Electronic keyboard freezes on startup.

[Yamin]

Hi guys, I have got a Nord Electro 2 in for repair.
History of the unit and fault:
The unit is around 15 years old. Around an year back one fine day when it was in use the player heard a pop from the inside with smell of smoke, it was turned off and kept.
After couple of weeks the owner turned it on and it was working for a bit and then the unit freezes. After cycling the power it works for a bit and freezes. Cycling the power sometimes gets it to work and sometimes it doesn’t.

The unit was brought to me and when I opened up I found what had smoked. It was C3 (Rifa caps in the power supply across primary side of the transformer).

I changed the both of the line filter caps. I also changed the electrolytic filter caps C5, C7, C9 as I noticed a bit of ripple when measured.
It didn’t rectify the problem. The unit freezes on start up and sometimes remains unresponsive. After cycling the power the same thing happens but random LED lights up.
Here is a link for a video showing what happens. Note that when pressing the one button other parameters changes and the on/off doesn’t turn off it goes from dim to bright.
https://www.youtube.com/shorts/whPIeDd0htk

I suspected that it was an improper reset pulse. IC U27(SP705) - I read the datasheet and on power up the reset pin should be low for 200ms, but I measure about 400ms of low pulse. Little bit of side note here - since I thought it was this IC which was faulty I removed it and wired in Arduino programmed for the 200ms low pulse when power up is detected. The waveform I measured was the Arduino output pin tries to go high after the 200ms but something in the unit tries to keep it low till the 400ms is elapsed.Its like a step wave. I resoldered the IC back.

So far I have checked the voltage pins at the controller MC68331 for ripple but looks clean. There is a clean sine wave at the crystal. One thing I have noticed is that the chip select doesn’t cycle through it remains static. After cycling the power the chip select might shift and remain frozen.

I was wondering if you guys could point to what to look for next and troubleshoot the issue.

Thanks in advance.