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Subject: Re: [yamahacs80] CS-80 tech question: initial velocity problem on TSB board

From: David Rogoff <david@...>
Date: 2018-01-01

Hi Jonathan!

Happy New Year and sorry about your CS80 problem.  It sounds like you’re doing a good job trying to isolate the problem.  However, I don’t know if you can do this with just a multimeter since the signals are multiplexed by the 4051.  I think a cheap, used oscilloscope would really help.  The fastest clock in the digital logic is only about 100KHz so even a 1Mhz or 10MHz.

If IC4, pin 3 is really stuck at +15v, that  would give you max velocity (initial touch) and aftertouch on those 6 notes.  What if you remove IC4 and leave the socket empty?  Is it still +15?  That would indicate a short on the board.  If not, then your idea of the op-amp, IC7 having a problem could be it.  Try replacing/swapping/removing that.  It’s just a common 4558 dual op-amp.  If it’s not one of those or a board short, I’m not sure what’s going on.

Good luck!

 David

On Dec 29, 2017, at 11:27 AM, jonathan4051@... [yamahacs80] <yamahacs80@yahoogroups.com> wrote:



Hello,
I am looking for technical help for a Yamaha CS-80. The issue it is having is: a cluster of six notes (G2 to C3) generate full volume at all times when keyed, no matter how softly they are played. I have traced the problem to IC4 (a newer 4051 multiplexer) on the TSB circuit board, and have tested the chip with a multimeter. The INH pin 6 is not stuck high and is toggling as it should. Pin 3 (Com in/out), however, is stuck at +15V for some reason, and I can't seem to figure out why. I believe this voltage should be around 5V as initial velocity is triggered and approach 15V as the poly aftertouch is applied. I tried swapping in another 4051 from IC3, and the problem remains exclusive to the IC4 socket and the same cluster of keyboard notes. I've also checked the solder underneath for shorts and opens and compared it to the other 4051's on the board and haven't found anything out of the ordinary. Does anyone have any idea as to where I should look next? Could the IC7 (a JRC 072BD dual op-amp, also a recent replacement of the original 4558) that pin 3 goes to be involved? Could a faulty TKC board component be causing the problem? Any help would be greatly appreciated. Thanks!
Cheers,
Jonathan