the contact PCB. Any ideas what component from this board could cause the dead key?

i'll try to resolder all solder joint incoming from red connector , i realize that it is an empirical solution but considering that we do not have the schematic ...

curious?
 

did you ever find fix? similar issue here

Have you tried this thread?:- Indigo ---------> 1 key not triggering <---------