FISHERS (project pt.1)

 1989 FISHER PRICE 3810 "REAL-SOUNDS PIANO" 

The Fisher Price 3810 Piano was a common toy during the early 90's. They were well made, and are still available in good condition on ebay, and sometimes can be found at thrift stores. The keyboard measures around 23 inches long, 8 inches wide, and about 2 inches thick. There are 32 keys that are slightly larger than standard mini-keys of the era. Despite the sturdy build quality and uniquely 90's design, the sound quality and functionality of the toy is curiously limited...

On the right hand side of the keys are located the volume control wheel, and five push-buttons to turn the 'Power' on/off, play the 'Demo' songs('Saints Go Marching' & 'Around the Mountain'), 'Record' a melody(40 notes), 'Correct' notes in record mode, and a 'Playback' button to play the recorded melody one step at a time. 

There is only one sound available. It is an 8-bit sampled piano that is actually two very different samples split at C#4 and D4. The piano voice is heavily filtered to alleviate the presence of aliasing noise from low-cost components and circuitry. The low quality sound gives the keyboard a dark and endearing tone that sparks the imagination–despite its flaws–and sets itself apart from other hardware-heavy toy instruments of the time like Casio's, Yamaha's, Vtech's, and even GPX's to name a few. 

The Fisher Price 3810 has 4-note polyphony which is pretty useful, and more sophisticated than other low-cost electronic-toy-organs available at the time, which tended to have a single square wave voice that could be recorded and play back. The playback function of the 3810 is unique in that the recorded melody loops with no breaks, so it is possible to advance the sequence continuously with the playback button, or with the use of an external clock. It is also possible to play notes from the keys over the sequence while it is playing, although it is a little choppy sounding. 

The keyboard also has an auto-shutoff feature with a warning beep prior to shutting off. It can be powered by by four 'C' batteries, or a standard 9 volt AC adapter(tip negative).

-REVERSE ENGINEERING:

Opening the enclosure was pretty simple. There are 10 screws on the back that release the top and bottom pieces from each other. Inside, there are two separate circuit boards. One is fore the key switches, and volume pot, and the other is the main-board that holds all of the voicing circuitry. I spent a few hours reverse engineering the whole circuit into a clear schematic to get a better idea of how everything works. The number of through-hole chips in this keyboard seemed very promising for circuit-bending potential. Unfortunately and surprisingly, there were really not many(if any) useful bends...

The main processor chip SC80C31 scans for key-presses, and writes to an 8-bit latch(74HC374) that outputs 8 multi-pulse signals to an R/2R resistor network used as a low-cost DAC. The SC80C31 also writes to another latch chip(74HC373) which sends signals to the proprietary ROM chip(AMI8929) that then sends back 8 bits to the SC80C31 chip. I suspect this is where the piano sounds and program memory are stored. Both 74HC374 and 74HC373 latch chips have their own clock and latch-enable signals coming from the SC80C31 chip, respectively. The summed signal coming from the R/2R resistor network is sent to a buffer/filter stage before being sent to the volume control pot, and then to the TDA2822 amplifier chip that drives the 4 ohm speaker. 

The power supply circuitry is interesting. The keyboard turns on or off when the power button is pressed successively. If no keys are pressed while the power is on for 3 minutes, the keyboard will beep for another 3 minutes, then send a low signal from pin 17 of the SC80C31 chip to a comparator circuit on the LM324(U5D) that will then send a short pulse to the reset pin of the SC80C31 chip(pin 9), which will also cut power to the LM2931 5 volt regulator via the 2N6726 PNP transistor.  Pressing the power button again will toggle the reset pin again, and pin 17 will return to a high-state, allowing power to flow to the regulator again. It is good to know how the auto-shutoff works so that it can be disabled. 

The keyboard matrix is pretty standard. There are 8 outputs, and 5 common inputs to gate 32 piano notes(F2~C5), and the 'Demo', 'Record', 'Correct', and 'Playback' functions. There are four additional key/bus combinations(K7, K6, K5, and K4 to 'E') that have no function. The SC80C31 chip operates at 12Mhz that comes from a ceramic resonator. 

Part 2 coming soon...