Just recently my school’s Yamaha CS-15 started the weird behaviour of never stopping any notes. When I turn it on, it is silent, but at first keypress the note never stops. When new keys are pressed it changes them fine, but seem to regard them as played legato.
It does not seem to be related to the envelope generator, and affects both oscillators.
Does anyone have any idea what cause this issue? I purchased it about a year ago, and it has been working fine until now. If re-capping it will solve the issue, I might attempt it myself, but only if that seems likely to fix the issue.
Thank you! 
JayTee
First off, what a cool school!
If you’re able to run a few tests then this will help identify where the problem lies. I’d check what’s happening at JK-5 and JK-4 (pins C4-2 and C4-3 on the CPB board) to make sure they’re going low when the key is released. They probably are. I’d also check what’s happening at pin 3 of IC3 on CPA (I think).
Last check is what’s happening with the external trigger. If you’ve got something that can trigger it externally (like an Arturia Keystep Pro) then you can confirm whether it’s the internal latch that’s not working or something else.
Once you know where the fault lies then it should be relatively simple to either diagnose it or just throw parts at it until it works. The only thing I’m scared about is if the key assigner IC has died. That’s an unlikely but potential culprit and you’d be pretty screwed if it’s dead.
Also, take all of this with a grain of salt and I’ll happily defer to someone who knows more.
Best,
Mike
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Thank you for your reply!
I’ll test with a Keystep Pro tomorrow, that’s a very smart step to cue in on the problem. If that works, I’ll read up (or rather, YouTube up) to know precisely how to check the parts you mention. I’m far from a technician, and the most advanced stuff I’ve done is to re-cap a Poly 800 and replace the internal battery with a battery holder. But, I’m up for any not-too-delicate stuff.
And yes, let’s hope it’s not an IC. I guess replacement IC’s can only come from vintage Yamaha gear, and that seems expensive and hard to find.
My school is great, I applied for (and got afforded) a grant to purchase synths for the music program. So, in a year I’ve bought 24 synths! Several old ones (CS-15, Jupiter-6, Juno 60, DX7 etc), some new ones (Prophet 6, Hydrasynth, MicroFreak etc). And due to cost/maintenance also lots of new clones (Behringer stuff; Poly D, CAT, TD-8, 2600 etc). Never been more fun to go to work! 
I’ll let you know how the Keystep-testing goes.
Thanks,
JayTee
That’s fantastic! What a cool thing you got to do with your school. Funnily enough, I have almost the same set of vintage synths: CS-40m, Jupiter-6, Juno-6, and DX7ii (and a few others). If you can find a JX-3P for cheap then that might be a nice addition. It doesn’t sound like either the Juno or Jupiter and has a great sound unto itself.
The good news as far as the ICs go is that most of them are easily replaced. Almost all of the op amps are still available and the 4000-series ICs are easily available. If it comes down to needing a 4000-series IC then I recommend getting an assortment package from Amazon rather than buying them individually.
One thing to know about this era of Yamaha equipment is that they seem to suffer cascading failures. Yours isn’t exhibiting this, but just be aware of it. Something can go wrong and then poof, tons of crap gets fried. I ended up replacing over 200 components in my CS-40m due to this problem and had to swap guts between an FX-3 organ and an FS-500 (same organ, less cool case) due to all the component failures.
OH! One hot tip: Since you’re getting this stuff for your school then I highly recommend you look for cheap or free organs on Facebook. You can find some really good things for the cost of buying your friends some pizza. Look for Yamahas from the 70s through the early 80s and Hammond models M-3, M-100, L-100, E-100, T-500. The Yamahas E70, D85, FS/FX series, and a few others are all just amazing instruments that I know you will love. Can you imagine trying to buy a beat box, string synth, poly synth, analog synth, clonewheel organ, and a performance controller? You get it all with these crazy things and they’re usually free!
I’ve heard great things about the CS-40m, but they are rare (but it seems all CS synths are rare in Norway). Yes, the JX-3P was on my list, but after adding the Super-JX to the mix of synths, I thought it appropriate to focus on more diversity.
I didn’t have time to check the CS-15 today, sadly, so it’ll have to wait until next week. About the cascading failures, do you think for instance a re-capping could prevent some of those failures? If frying is a risk, it seems that new caps on the power regulator maybe smart risk management? sigh I think I need to learn a LOT about electronics real soon, haha.
And hey, great tip about organs! I’ve recently discovered what synth (and rhythm box) treasures they can be! I think our biggest problem now is space to keep all our instruments plugged in, but I’m sure we can find room for a 70s organ!
The electrolytic caps aren’t likely to cause any major problems, though that’s not guaranteed. The tantalums, on the other hand, can short and cause things to go haywire.
The other big culprit is the 4000 series ICs. I don’t know why but they seem to be more likely to fry in Yamaha equipment than anything else. Another culprit is the 311 op amp, but I haven’t checked if there is one in the CS-15.
Re-capping is good preventive maintenance, though. It’s a bit like changing the coolant hoses on an old car. The hoses may well be just fine but it doesn’t take much effort to replace them and ensure nothing will leave you stranded.
The general priorities on re-capping are the following from most to least critical:
- Always replace any old RIFA or Frako film capacitor (Yamahas don’t use these, at least, not to my knowledge)
- Replace small metal can electrolytics (e.g., Philips, Frako)
- Replace three pin tantalums
- Replace power supply electrolytics
- Replace tantalums near logic circuits (e.g., main processing board)
- Replace general polarized electrolytics
- Replace other tantalums
You usually don’t need to replace bipolar electrolytics.
Oh, and I’m torn about the CS-40m. I don’t know if I’ve found a great use for it yet.
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Good advice. Extra characters here just to meet minimum required to post.
Thanks so much, this is super helpful!
After checking the service manual, unless the NJM4558DN is the 4000 series ICs you refer to, the CS-15 may be in the clear of that culprit. Couldn’t find any reference to a 311 op amp either.
Your priority re-cap list is great! I’ll make sure to replace the small metal cans and the 3-pin tantalums (if I find any). Probably also the power supply caps while I’m at it. AND maybe find a soldering course, as I don’t yet really trust my skills a lot, haha.
Ok, this is embarrassing (probably). I may have found the cause in a YouTube-video (by automaticgainsay).
So, in short, the CS-15 has a feature I’m not familiar with, and it’s a sort of “hold” function on the VCAs. I THINK I turned the “Initial level”-knob down to 0 while spending 30 minutes testing everything I could think of, but Marc’s demo of it in the video is just too on point. I’ll test it on Tuesday when I get back.
I still count myself as a newbie on synths (4 years ago I had no idea what a filter or an oscillator did, much less what an envelope generator was), and I continue to learn. Your advice, Mike, are still excellent, and will come in handy down the line when (not if) the older synths start showing signs of their age.
I’ll let you know on Tuesday if the fault is the user and not the hardware.
Just to end this topic: it was indeed user failure, and not hardware.
So now I’m enjoying the CS-15 to the max (what a fantastic synth!), and have learned a lot. Thanks for all help anyway!
JayTee