Sir LJ89 . . . . .
OUR CONTINUANCE . . . . .
Now you are finding fault ! . . . . .
with that output pin 6 of the LA / CA5668, being now only at a 1.248 VDC level is being suggestive of it feeding out to a circuit and then encountering a short /or excessive loading on that branched off line, thereby loading down and pulling the +5V AVDD supply on down to that lower voatage level, instead of being the expected ~ 5VDC.
I magged up and then digitally enhanced one of your top board photos to give us the supplied reference and a relevant piecemeal researched schematic that agrees with your boards LC 7881C D/A converter , LM5218 dual op amp, used for filtering out of digital artifacts and trash , and then the final Dual Audio Output Power Amp of the LA4127.
My closest mark up reference is being at the CA5668 regulator chip and its RED rectangle designation marking and in its RED square foreground seems to show the last half of . . . ?'2 ohms printed on the PCB.
SET UP . . . . .
In a NO POWER /BATTS disconnected test, use metering in its low ohms function.
Then you measure from from IC pin 6 to what looks to be its associative resistor connection . . . .of one of the leads of that ( RED-RED-BLACK band, color code marking) 22 ohms resistor. Then you confirm that the touching of one of its leads results on an almost zero reading / like shorting the meters two test probe leads together. OR . . . . .you could just visually trace the direct foil path to the terminated 22 ohm resistor lead.
Then you go to the OTHER resistor lead and just see if it is being connected to the references small RED square marked up side of the smaller blue cased E-cap.
If so, then you ohmically test across that E-caps two leads to see if it is being dead shorted .
If so . . . . . . .then . . . . .as per the video, lightning actually CAN strike twice on Casio players of this design, using a 6.3 VDC rated E-cap with 5 + volts subjecture on it.
You can then unsolder and lift the defective cap from the board. Or if being a nonsolder-er-er . . . .as of yet!
Just a back and forth /side to side full bending of the cap case should let metal fatigue break loose its two wire leads after 363 full flexures.
Then a retest of the unit . . . .sans the bad E- capacitor . . .with batteries installed should hopefully have the unit making beautiful music for mama-san again.
Then you worry about getting / ordering a like capacitive value of E-cap at on upwards of a 16VDC rating, thereby providing a specs voltage cushion for the future..
If that is not the problem, then we move on to my top center VIOLET square markup as well as the right top sides YELLOW square markup.
BECAUSE like situations with 22 ohm isolative resistors and their following bypass caps also exist at those locations..
And there is being a final u/p reset IC located at the GREEN square markup, its also being associated with that +5V AVDD supply line .
AN INITIAL ASSIST . . . . .
Using another photo, I will now start you on the initial flow path of the pin 6 of the LA5668 and its +5V AVDD supply source.
Pin 6 only has one additional foil connection to its side which likely will be for a bare wire jumper on the other side of the board and it then ends up at the pin 10 area and that foil associated with the end of this jumper, it then only shares its foil with what must be THE 22 ohm resistor, then the other side of that resistor has its foil pad which only has two other adjoining pad connections, one of which must be the pad connection to the + wire of THE suspect E-cap and the other pad, must be associated with carrying on the path of the +5V AVDD supply ever on to its other connection areas of the system.
Now, I can't flop the board over . . . nor meter out . . . to confirm those routings , but I highly suspect that I am being correct.
Le REFERENCE . . . . . .
73's de Edd . . . . . .
View attachment 51296
Providing a quick update.
I did desolder the 6V220μF capacitor that you marked with the red square. And I fired up the keyboard with battery power. There was no change, still no sound.
I was under the assumption that the capacitor needs to be soldered on the board for the keyboard to work?
I will test the other components that you highlighted in your schematic. Thank you