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Yamaha CS-80 Technical Question: Initial Velocity Problem
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author Yamaha CS-80 Technical Question: Initial Velocity Problem
JonathanG
Hello,
I am looking for technical assistiance 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

jkjelec
Do the input signals at pins 13-15, 1,2,5 look reasonable (i.e. like other keys when you hit them fast or slow or push them for aftertouch?) If these are stuck at a high value maybe there's a broken trace on the aftertouch PCB (or a wire going to this bd) under the keyboard. It looks like if the -15V rail wasn't getting to the aftertouch board in this area, this would be the symptom
noddyspuncture
What I would do if all else fails, is check for open circuit wires going to those solder pads.

Years ago, when I put bypass caps on my TKC IC's, the constant turning and handling of the board created strange symptoms which I was chasing for a long time.

My measurements didn't seem to make sense... in the end it turned out to be broken wires... you couldn't see it as the plastic sleeving was still in the solder pad, so it all looked OK - but wires were broken inside.

Just a thought, and something to bear in mind...!

Let us know what you find,
Cheerz,
Tom
JonathanG
Thanks, gentlemen, I will look into those areas. I had done a cursory physical check of the web of wires going to the PCB pads, and they all seemed OK, but I will do a more in-depth continuity test.
noddyspuncture
JonathanG wrote:
Thanks, gentlemen, I will look into those areas. I had done a cursory physical check of the web of wires going to the PCB pads, and they all seemed OK, but I will do a more in-depth continuity test.


Mine "showed itself" when I touched and moved the wires gently.

Manipulating them will make your problem come and go so monitor those notes whilst gently moving the wires.

If that is your problem, you'll soon know..!
JonathanG
Pulled this photo off the web:
JonathanG
jkjelec wrote:
Do the input signals at pins 13-15, 1,2,5 look reasonable (i.e. like other keys when you hit them fast or slow or push them for aftertouch?) If these are stuck at a high value maybe there's a broken trace on the aftertouch PCB (or a wire going to this bd) under the keyboard. It looks like if the -15V rail wasn't getting to the aftertouch board in this area, this would be the symptom


I checked the input voltages at these pins (13-15, 1, 2, and 5), and they do not look reasonable. Whilst healthy keys rest at 6V and approach 13-14V as you apply aftertouch, the bad keys are resting at 9.23V and jump to 15.49V as soon as they are hit, without even applying the aftertouch. The voltages on some of them slowly drop back toward 9.23 until you hit them again and they jump back to 15.49V. Odd.
JonathanG
OK, so I’m getting some continuity between the ‘E’ or ground and the -15V rail on the TSB board, with about 60 ohm resistance between the two. Is this normal?
JonathanG
Well, the good news is I think I found the problem: an open trace on one of the poly AT boards under the keyboard. The bad news is my CS-80 groaned, wailed, and died when I went to turn it back on. Hoping I didn't discharge static or power caps into any fragile proprietary IC's. Ugh. Now to test the 26600 and 26700 chips..
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