This article was originally written by Raj at obsoletemachines.com. The site is not currently online but Raj has intentions to bring it back along with a whole bunch of new articles on repairing various synths. Well worth bookmarking!
Over time or more likely because someone has wrenched the flat-flex strip from the socket the carbon traces on the flex wear away. This causes poor contact and the aftertouch will not work properly as a variable resistor. Another issue can be a warped flex, say after you have removed the keybed and it has become trapped and this is very easy to do because the strip, after it leaves the keybed is far too long and delicate. In either case you will see very low readings in the diagnostics page: Voice + Bank ‘D’ + 8, then no.16. Here you should see 0 and when you press down you go to 20-80 comfortably and with some exertion you can get to the 95-99 range. If there is a problem you will not see any readings or there will be a steady aftertouch of 7-10
Open up the SY99 and identify the small PCB secured with two screws just under the Power PCB and on the keybed. There are two trimmers VR1 and VR2, which we will return to.
Pull up the molex connector where the strip enters and gently remove the strip. Take a look at the carbon traces and see if they are dull, worn or even stripped bare.
If so, then one solution is to jump these traces and I used an 4-way FCI flex header connector for that. The FCI flex header connector makes a secure connection to the carbon strips. I remove the two outer ‘teeth’ and connectors as you only need 2-way or purchase the 2-way version.
Then cut off and trim the end of the strip, you don’t need to go too far back you need a sufficient amount the strip to enter the FCI header so the connectors can get a sufficient purchase.
Then you need some jumper cables and i soldered them directly on the underside of the aftertouch PCB like so.
Check continuity with a DVM and then check resistance too. When you press down the keybed you should see resistance appear in the 100k range. Next power the SY99 up with the back still off, be aware of electricity running but you need to do this so you can re-tune your aftertouch.
Go back to the diagnostics page and no. 16 down and it will say 20-80 and if you get to this range it will then go to 95-99, again if you can get to this range it will then revert back to 0-20, where upon you release the keys. If it is moving too quickly for your liking or there is a aftertouch reading even when you are not pressing the keys you need to adjust those two trimmers. One trimmer gives you initial gain the other is the sensitivity, how hard you have exert pressure for it to move up the aftertouch range. Adjust this to your personal taste and continue to enjoy your Sy99.
More information including a pdf version of this article over on the forum :