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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:
  • It was a European-spec unit with a 220V power supply.
  • The display was dim, a common problem due to the LCD backlight wearing out.
  • The floppy drive did not work, another common problem due to the belt drive failing.
  • Additional patch memory is stored on data cards, which are next-to-impossible to find.
  • It did not have any of the optional sample expansion RAM (also rare to find).
  • Many of the panel switches did not work reliably any more.
  • Several of the red LEDs were dim after years of use.
  • The memory retention batteries were weak and needed to be replaced. 
Here's the story of how I addressed each of the issues:

Power Supply Upgrade

My SY99 was brought to the US from Europe at some point because it had the European-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 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

I replaced the old LCD 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 here 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 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 schematic of the floppy emulator is shown here.  The first page of the schematic has my markups for the SD Card breakout board connections as well as some LED display modifications (more on that later).


SD Card Floppy Drive Emulator

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.  Fortunately, the two can be interconnected as shown in the drawing.


SD Card Floppy Drive Emulator

The photos at left show the floppy emulator along with the 34-to-26 pin floppy drive adapter and the SD card breakout board adapter.

SD Card Floppy Drive Emulator

The SY99 floppy mounting tray was modified to allow mounting of the emulator and adapters. 


SD Card Floppy Drive Emulator

The tray with the boards inside mounted in place is shown here.


SD Card Floppy Drive Emulator

Here's the SD card breakout board and connecting cable attached to the front panel mounting blocks.

SD Card Floppy Drive Emulator

Here's the SD card breakout board and the floppy emulator LCD display mounted to my SY99 disk drive panel.

Below that are a couple inside views of the breakout board and panel mounted.  You can see the front panel pushbuttons in the bottom photo. 

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.

SD Card Floppy Drive Emulator

Here's what the panel looks like from the front of the SY99.
Dual Internal Data Card Banks

The SY99 used removable a 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 at left 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.

The overall interconnection schematic of the SY99, floppy emulator, and dual internal card data banks is shown here.
Dual Internal Data Card Banks

Here is the dual internal data card bank PC board mounted to the top of the floppy drive frame. 

Dual Internal Data Card Banks

Here is the complete unit 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 at left 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.
Sample RAM Expansion

Here's a photo of the circuit board made from the schematic.  This 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.

Below are the CY62127's soldered onto the board.  The pins on these chips are spaced a half millimeter apart!

Sample RAM Expansion

The circuit is designed to plug into the same connectors that the original SY99 expansion memory board used.  CN6 and CN7 are the same data bus and control bus connectors.  CN8 and CN9 are the power in and out.  The SY99 "CN" board provided the "donor" cables.

Here are the cables and all the parts attached to my board.

Sample RAM Expansion

The new board fits onto the same mounting bosses that the "CN" board used to mount to.  The sample memory slot unit is not needed any more and is removed.
Sample RAM Expansion

3MB sample RAM available!
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 PTS645SL43LFS switches from Digi-Key.
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.
LED Replacement

Here are pictures with the original red and green LEDs.

LED Replacement

Here are pictures with the new blue and green LEDs.



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.