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CS-80 tech question: initial velocity problem on TSB board
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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 |
开云体育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
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Hi David,
Thanks for the info and happy new year to you as well! This is actually the second time you've helped me, as I've been going by the CS-80 schematics and block diagrams from your website - thanks for uploading those! I do have an oscilloscope, so will give it a whirl, and will try removing IC4 and see if pin 3 is still +15v. I'm also wondering what effect the TKC board has on the A, B, and C inputs of the 4051, and if that could be the culprit. One guy on Muffwiggler suggested a possible broken trace or wire going to the aftertouch PCB under the keyboard, and that if the -15v rail wasn't getting to this board, +15v at pin 3 could be the symptom. Another said to simply wiggle all the spider webs of wires going to the solder pads, as they can detach over time. So I have a few things to try. That thread can be found here: https://www.muffwiggler.com/forum/viewtopic.php?t=193847 For some reason, the last tech that serviced this board installed JRC 072BD's in place of the old 4558's. Am not sure if this is a common swap, but the design on their respective data sheets looked pretty close. Pin-outs are the same, just some subtle differences in the actual circuit. Cheers, Jonathan |
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I checked the voltages at the 4051 key input pins
(13-15, 1, 2, and 5), and they do not look reasonable. Whilst healthy
keys on other 4051's 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.
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