Yamaha PSRS 650 keyboard

[bsco]

Somebody dropped off this keyboard for repair. The problem is the keys are not at the same volume......some of the keys are louder than others...as an example, the fisrt key on the left of the keybed has a certain loudness when you strike it with a certain force...however the next few of them will be much lower in volume..this is happening all over the keyboard.....and there are a couple that make no sound at all until you strike them a couple of times.....I dis-assembled the unit and removed the keys to get to the rubber actuator strips..I cleaned both the board and strips and re-tried this but the exact same thing.I even went so far as to use different rubber actuators in different positions and there was no change...so is this an electronic problem(which to me points in that direction), or do I still have an issue between the rubber actuators and associated circuit board. I had also checked for loose screws, cables and connectors etc.....but everything seems fine....any suggestions very welcome.....
Cheers,
Bernie

[Enzo]

I don't know that specific unit, but I assume it has velocity keys? That is to say dual contacts per note? If cleaning then doesn;t help, then look for open or damaged diodes on the keybed contact board. Also the ribbon from the system to the keybed. Is that two ribbons? And can I assume the length of the bed is more than one board, so there are maybe short jumpers end to end on the sections?



Now look VERY closely. is ther ANY chance the rubber strips were installed backwards? Was anyone into this before you? Or was it honestly in the initial as-failed condition? They keybed is grouped in sections of keys, and if the rubber strips are installed from the wrong end, the spacing will be off and grouping can be affected....in some systems. Also, in many the two contacts on each key will have different heights. SO the rear one hits first, then the forward one. The timing between accounts for velocity. If they are in facing the other way, the tall one is up front and it screws things up.

[bsco]

Thanks Enzo. It does have velocity keys...The rubber actuators are installed correctly...I noticed a pattern here. The keys are grouped together in a set of 12...7 white ones and 5 black ones and there are 5 sets plus one white key at the far right side.The first Left white key has a certain volume when you strike it. Then every white key in that group are much lower in volume but they have the same lower volume. Then the third black key is as loud as the first white key...the rest of the black keys in that group have the same volume level as the other white keys with the low volume. if you move to the next group of keys this pattern repeats itself and does so for all five sets of keys...the final key is a white one which is on it's own..this too has the low volume....Also.......when striking any of the low volume black or white keys they sometimes do not make any sound.....if you press it again then it will make sound...but it is not always the same keys...this second problem is random.I did remove the keys, cleaned the board contacts and rubbers and re-installed. Same thing...I placed a small piece of foil on my finger and tapped on the ckt board contacts but same problem.... I checked the cables as there are three of them and the contacts are spread over to boards connected by a short inter-connecting cable. Everything looks good there. Can't see any bad connections either. maybe I have two problems....or maybe two problems being caused by the same failure??? but I kind of doubt that one...As far as I know this is the first time it has been in for repair....so everything is installed and connected properly...I'll check out those diodes...There are quite a few of them there so I will have to check them all....It is 2:30 in the morning here now so I'll check it out later tomorrow...Thanks for the reply.....
Cheers,
Bernie
P.S. I just thought of something..maybe my problem is with the loud keys and not the lower volume ones...That would probably explain the pattern over the entire keyboard....I guess these keys are on a matrix of some sort so there might be a single broken connection somewhere in the cabling or one bad diode maybe??? I don't have a schematic but shouldn't be too hard to figure it out.... I'll get back to you later....If you have any more ideas fire away....

[Enzo]

I am concerned over your 12 key arrangement. The twelve keys you see are the octaves of the keyboard. That is how the piano is arranged for the player. But internally the thing is not wired that way. Inside, your keys are wired in groups of 6. That is why your first key is funny and then six keys later that key is funny (the first of the group of three black.) Every sixth key will be funny. Do the quiet keys respond to velocity? That is, if you hit them harder do they play louder? I suspect so, and i suspect your loud keys are in fact the problem. I suspect they are stuck on the highest level because their velocity contact line is stuck.


The way this works is one contact closes, then the second, and the computer determines how long the time was between them. But if the second contact is stuck on, then as soon as the first contact closes, then the second is already there, so the system thinks you hit it at maximum velocity. So it is louder than the rest.

I would check all those diodes, a shorted one could be at fault. I would scope the lines coming to and from the keybed, looking for one that looked unlike the others, probably just flat. You could have a bad key assigner IC, IC16 on the main board, but if the problem is on that board, I'd find it more likely to be a bad resistor or bad segment of a resistor pack.

[bsco]

Thanks Enzo...I'll check that out this afternoon...I was not aware of the group of 6.....I am not sure if the quiet keys are touch responsive...they don't seem to get louder if struck harder...The loud ones are just loud...doesn't matter how hard you hit them...I'll check it out after and let you know what I find....Thanks for the reply...
Cheers,
Bernie

[bsco]

Hi Enzo. Checked some more and the quiet keys are touch sensitive....I did not find any shorted diodes on the keybed boards.....Scoped the connectors and noticed that one line was flat....It was the main cable running back to the controller board.....pin11 of CN6 to be exact.......so I scoped the other end on the controller board....same thing.....all the other wires in this connector has a signal on them and when you press a key the scope display shows it.....I can't tell where the traces go from there but around IC16 there are a bunch of pull-up resistors...I checked them and could not find any of them open.....most of them read around 100 ohms but there are some around 1K....I don't have any schematics so I can't say for sure if these values are good...but most likely they are....I will have to contact Yamaha and see if I can get some schematics for this....If you have any other ideas I'd love to hear them...
Cheers,
Bernie

[bsco]

Found the problem.There are three cables coming from the DM board to the keybed..One of these cables has 12 pins...there was one wire which was folded over in the connector and was not making a good contact...and made it look as if the problem was on the DM board..once I got that sorted out the keyboard worked good..thanks for all the help you gave me....I learned something new again....
Cheers,
Bernie