Yamaha SY99

Back in the 90s, I had a Yamaha SY77 digital synthesizer.  It had a combination of DX-7 frequency modulation (FM) synthesis and a sample playback synthesis engine, along with a powerful sequencer.  I recorded many sequences and played it in a variety of bands and venues over the years.  When I got my Roland Fantom-X7, I sold the SY77.  The Fantom is great, but the SY77 just had a great sound and texture that I have missed ever since.  In 2013, I came across a used but workable SY99, the SY77’s big brother, on ebay, and won it for a great deal.  The SY99 has all the features of the SY77, but with a bigger keyboard (76 keys), better effect processors, user sampling capability, and better sequencer storage.  Unfortunately, my unit also had several problems that I addressed as described below.

Power Supply Upgrade

My SY99 was brought to the US from Britain at some point because it had the European (British) style 220V power supply.  It’s a linear power supply design – a transformer powers bridge rectifiers and +5V and +/-12V regulators.  The total power consumption is rated at 35 watts.  To convert for 110V use in the US, a different transformer is required but it is no longer available from Yamaha.  Based on the specs and the dimensions of the original transformer, a 110VAC input 56VA size “7” triple output transformer (rated to drive a DC output of 5V and +/-12V) appears to be physically and electrically compatible what Yamaha used for US models.  I bought a Signal Transformer (now part of belfuse.com) MT-7-12 along with a new US power cord.  The transformer mounts in the same location as the original using the same screws.  In addition to the transformer change, jumper J1 is added to the power supply for proper grounding in the US spec model, and jumper J2 is replaced with a 3A fuse.  The fuses in the power supply are after the transformer and rectifiers, which didn’t seem like a good design, so I added another safety fuse inline with the AC source before the transformer. Problem sorted!

LCD Replacement

The display on my unit was dim and hard to read, a common problem due to the LCD backlight wearing out, so I replaced it with a “Cool Blue” display part number JHD24064C that I also bought on ebay. Because the new LCD has a slightly different contrast bias spec than the original, the adjustment resistors were changed. The schematic excerpts below show the contrast adjustment pot on the “JK” board and the R195/R196 resistors on the “DM1” board.  The new display has an LED backlight, so the inverter circuit on the “JK” board that powered the old EL (electroluminescent) backlight was disconnected. The new LED backlight is rated to connect directly to 5V and draw 120mA, but is too bright at those levels. Connecting the LED supply through a 22 ohm resistor reduced the current draw to 58mA, which is very close to the OEM EL inverter’s original current. The schematic excerpts below show where the inverter was disconnected and the new power leads were added.

SD Card Floppy Drive Emulator

The SY99 uses a 720KB floppy drive to store patches, samples, and sequences.  Yamaha used a belt-driven unit, and the belts eventually dry up and fall apart, rendering the drive useless.  Replacement belts can be found, but then you still have floppy disks to contend with.  I found a very cool floppy emulator kit to use in place of the floppy drive – the SDCard HxC Floppy Emulator.  This emulator uses a modern SD Card as the storage medium, one of which can hold the equivalent of hundreds of floppy disks.  The kit normally has the SD Card connector on the board but I wanted to mount it to my SY99 front panel, so I used a SD Card breakout board from SparkFun electronics.  The photos below show the floppy emulator and the 34-to-26 pin floppy drive adapter and the SD card breakout board adapter.  The SY99 floppy mounting tray was modified as shown to allow mounting of the emulator and adapters.  Also there are photos of the SD card breakout board and connecting cable attached to the front panel mounting blocks, and as mounted to my SY99 disk drive panel with the added front panel pushbuttons.  A few holes needed to be drilled for the emulator control switches and bit of the internal frame needed to be cut away to make everything fit.  The schematics below has my markups for the SD Card breakout board connections as well as some LED display modifications (more on that later). The SY99 floppy drive uses a non-standard 26 pin interface plus a 4 pin power connector, while the emulator uses the standard 34 pin floppy interface – the two can be interconnected as shown in the last photo below. 

Dual Internal Data Card Banks

The SY99 used a removable 64KB battery backed RAM card to store an extra set of patches. Used cards can sometimes be found on ebay for around $100 each. But SRAM is cheap, you can get twice that (128KB) for less than $5 in chip form. I designed the circuit below to emulate two 64KB cards, selectable via a pushbutton switch on the front panel. The HM628128 is the 128KB SRAM, and the DS1210 is a power supervisory circuit that uses a CR2032 coin cell to provide battery backup for the SRAM. The CR2032 will last for at least 4 or 5 years based on the standby current draw of the memory. An interface is also provided for the floppy emulator LED outputs to combine the signals to drive one dual-color LED instead of two separate LEDs. My circuit allows for the insertion of a standard memory card in the external SY99 slot, doing so will temporarily disable the internal data card banks while the external card is in place.  Below are a couple photos of the dual internal data card bank PC board mounted to the top of the floppy drive frame, and as installed in the keyboard. One of the selector switches seen on the front panel photo chooses the internal data card A or B bank. 

Sample RAM Expansion

The SY99 has slots to install up to five half-megabyte sample expansion RAM cards.  These modules are next to impossible to find nowadays.  Sector 101 makes a clone of these boards but they cost about $62 US each, or about $300 for a set of five.  As was said before, SRAM nowadays is cheap.  The Cypress CY62167 is an excellent 2MB SRAM chip available for about $16 US each.  The SY99 sample RAM is 16-bits wide, so two of these chips is just the ticket.  I designed the circuit below to use two CY62167’s and a DS1210 power supervisory circuit (same chip as was used on the internal data card bank circuit above) to provide backup power from a CR2032 coin cell to retain the memory contents.  The tricky part about this design is decoding the proper memory addresses for the expansion RAM from the signals available on CN7.  Fortunately, one 7402 quad NOR gate provides all the logic necessary.  Below are photos of the circuit board made from the schematic.  The board is a one sided PC board that I etched using the PNP-Blue toner transfer method.  The CY62127’s are surface mounted chips attached to the copper side of the board.  The pins on these chips are spaced a half millimeter apart! The circuit is designed to plug into the same connectors that the original SY99 expansion memory board used. The SY99 “CN” board provided the “donor” cables shown attached to my board. The new board fits onto the same mounting bosses that the “CN” board used to mount to.

Update: I have recently been collaborating with Joan over at wohmart on her new SY99 8MB RAM expansion board.  This board fills the sample memory all the way to address 007FFFFFH to give the maximum 8MB possible in the SY99 – check it out!

Panel Switch Replacement

As is usually a problem with equipment that’s 20 years old, the tact switches behind the panel buttons wear out.  I replaced all the switches on the panel PC boards with PTS645SL43-2LFS switches from Digi-Key.  Note: the original Yamaha parts list (part of the service manual) lists the OEM switches as part number SOA-111HS, which unfortunately are no longer available. These switches are 130g actuation force according to the datasheet, which also says they are good for 500,000 actuations.  The PTS645SL43-2LFS switches are also rated at 130g, but they are rated at 100,000 actuations.  I could not find any currently available switches spec’d for half a million actuations, but somehow I doubt the original switches were used that much anyway before they wore out.

LED Replacement

The SY99 uses fairly dim bicolor red and green LEDs for most of its functions. To update it to a 2010’s look, I replaced all the LEDs with new bicolor blue and green LEDs. My blue/green bicolor LEDs came from an ebay seller in China. The new LEDs are so much brighter in fact that I also had to change all the current limiting resistors. The red LED element resistors, which now power the blue LED elements, were all changed to 2700 ohm. The green LED element resistors were all changed to 5100 ohm. Below are pictures with the original red and green LEDs, and the new blue and green ones.

Battery Replacement

The SY99 has a couple batteries that are directly soldered in. I replaced them with battery holders so future replacements can be done without a soldering iron.

Update: This page was featured on Hackaday!

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