Casio CTK-611 8 keys not working

[mygrove]

I have acquired an (old) Casio CTK-611. All seems to work as expected, except that 8 keys in the centre of the keyboard do not work. The keys are G#4, A#4, C#5, D#5 and A4, B4, C5, D5. In fact, when you first switch on they can be used once but only sound very faintly, then nothing. I have had a look under the keys to see if it was perhaps a dust issue - it wasn't.

 

Looking inside it is the first 8 notes of the second half of the keyboard on pcb KY2M. I understand this board was a common component of many Casio products. I get the same nil response by pressing the pair of rubber pads at the sametime.

But is it this pcb that is likely to be the problem or something else? Any clues on fault finding are appreciated.

 

(I have posted a similar question elsewhere but it has been suggested there may be more folk with knowledge of fixing Casio keyboards here - let's hope so!)

Thanks.

Edited August 9, 2023 by mygrove

[Jokeyman123]

Can only guess from this far away. Sounds like a data bus line connected to that portion of the keys-usually when an octave (or close to) that is simultaneously out-without seeing a diagram showing the scanning matrix of this Casio-and assuming it uses a scanning matrix, which most keys past early analog used to scan each key-and even early analogs actually-I'd need a schematic to trace which chips and data lines are connected to this section of the keyboard. Could be as simple as a broken circuit trace somewhere, possibly on the key contact IC board or deeper into the mainboard if it has one. to as complex as finding which chips are used to create the scanning matrix and finding a possibly defective chip. Again, just my best guess.

[mygrove]

4 hours ago, Jokeyman123 said:

Can only guess from this far away. Sounds like a data bus line connected to that portion of the keys-usually when an octave (or close to) that is simultaneously out-without seeing a diagram showing the scanning matrix of this Casio-and assuming it uses a scanning matrix, which most keys past early analog used to scan each key-and even early analogs actually-I'd need a schematic to trace which chips and data lines are connected to this section of the keyboard. Could be as simple as a broken circuit trace somewhere, possibly on the key contact IC board or deeper into the mainboard if it has one. to as complex as finding which chips are used to create the scanning matrix and finding a possibly defective chip. Again, just my best guess.

Thanks @Jokeyman123. You prompted me to study the Service Manual a little more closely. I have annotated the relevant areas, but when it gets to the logic and circuit I'd need some help. If you think there should be any more useful pages I'll attach those too.

 

I can't see how you attach files here so here is an external link to the pdfs Casio CTK-611 pdfs

[Jokeyman123]

Let me know how it goes. I use a jeweler's loupe or other magnification to check for hairline cracks in solder joints and circuit pads and traces. I have even seen solder bridges between 2 connections inside keyboards-which may not cause problems initially but over time act as short circuits.

 

One fairly easy trouble-shooting circuit tracing I use, without using an oscilloscope to check for specific signals present or not present in a chip or circuit-using the schematics-check with a multimeter to see if voltages are running through areas connected to this set of keys-especially the voltage pins on any IC's, usually 3V or 5V in Casios. 

 

I rarely find dead diodes in the keyboard IC boards which would usually only affect a few keys at best, but I have often found broken, cracked or corroded circuit traces on these boards since these take almost all the shock of the keys hitting plus possible contamination as minor as it might be, especially in older boards. If you're lucky this could cause a group of dead keys. if all this fails, I'm thinking back-tracing with an oscilloscope might be needed to see/compare data lines from the chips directly connected to the matrix back to the LSI's, to determine if data is communicating through the signal path to these keys, not an easy task. Looking at the schematics, I am hoping it is something simpler than this, as there could be a broken data/signal path, or even a voltage path or short to ground in even one trace in one chip. Why most techs will not bother repairing mainboards at the component level anymore, for keyboards or computers.

[mygrove]

Thanks again @Jokeyman123 I'll try some magnification over the solder at the weekend. I'm sort of OK understanding the schematics but I find the challenge interpreting this on the actual board. Perhaps one point you could clarify for me is how the signal from the keys being depressed is combined and then passed over the ribbon cable. 

 

As I know that the silent keys precisely align with the F14 and S14 rows from the Key Matrix, it seems a component or joint that is common to them that is at fault.

 

I also wondered whether the faint sound that is heard when first switched on and depressing one of those keys is a symptom of a failed capacitor. Thoughts?

[Revenant]

The FI and SI signals work together to let the keyboard measure keypress velocity. If one of them is having issues, it can cause notes to play at the wrong velocity, and if you're only able to press each key once, then the keyboard is also probably not able to tell when the key is being released.

 

I'd just check the FI4 and SI4 solder pads on both sides of the ribbon cable and make sure they're not damaged or shorting out with anything nearby (or shorting to ground).

Edited August 16, 2023 by Revenant

[mygrove]

6 hours ago, Revenant said:

I'd just check the FI7 and SI7 solder pads on both sides of the ribbon cable and make sure they're not damaged or shorting out with anything nearby (or shorting to ground).

Thanks for that. I am guessing you meant FI4 and SI4 as this is the block I'm having problems with? These are the pads each end of the ribbon cable on the keyboard pcb and main pcb?

[Revenant]

Oops, yeah, I meant 4, not 7. It should be pins 7 and 8 on both ends of the ribbon cable (pin 1 should be marked on both PCBs).