Casio (Music) Keyboard CTK-990

Thread Starter

BerryG

Joined Mar 2, 2022
2
My wife's which she uses as a piano. We've had it about 14 years. Maybe it doesn't owe us anything. Managed to fix quite a few snags but a bit stuck now despite my electronics background as I just cannot find a specific Schematic. This is weird! It works perfectly well on external power yet when batteries are installed (PSU unplugged) is more spasmodic in its operation. Works OK for a while but when switched off it will not come back on. Revert to external PSU & no change. Remove batteries and wait. Reinstall external PSU (9V) Now works faultlessly again every time day after day. Need I say batteries checked - no snag with them on a test load.
Inside the unit are two PCB's with mounted components. One is the "logic board with LSI chips, flash memory etc. The other is the DC power and audio PA pcb. There are two more Paxolin boards that contain the graphited switch contacts and traces for the matrix that interrogates which button is pressed.
I can see power components but without a schematic and layout its almost impossible to figure how this part works. The power On/OFF switch is a MOMentry type with a single graphite button. You need to hold it down >2secs then release to bring up the logic display. (Or turn it off). Its part of the matrix going back on X & Y ID lines to the logic pcb. A ribbon cable connects from the logic to the power board and I assume signals the power section. (As well as routing the audio to the PA section and speakers.
Is there any bright spark out there who can take me by the hand and lead me further into figuring how this works?
I do have some photos - which might help. (Attached). In anticipation..... and grateful thanks. BerryG
 

Attachments

Thread Starter

BerryG

Joined Mar 2, 2022
2
I can now answer, at least in part, how to get a Service (not Operators) Manual for this keyboard. I noticed that other Casio models did provide Service information and began to wonder if I might glean anything from similar models. Of course the block drawings are bound to be similar if they use a micro-processor, SCRAM, RAM, and "modern" logic chips. I needed something more than that. To my surprise I found a very similar hardware provision in the model known as CTK691. The software provision might be the only difference. I have looked at the spec and its all remarkably similar and the power supplies and ON-OFF circuitry looks to be identical. The drawing isn't very well laid out in this respect but I am in the process off redrawing it.
Very roughly I can describe the operation as follows: The silicone rubber ON/OFF button has, along with all the buttons and keys, a contact of a graphite like material. It places a ground to the line which traverses the button PCB via a copper trace to a ribbon cable (JF pin11) that carries it to the logic PCB and pin 97 & 83 of the uProcessor (IC110). It is identified as NMI/STBY/MUTE. You definitely need the Service Manual drawings to get this far. It looks, (my deduction at the moment), as if the uP is put into sleep mode (STBY) when powered off. This reduces the power drain to a trickle of 0.08mA. Is that likely?
I was able to locate a cracked trace near the push switch - probably the result of pushing it too hard in frustration of intermittent operation. I repaired that, replaced the graphite pad, and washed off the PCB after there was a lot of spasmodic cycling between the modes. This cured that so I assume cross-coupling of the fine traces. Maybe capacitive? The mode button, like all the other control buttons, is interrogated by an XY matrix. The piano keys are also accessed in the same way BUT by another XY array and a separate logic chip. In fact there are two contacts on each of the 61 music keys. One is shorter than the other and the time lapse between their making contact is used to decide what the attack wave-font of the note should be like. "Piano/Fortissimo"
So far then, it is working again. I'm more than pleased as it had threatened to beat me.
 
Top