What is the purpose of the mysterious 'strip' found on keybeds of digital pianos

Thread Starter

Yami

Joined Jan 18, 2016
332
Hi guys, I have always wondered what the purpose of the mysterious strip found on the keybeds of digital pianos are. There seems to be a carbon line in the middle and soldered at both ends and in the middle. Doesn't seem to be connected to anything important :S and nothing is placed on top it when the keybed is assembled.
Thanks in advance.
Strip1.JPG
Strip2.png
 

MisterBill2

Joined Jan 23, 2018
7,047
I am guessing that it is actually a tapped resistor that has some important function, such as calibration of the keyboard somehow. It may just be a jumper between points on the underside, since it looks like this may be a two-sided circuit board. And also, it might be a production quality-control item that has no use after the keyboard passes it's inspection.
 

Thread Starter

Yami

Joined Jan 18, 2016
332
Could it be a conversation starter?
Hehehe nice one!

Perhaps a pressure sensor for aftertouch effects?

Bob
The pressure sensor is underneath the red felt material. The keys touch that, but nothing touches these strips.

I am guessing that it is actually a tapped resistor that has some important function, such as calibration of the keyboard somehow. It may just be a jumper between points on the underside, since it looks like this may be a two-sided circuit board. And also, it might be a production quality-control item that has no use after the keyboard passes it's inspection.
Yeah I am also starting to think that its some kind of a test strip used after manufacturing. There is no indication of this strip in the schematic and does not include any test procedures in the service manuals which uses this strip. I am trying to figure out a way to confirm this though.

Thanks everyone for the help and input.
 

Thread Starter

Yami

Joined Jan 18, 2016
332
Hi guys,
I am working working on a digital piano which is having some issues with the key scanning (my suspicion). Anyway this is a general question about dirty pcb.
I have attached a photo of the PCB (keybed). As you can see some parts of it has got some kind of black spots and tarnishing and there is another spot which has got some greenish/turquoise colour I am assuming its some kind of corrosion.
I have measured for continuity from these questionable traces, and to the closest points it showing almost 0 ohms and to the furthest point which this trace goes it shows around 0.7 ohms.
My first question is would this tarnishing cause any problem? I know its not ideal but seems the connections are intact. Would something happen when the circuit is in use?
My second question is how do you recommend cleaning this kind of tarnish, I have seen similar kind of tarnishing on other boards. I have used IPA and a brush, not much improvement.
I have a suspicion that key scanning is hiccuping due the dirty pcb and its affecting only some keys. I measured the voltage at the diodes of the 'faulty' keys and I get a reading of 4.3V and I get close to 5V at the diodes of the 'good' keys. I know the scanning process would involve a square wave and I can't directly measure it using a DMM, but 4.3V measured at the faulty keys cannot just be a coincidence. However I measure 4.3V at the microcontroller pin which leads me to disregard voltage drop from the dirty traces.
Thanks in advance.
Tarnish.jpg
 

shortbus

Joined Sep 30, 2009
7,961
I am working working on a digital piano which is having some issues with the key scanning
Not sure what you mean by scanning? It looks like the key touches the area/pad next to the yellow circles(don't know how to describe the inter locking prong shapes). Those are real similar to what is used in TV remote control buttons The bottom of the key should have a black colored hard rubber or plastic pad that completes the circuit when it touches that area of the circuit board.

With time and age the pad on the key gets worn or even coated with sweat or what ever. That pad is conductive material so I have had luck using alcohol to gently clean those type pads, with a Que-tip. They make stuff to repair them, a couple of kinds. One is new pads, another is a type of paint. Here is a link to what I mean - https://www.google.com/search?client=firefox-b-1-d&q=conductive+pad+repair
 

MrChips

Joined Oct 2, 2009
21,893
I would clean the entire surface with a soft paper wad dipped in IPA (isopropyl alcohol).
Next coat the tarnished traces with clear nail polish. Wait for a few hours for the polish to dry.
Then go over each key contact lightly with cotton swabs dipped in IPA. Clean the underside of each key that makes contact with the pads using IPA. Make sure that there are no specks of dust remaining on the contacts on the key and on the PCB.
Reassemble keyboard.
 

Thread Starter

Yami

Joined Jan 18, 2016
332
Thanks everyone, yeah I have had good luck with fixing key issue with IPA and a cotton swab and brush on many units, but this particular unit is a head scratcher. Everything I check seems to be in order as in can't find and shorts or open connections and to make things even more harder the problem seems to be intermittent. I have to play several keys fast for the problem to occur eg. if I play g,a,b,g,a,b and all of a sudden the b note comes out with full velocity. It remains at full velocity for a second and goes back to normal and then I am able to play all the keys normally.
Only thing I could remark is that on the 'velocity' detecting path (I hope I am making sense) when I measure at the diode to ground (of the 'b' note) the voltage keeps going down from 4.3V to around 3V (does not stay stable). With all the other keys I measure something close to 5V.
 
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