Yamaha PSS-680 Keyboard Teardown

I recently took apart my Yamaha PSS-680 keyboard and had a look inside and took some photos. The reason for this project was that this keyboard was in need of refurbishment. Technically speaking, a cat had sprayed on it and it was in absolutely filthy state, but it still worked fine. I took it apart and did a very thorough cleaning and took some pictures inside.

Here is the keyboard, on top of my rather cluttered desk

Aside from the cleaning, the first thing I did was open up the case to see what damage might have been done to the PCBs. Fortunately they all seemed to be in excellent shape. There is a single pair of wires going to the bottom half of the case for the battery compartment (6 D-cell batteries, very old school) which you must be careful with when taking the unit apart.

Inside the keyboard, keys are towards the top of the image

One of the first things I noticed is what a massive ground plane this main board has. The keys themselves are attached to a piece of sheet metal. Since they went to the effort of attaching this metal to the ground in several places, I'm betting that it serves to provide RF shielding from the user. You wouldn't want any stray RF coupling into the sound mixing circuitry. The main keys and drum pads have individual ribbon cables connecting them to the main board and the drum pads are separated into two sections. If you happen to have a keyboard where one of these sections happens to be malfunctioning, this could explain why.

Isolation Diodes

Each key has its own individual diode. These isolation diodes are used to isolate the keys from each other to prevent rollover so that you can, if you want, play as many keys at once as you want. From what I recall, very old computer keyboards also required isolation diodes to prevent static electricity from getting back into the computer itself.

Sound processing board?

I wasn't able to remove the board shown above, but we can guess at its function based on the connections going to and from it. The power input goes directly to this board, both from the battery bank and from the DC adapter input so we can assume there are probably power regulators and filters on this board. The main speakers, headphone and line out jacks also connect to this board, so there is a good chance that there are audio amplifiers on this board as well. Aside from that, I'm not really sure. Are the actual FM synthesizers and mixers on this board?

From left to right: 8830PD XE405B0-070, Sanyo 8K4 LB1214 and another Sanyo 8K4 LB1214

Shown above are three ICs near and slightly tucked under the sub-board shown earlier. As for the first IC (let's call it XE405 for short) I can't find any information. Any help on that would be appreciated. The dual Sanyo ICs are general purpose transistor arrays (exciting, I know) with 7 channels of common-emitter arranged transistors including some internal resistors each. There are two more transistors in this area, so I guess they needed a total of 16 transistors here.

From left to right: PC900, 8836PD XE416D0-093 and a Toshiba TC5565APL-12

<The TC5565APL-12 is a 8k Word RAM. The PC900V, interestingly, is a photocoupler with 710 Volt isolation. It actually has a powered amplifier with hysteresis on the output side.

Note: this post was previously unpublished. I have decided to publish it as "good enough".