Yamaha KX88 key group not working

[stevegkeezer]

I have an old Yamaha KX88 keyboard that is having some key group issues. The G keys on the right side of the keyboard are all not working in all octaves. Keyboard also is stuck in dual mode which I am having trouble finding a solution to get out of.

I was curious if anyone has had any experience as to common failure points of this device. I went through and cleaned all the contacts within the keyboard which seemed to have no effect. I wasn't sure that would solve it, due to the dead notes spanning an octave. Since they all failed my guess was that these notes are probably all bussed together in some way.

My next plan is to replace the ribbon cable. Has anyone had a similar issue with this keyboard could suggest me some troubleshooting ideas?

 

[user 130681]

Being from 1985 this model has little service information but I usually find it is a leaky, short circuit capacitor or 20.
If I wanted to keep it going for a customer, I would replace all the electrolytics and then start fault finding. Leaving an open cheque to pay for my workshop's many hours of labour.
The G oscillator is just the one and goes throus octave dividers to produce the lower octave notes. There will be a duff capacitor in one of the dividers or more.

 

[stevegkeezer]

Thanks for the insight, Jon. Without a doubt the caps are probably on their last legs. Sounds like the most promising place to start. Being that the keyboard worked previously, and was hardly moved or bumped before being stored, I am really doubting the cable was damaged in that time.

To address dual mode if anyone cares, I read that changing out of dual mode is more than likely due to that circuit being open or to a contact being stuck and continuously sending signal to the board.

 

[user 130681]

My money is on short circuit capacitors as, like a car battery, if they are not used for a long time, they deteriorate and eventually go short or open circuit.

 

[radardoug]

Keyboard problems are usually in the key matrixing, look for dead diodes or breaks in tracks.

 

[stevegkeezer]

I doubt there have been any breaks in the actual traces themselves considering the keyboard wasn't rough-handled and kept in a decent condition. I think Jon's solution would be more common. Being an old keyboard, the capacitors on these boards are about 40 years old, those I suspect would likely the first element to fail on this board. I am not sure what you are referring to about the dead diodes, can you elaborate on that?

 

[rock soderstrom]

As far as I know, the KX88 is a master keyboard without its own sound generation. This means that G oscillator, octave dividers and everything around them are ruled out as a source of error.

I think it is a contact problem in the keyboard matrix of, one would have to find out how this is organized.

Broken diodes, capacitors, ribbon cables and PCB traces are all possible.

Only signal tracing will help.

Are there any photos and a service manual?

 

[stevegkeezer]

Here's a link to the service manual https://www.manualslib.com/manual/898831/Yamaha-Kx88.html

 

[rock soderstrom]

Thank you. Unfortunately, the service manual is not very informative for our topic.

I think the PN board is responsible for the keystroke scanning and leads these signals to the DM board.

Check if this assumption is correct? Then compare the signal flow for the G's and other notes and find out where the signal is lost.

 

[stevegkeezer]

Thanks, I will give that a look as soon as I get back to my shop. Appreciate your help!

 

[radardoug]

You want to bet on caps being the problem? I'll take the bet that it is a diode or a track broken.

 

[stevegkeezer]

@radardoug I am pretty unfamiliar with these keyboards. I am just going on what I know working on old mixing consoles so please forgive my ignorance. I will be sure to give the diodes and tracks a thorough look. My experience from old electronics is that caps are a ticking time bomb which I always assume are the first to go.

Do you have any recommendations on a good place on the board to check first? Being a trial and error solution, I value and appreciate all of your input, so any advice you can give me will be helpful to eliminate some variables.

 

[radardoug]

Look at the pads on the missing keys, and trace them out. Look for the connected pattern. Then meter between keys.

 

[stevegkeezer]

Okay thanks, I am going to start there. Just for clarification when you mentioned the diodes, are these the type of diodes that you are referring too? Wasn't sure if these were diodes or resistors since I didn't opened it up yet.

 

[radardoug]

Thats the main board, your problems will be on the keys board.
Here is a Youtube video which will give you some ideas.

The diodes are on the board shown, in this case on the back.

 

[stevegkeezer]

Well, I took the PCB off the keyboard and checked all the diodes with my multimeter and they seemed to be functional. Went through and checked the keys again and now everything works all of sudden. Wish I had an answer as to why, maybe I had a bad connection on one of the diodes or something. So mission success, thanks for the help all.

 

[stevegkeezer]

Well this issue came back unfortunately. It worked for a while, the keys would work again, then go out again.

Since this seems to happen intermittently should I be able to rule out a broken trace? I am thinking it must be a diode that is faulty and going intermittent. I don't have my multimeter to check with me at the moment. Does anyone happen know the diode value that I should buy?

 

[rock soderstrom]

Well this issue came back unfortunately. It worked for a while, the keys would work again, then go out again.

Since this seems to happen intermittently should I be able to rule out a broken trace? I am thinking it must be a diode that is faulty and going intermittent. I don't have my multimeter to check with me at the moment. Does anyone happen know the diode value that I should buy?

As I said, such an intermittent contact can have various reasons. I would not rule out broken PCB traces, since mechanical force is applied on the PCB by the keystrokes.
Bad solder joints, corroded contacts on the connectors or broken ribbon cables can also cause such a behavior. Defective components such as diodes (also capacitors) are of course also possible. Without measuring devices and systematic troubleshooting, however, this is just fishing in the dark.

I would start with contact cleaner on the connectors of the PCBs and try to provoke the error by wiggling the connecting cables.

Good luck!

 

[JohnRoberts]

a bad solder joint could behave like that, if you can figure out what node is open in the circuit it will be easier to find physically.

JR

 

[stevegkeezer]

@rock soderstrom I didn't see any corrosion on the ribbon cable, cleaned all the pins and sprayed the connector with contact cleaner. I also did a bunch of wiggling of the ribbon cable to try to reproduce it but nothing.

The only time I thought I had the issue pinned down is when I cleaned the PCB which is what seemed to fix it the first time. I wasn't sure if it was due to cleaning it or hitting the diodes a bit that made it start working again. I've also gone through and individually wobbled the diodes as well.

I do see some areas on the board that look like partially cold solder or possibly corrosion.