Yamaha PSS590 repair: bad IC ?

[ratatax]

Hi,

I'm trying to repair a PSS-590 keyboard, it worked well a few months ago but today it keeps creating random noises and random display when I power it on.

So opened it up then I reworked all the solders. Now it starts with the right display ("000" on the 7 digit displays) but no sound and the interface doesn't respond at all (pressing any key does nothing).
I also noticed two leds randomly flickers with varying intensity while they should be completely on or off.


Power supply is OK (+5V), the sound amplifier part too. The keyboard scans the button matrix I see the pulses. But I noticed that some 8-bit communication between two IC's YM7138 (sound chip) <=> XH748A0 (mask rom I think) is totally weird, see what I probed with my scope (attached pic)


It's the same on all the 8 data lines. Random levels instead of clean 0/5V pulses.

I'm not very experienced in repairs but does that mean the IC went bad (for no apparent reason)? I'm short of ideas, I probably could remove some caps to test them for high ESR (although they look perfect) but if you have more ideas to try let me know

[gnif]

Cihazı kapatarak single finger ve fingered tuşlarına aynı anda basılı tutarken  cihazı açın ve reset işlemini takip edin.

This is an English forum, please reply in english.

[sashko]

Hi,
I know it's been a while but... I guess I've found the issue.
I recently encountered the same symptoms with my Yamaha PSS-595.
None of the buttons and keys are working. Both displays show "0".
The power amplifier looks good.
All electrolytic capacitors that showed high ESR and rather low capacity were replaced.
After some poking around with oscillograph and multimeter I've come to a conclusion, that the System ROM chip (IC5) is a faulty one.
It wasn't hard to find a dedicated firmware dump for that EEPROM IC and the programmer for flashing it, but getting a replacement EEPROM chip is a tough one (no luck yet).

[aliarifat794]

Make sure there are no short circuits on the PCB, especially around the ICs you've mentioned. Also, check for continuity on all the traces connected to these ICs to ensure there are no breaks or damaged traces.

[xvr]

>  But I noticed that some 8-bit communication between two IC's YM7138 (sound chip) <=> XH748A0 (mask rom I think) is totally weird, see what I probed with my scope (attached pic)

Looks like ROM chip is out of order Just in case - try to check power pins with oscilloscope on both chips.