United States
Total Posts: 7
Joined: November 17, 2015
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Hey everyone - I have been battling the D2 key on my SX-PX55M digital piano for about a year. It's normal state is that is makes a very faint low velocity sound when pressed, however hard or soft, and then when it's not pressed, it makes ghost notes with imperfect regularity, but constantly - also very soft - like, donndonn...donn...donndonndonn..donndonn.. etc. As a workaround, I put a weight on the key to keep it depressed all the time, which makes it quiet, but of course causes playing issues beyond not being able to play the note, because it's in the way.
I've tried to fix it three times by disassembling the keyboard - my approach was pretty much the same each time - take it apart and see if anything looks bad, then clean it and reassemble. I have never seen anything unusual. Each time, I have watched videos about cleaning keybeds, and have gone a little further - this last time I gently scrubbed the contact surface with a toothbrush and isopropyl alcohol.
When I reassemble the keyboard, things are temporarily different, but then revert to the normal state within an hour or so. One time it seemed like it was fixed at first, but this last time it seemed like it was completely dead - no sound when pressed and no ghost notes, but then an hour later it went back to the very low sound and ghost notes.
I have the service manual thanks to this site (thanks!) and I can see that the circuit boards under the keyboard are no longer available anywhere, of course.
Could it be one of the diodes associated with the key? If so, I could probably source and replace both of them. Any other thoughts? Thanks!
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Posted on February 13, 2017 at 2:32 PM
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United Kingdom
Total Posts: 166
Joined: January 12, 2015
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Re: dodgy key on SX-PX55M
Hi there
You may have an issue with the rubber contact itself. It is difficult to tell exactly but I have found that repeated failures usually means that the conductive rubber is either not making a good contact onto the PCB contacts or the rubber itself is no longer conductive enough to work. The little rubber pad should be around 300 ohms if you have a meter to measure it from one side to the other. Very faint sound is due to the first contact not closing so the D2 key contact closer to the fulcrum is the one that is at fault. This contact starts a timer the second contact nearer the front of the key stops the timer and the total time is the speed of note movement and hence the volume of the note. This might make it easier to find the issue.
It is very unlikely that any other part of the piano is at fault. Even the diode matrix that sits on the same circuit board is very unlikely to be the problem. They are only 1N4148 signal diodes so you can try replacing them but because the scanning of the keys and its output multiplexing is quite complex I would think that if a diode is faulty it would have other effects across the keyboard and not just at D2.
So how to fix the key if it is a contact resistance issue? There are unlikely to be any contact strips available. Even those that you see occasionally on various web sites, if they have been removed from old instruments are only likely to add to your problems. You can move the strips around as they are not specific to one part of the keyboard. I would suggest to check that it is the contact strip move it down to the very bottom of the keyboard and if the fault moves with it you know it is the rubber strip that is at fault. Beyond that I have found that a conductive paint is sometimes a good enough solution. you need a very small amount of it and be aware that it must be thoroughly dry before you reassemble the keyboard. The contact only has to bridge across the PCB tracks so it only has to drop the resistance to get an output to the rest of the electronics. In any case its almost worth a try! It comes in tiny little pots almost like nail polish and is available on EBay as well as some electronic suppliers for printed circuit board repairs.
Hope this helps. Let me know how you get on.
Regards
Dave
Posted on February 16, 2017 at 5:31 PM
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United Kingdom
Total Posts: 166
Joined: January 12, 2015
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Re: dodgy key on SX-PX55M
On your repeat key issue I think that cleaning the contacts may have made things a little worse especially if the cleaner has caused leakage across the printed circuit board. I rarely use anything other than electronic type cleaners as most other types tend to cause issues. When out in the wilds dealing with things like church organs for example I found that by far the best cleaner was a product called servisol. I never spray it just using a tiny dab of it on a tissue. So many standard liquids can leave trace effects that can be hard to remove. Even some PCB cleaners (those containing isopropyl alcohol) leave a residue that you won't easily get rid of.
Regards
Dave
Posted on February 16, 2017 at 5:41 PM
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United States
Total Posts: 7
Joined: November 17, 2015
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Re: dodgy key on SX-PX55M
fantastic information - thanks! I will see what I can do this weekend, and I will not attempt to clean again
Posted on February 16, 2017 at 11:17 PM
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United States
Total Posts: 7
Joined: November 17, 2015
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Re: dodgy key on SX-PX55M
well - the problem did not follow the rubber pad - I switched it with the one on the very bottom, and the very bottom keys still work fine, but the problem key is still the same.
Unfortunately, though, I must have done something else wrong, because now other keys are not functioning (more critical ones, unfortunately). I will leave it on for a while and see if they come back.
edit: after a couple of minutes, they did come back - yay! Almost like they needed some time to build up a capacitative charge or something. Anyway, no progress on the problem, but at least no additional damage.
One thing I noticed about the circuit board underneath the rubber pad for the problem key (and the ones next to it) - it seems I rubbed away some silvery coating, exposing some circuit traces to some degree that is not the same with other portions of the board that I didn't clean. Argh... I swear this one was thinner to begin with - I don't remember it looking as solid-silver-gray as the other sections, but it's too late now to be sure.
Posted on February 17, 2017 at 6:03 PM
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