Dead SY99 PKS (IC125)

[peter_kre]

Hi Forum Community,

i hope, that somebody can help me here - my SY99 is dead.

After debugging the device some time, i think that the PKS Chip IC125 on the DM1 board is the faulty part. None of it's pins - except clock and E - shows signs of live. Neither the keyboard nor the button matrix is polled and the display show only black dots. There is no communication between the main CPU and the PKS. But the main CPU is running, i can see heavy traffic on it's data and address busses.

The PKS chip is a mask programmed HD63C01Y. Yamaha Part number is XG950C00. I called Yamaha Germany Support and asked them, if they have some of these Chips in stock. But the situation is hopeless, no chip available and no idea, how and where to get one.

Is here somebody, who knews where such a chip is to sell ?

I think, that if somebody has a dump of the PKS chip firmware (16k rom) for download, i'm able to burn that image in an empty HD637B01Y (eprom version)

It would be awesome, if someboy has a hint how i can recover the wonderful synth.

With best regards from germany,

Peter

[shadowmask]

Hi Peter,

Small world - we were talking about this on the SY Facebook page last week. Any result from the guys at http://music-service.nl ?

Brian

[peter_kre]

Hi Brian,

yes, music-service.nl answered my question about a replacement for the XG950C00 chip. They told me, that this chip is not available anymore. But there is an official alternatve: XG950B00 That is the part number of the SY77 PKS. Sadly, the delivery time is 6 to 8 weeks and the chip is really expensive. So, i thought about a plan B. Last week, i bought a SY77 with some defects. (Floppy belt and one broken key - easy to fix) And i ordered some HD637B01Y in hong kong for a few euros.
I hope, that i can dismount the PKS out of the SY77 and dump the firmware to a file. If this works, i have the image to burn into the new HD637B01Y chips. I also ordered some shrink dip sockets for remount new PKS chips into both boards. If the SY77 survives that procedure, i'll sell it repaired And have hopefully won a usable PKS for my broken SY99.
In a few weeks, i'm able to talk about the results.

With best regards,

Peter

[peter_kre]

Hi,

to start with the positive message: the sy99 works again ! What a journey...

But the negative message is: it's PKS wasn't the faulty part. After the SY77 parts donator arrived, i opened it the same day. Surprisingly, his PKS is also a XG950C00 and not the expected XG950B00. Ok, i decided to desolder the PKSs in both Synths and exchanged them. Result: the SY77 booted up, the SY99 not - same symptom, only black dots in the display. I learned: the PKS of the SY99 is okay and there must be an other fault. So i did a little bit brainstorming, in which direction to go next.
I exchanged some bus drivers (ALS245 out, ABT245 types in) to eliminate dirty signals, timinig and bus errors. I exchanged the SRAM IC 119 to eliminate memoy errors. I examinated the complete board with a microscope camera square cm by square cm to find hair cuts and corrosion. The bug didn't show up and i became a little bit frustrated.
I started to disassemble the processor and mainboard rom to understand, where the cpu loops. I could see with my oscilloscope, that the cpu loops somewhere. The idea was, to record the address and databus bus signals with a little logic analyser. What adresses appear at the cpu address bus, what data is on the data bus and where in code the cpu loop occures? I hoped, If i understand that scenario with the help of some disassembled rom content, i have the bug.
During the analyzer recording session, i noticed that the D7 timing at the CPU side of the IC122 looks a little bit different than D0-D6. D7 showed longer low and high levels and fewer level changes than all the other bus lines. In that moment, i thought: ok bug, i see you After that, all things went fast. The D7 pin of the cpu had not sufficient contact to the contact spring of the new PLCC cpu socket. I pulled the cpu out, removed a little bit oxide from the pin and corrected the pin position a litted bit.

YYYEESSS - this evening, i went out of my basement very satisfied !

One day later, i did the blue display mod and some finishing. And here he is in his full glamor:

Thank you very much for your support.

With best regards,

Peter

[Miks]

Hi Peter,

wow - what an odyssey to track down the culprit! Congrats! Good to see that another SY is 'back to life' and up and running. And looking good with the new 'Cool Blue' !

[Derek]

Well done. Well worth the effort to resurrect this amazing synth, and it shows that the good thing with the older boards that electronic skills are still valuable.

[JK1974]

Hi Peter,

congratulation and thank you for your longer explanation. This might be helpful for all other SY77/SY99 users that once might suffer from a similar problem.

[ledan]

Awesome Peter. That story was a real "nail-biter"