EML (ElectroComp) 500 keyboard issues

Hi! New user here. I’m a novice at this; my wife and I are taking apart an old EML 500 synth and trying to get it working. Seems to mostly be okay at this point (cleaned/lubed the pots and faders and I think they’re okay). We’re having some trouble with the keys not working quite right. When I depress a key, it plays the right tone, but when I release it, it tends to momentarily play the wrong tone and then stops. We tried cleaning up the busbar a bit but we’re wondering if there’s anything else we should be doing, and what else the problems could be from besides just wear and tear and corrosion. Since it’s not just one or two keys, I don’t think the problem is any one of the actual resistors, and they look pretty clean and good. Also, is the keyboard something that could/should just be replaced? It looks like the keyboard is only attached via a few wires and a few screws, and it’s very similar to other EML keyboards and possibly other keyboards of the era. I’m wondering if it would be easier to just swap it out for a known good set of keys. We’re willing to take it all apart and clean it really well if that’s likely to the be the problem, but I’m a bit suspicious and would like to know more before we take this too far.

Apologies if I’m using incorrect terminology or not providing enough information. We’re new at this and learning as we go. Thanks!

Welcome to the forum!

I’m not particularly familiar with these but I imagine it’s a similar keybed to other stuff from the era. I recently worked on an Arp Odyssey that had a similar problem. Cleaning the bus bar with a q-tip and isopropyl alcohol and correcting a few bent j-wires solved all of the problems. If this is a keybed with j-wires, make sure none of them are defaulting to touching the bus bar without a key press.

So the unit plays the correct tone when you hold down a key, but when you release it plays the incorrect note. Does it always play the same incorrect note when you release or does it always raise or lower in pitch by a certain amount or is it pretty random? Does every single key on the keyboard behave this way?

These are the kinds of clues that can help us figure out what’s up. If you notice any other odd behavior let us know because it may help with diagnosing the problem. Most likely the bus bar just needs a good cleaning everywhere that the j-wires make contact to it. You’ll want to clean the bottom of the bus bar, specifically the side the j-wires touch.

Hi Travis!

Thanks so much for your reply!

I did some more testing. It USUALLY plays but does not ALWAYS play the correct tone when I press down; sometimes the tone “wobbles” and starts playing lower tones. When I release the key, I assume it’s doing the “decay” part of the sound; at this time (during the decay) it OFTEN but NOT ALWAYS wobbles and plays a lower tone. It is not consistent; sometimes it’s one or two tones off, other times it’s an octave or more. So it’s just inconsistent.

Based on what you’re saying, it sounds to me like I really need to get at it some more and clean it up real good. I’m just wondering if just having ANY corrosion anywhere on the bus bar is enough to make it inconsistent anywhere along the bar.

I think this isn’t a traditional j-wire setup exactly, although it’s pretty much the same thing. It looks like a pair of wires, and they look more like little springs sort of, rather than the usual wire. But it pretty much works the same way with it pulling the PAIR of wires over a PAIR of bus bars, which is pretty cool actually. I’m attaching a picture showing what it looks like when the key is depressed. And… looking at this picture, it all looks pretty dirty still, so I probably need to hit it a lot better with some alcohol or maybe contact cleaner.

Anyway, thanks for your thoughts! I’m curious to know if you have any other ideas after reading this or looking at the picture.

This is very similar to the keybeds in the first versions of the Arp Odyssey which is what I was having a very similar problem with. I cleaned the bus bar and the gently scrubbed the springs themselves. The springs in the Odyssey I was working on had turned quite green where they contact the bus bar but after enough time of gently scrubbing with a q-tip and isopropyl alcohol they began to play like normal. Also the mechanical wear of playing the keys after you’ve scrubbed with some IPA will help break up more corrosion. If IPA doesn’t clean it enough, you may need an electrical contact cleaner.

If after that you’re still having problems it could be something with the sample and hold circuit that stores the keyboard voltage but judging by the symptoms I bet it’s the bus bar.

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Fantastic! I will do as you say and clean it up as best as I can and I will report back the results. Thank you so much! :smiley:

I wrote an elaborate reply and deleted it, as I realized I still have some more tests I can run to see if I can figure out what’s going on. I did some cleaning but I still have more work to do.

Okay I finally have an update. I cleaned the heck out of everything with isopropyl alcohol (91%) and that didn’t really help a whole lot. I then went at it with actual contact cleaner (really strong stuff) and it did help SOME of the keys, but others… well, here’s a video showing what’s going on. You can see that three out of the four keys are just not working right. However, one of the keys is perfect. They’re all right next to each other; all subjected to the same cleaning. Any ideas?

You scrubbed the places of contact with a cotton swab too? Usually that does the trick with isopropyl alcohol but I’ve had a time where it hasn’t. You can try a wire scrubber, just take care not to scrub too much. Some bus bars are plated and too much abrasive scrubbing can take off the outer layer. Some people use pencil erasers too but I’d try that as a last resort.

If you can’t get it working well with any amount of cleaning it may be time for replacement. A bus bar sold as a replacement for an Arp Odyssey or Minimoog may work.

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Yeah, did it all with a cotton swab several times. I also did the best I could to clean the springs as well. I could try a wire scrubber and see if that helps, although those springs are pretty fragile and I wouldn’t want to go too hard on those.

I’ll look at what it would take to replace the bus bar. Might be worth just doing that if it’s not a nightmare. Thanks again for the help! I’ll report back when I know more. :slight_smile:

Is there a schematic for this keyboard somewhere?

I’m trying to wrap my head around what I’m seeing and hearing. I see two bus bars. When a key is struck, our spring makes contact with one. On release, we make contact with the other. What is that second bar?

Oddly, that second note varies. That doesn’t make sense to me. Each key should be fixed to one note.

I’d be tempted to say that another spring is vibrating into contact with a bus bar, but it’s not the case for every key. If that second bar is to clear the key, maybe some of them are not making contact with it when at rest? A good suspect would be any key that isn’t making a second note on release.

Some of the controls on the panel look interesting. Sampler? Make sure that you don’t have something re-triggering

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So there seem to be two bus bars, and I think that when it contacts both bars that’s when it triggers the sound. In theory when it releases contact with both bars then it starts the decay. What’s weird is that when it starts the decay, that’s when the tone shifts and it starts to play the wrong tone. So like, if it were a contact problem, if it weren’t clean enough or something like that, then I’d think you’d have problems with the first tone and it wouldn’t just be on release that there’s a problem.

I’ll see if there are any keys that aren’t triggering the decay. I have seen that symptom but it was like, one key, intermittently.

I could try turning off some of the features of the synth one at a time and see if any of them might be the cause. Like, I know I could probably just turn off the decay and it should just work.

But yeah, I’m really having a hard time understanding this, as I’m an amateur and I don’t do this for a living. I could try replacing the bus bar and cleaning it some more, but like I said, it looks to me like it’s making solid contact, and the problem is only on release, and that isn’t making sense.

It’s possible that the springs themselves are electric and serve some function, although it doesn’t really look that way and I don’t think that’s how these things work.

There’s also the resistors which could be part of the problem, but again, why just on release?

I’ll try some more stuff and see what I find. Thanks all for the comments. Really appreciate it. I feel a little less crazy :slight_smile:

I did some more tests. I am pretty sure what’s happening is that for this to work, the springs have to make contact with the pair of bus bars at pretty much exactly the same time. If they are hitting at slightly the wrong time, or releasing one at a time, then the tone shifts and it warbles or the tone is just wrong. So I think the problem is they aren’t releasing at the same time because they are loose or otherwise not adjusted properly. So I think it may just come down to tightening or possibly replacing the springs (similar to but not exactly like j wires) somehow, but I have literally no idea how I would go about doing that.

Three bars, actually. And the springs are the electrical contact. You’re completing a circuit.

I think the bar at the front is for the note. That’s the part that doesn’t make sense to me. I imagine that the bar is connected to +5v, and when the spring hits it, it sends +5v to a voltage divider on that key. (Unless this uses a top octave circuit and dividers…)

The forward bus bar at the rear must be gate. This is what triggers the envelope Attack.

When released, the spring at the back will break contact with the gate and strike the rear bus bar. That’s where I’m confused, because the absence of the gate means that the key has been released. It could just be a way to keep the key from floating electronically. (In which case I’m completely wrong in my assumptions, and the issue really is that other keys are not contacting this bus)

From what I’ve read, the EML500 is supposed to be able to remember the last key hit, and repeat it if the trigger switch is set to SAMP. I’m wondering if that’s what’s going on somehow

I don’t want to distract. Just trying to figure out if there’s something else that could be causing this issue. I have literally nothing in front of me related to this keyboard. Hopefully I can give someone some ideas

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One bus bar is probably for gate circuitry and the other for pitch CV (this is how the 2 bus bar keyboard in the Odyssey is). The last bus bar is probably to return the gate to ground but hard to say without a schematic. A dirty bus bar can lead to pitch drift like this but it is weird that it’s only on the release. I’d still have to believe that the most likely problem is the bus bar or springs being dirty or heavily oxidized.

The resistors on the keybed circuitry form a voltage divider, each key has its own respective voltage level that represents that note. If the spring doesn’t make perfect clean contact with the bus bar the added resistance appears as the pitch being lower than it should. It could be while pressing the key fully down the spring makes good contact with the bar but while releasing the electrical contact is not as good so the circuit understands this as a lower note. That especially noisy sound around 39 seconds into your video is a tell-tale sound of poor contact with the bus bar, at least in my experience. I’ve worked on an Odyssey with a stubborn problem like this recently but enough times through cleaning with isopropyl alcohol resolved it for almost all keys. There’s still two keys that have pitch drift on that Odyssey, but I’ve yet to hit it with a wire scrubber or eraser.

In any case, I hope you can get it figured out!


A good troubleshooting method would be to focus really hard on cleaning just one set of springs and area of contact for just one key that is misbehaving. If you can’t improve that one spot then either something else is going on or the bus bar is too oxidized.


I just had to look at a schematic for that Odyssey…

The two bus bar configuration was my first thought, but I dismissed it. Hitting any two keys will short out one or more resistors and produce the wrong voltage. But the resistors along the front support this, and it would be the easiest way to design it.

I noticed the scratching. There’s also a bit when the third key is slid from side to side where I think it’s getting triggered.

To be honest, that second key (the one not playing a second note) is the one that sounds odd. Maybe it’s the rapid playing of it, but something sounds peculiar. I couldn’t identify the attack and release.

Something else: That second note has a definite attack.

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I (sort of maybe) FIGURED IT OUT. The springs seem to be worn out and stretched and my GUESS is they are not maintaining a current along the length. I don’t know for sure, but I THINK they might be carrying their own current which it uses upon release of the key. [EDIT] Based on the patent info below, I think that’s incorrect and they only carry a current when connected to the bus bar. [/EDIT]

Anyway, I tested a way to improve the connection along the spring by wrapping one of the springs that wasn’t working well with aluminum foil in order to improve the conductivity along the spring and - IT WORKED. The key works perfectly now. It’s a temporary solution but it’s interesting. That says to me that the problem isn’t the bus bar and is definitely the spring. I’ll report back with more detail as I progress. Thanks all!


Aluminum foil is too difficult to apply and too unreliable, as it turns out, but it does seem to confirm that if you can get a better connection along the spring, the problem disappears. I don’t know what this means in terms of a permanent solution - aluminum foil isn’t it - but this is interesting. Might be able to add a bit of solder or something, although I hesitate to do that as that could just screw everything up.

Check this out. Here’s the patent for the mechanism.
Includes schematics.

Got an answer from a repair shop as to what they think is wrong. I’m going to have them take a look.

“The alignment between the two springs is off. The cv spring is releasing before the gate spring, causing the pitch to drop out before the key is fully released.”

So I think @ProfTrudo was on the right track.

More info in video form.