Yamaha MDF2 is an impressive little piece of gear. It doesn't do much, but it does what it's supposed to do, it's pretty robust and has not failed on me yet. The main problem with it (besides 720K DD floppy disks) is that it's impractical on stage - the 1x16 LCD display does not have backlight. Fortunately, backlit LCDs are very similar to the ones without backlight so an improvement here shouldn't be that difficult.
Opening up the MDF2: remove six screws on the bottom, three on the back, release three ribbon cable connectors with your fingernails - and the MDF2 pops open into two halves. On the left, right under the floppy disk drive sits the LCD display module with a 14-wire ribbon cable soldered to it. The module is fastened with four screws. Most of the 1x16 (and 2x16) character LCD displays are based on HD44780 controller or one of its numerous clones (mine has KS0066) and have a standard 14 or 16-pin connector, so almost any 1x16 backlit LCD display should serve as the replacement - HD44780 compatibility is the magic word here.
Although not required for this project, you can peel off the foil to reveal the system board. HD64180 is a Hitachi Z80 clone family, not very typical to a Yamaha product of that era that usually employ a Motorola 6800-based HD6303. XL280B0 is the firmware ROM, TC51832 32KB static RAM provides user memory and HD63266F Floppy Disk Controller, well, controls the floppy disk drive. Four cables connect the system board to the upper half to provide communications with LCD display, buttons, LEDs and the 720K floppy drive. Try to be gentle with them.
After removing the four screws LCD module comes out. Desolder the black ground wire. The next step is to desolder the 14-wire ribbon cable. A desoldering pump would help here. This is not easy but making a replacement cable without proper equipment would probably be even less easy. As a 'bonus' inconvenience, the original module has 2mm contact pads (and consequently the flat cable has 2mm pitch) while the contemporary LCD modules tend to have 0.1 inch contact pads. So the flat cable has to be 'sliced' for about 5mm in one end so that the wires would fit into the holes in the contact pads of the new LCD. Now, solder the 14 wires to the LCD.
My replacement LCD display came off eBay for 4.95 Euros and is a plain vanilla one with 14 contacts. LCD modules with backlight LED need two extra lines for LED supply voltage. In MDF2 the ribbon cable from system board does not provide those signals so they have to be added. Somewhere on the module (usually on one edge) are two spots labeled A (anode) and K (cathode). If the module has 14 contacts, solder a wire from pin 1 (GND) to K (black wire on the picture below). The LED voltage, however, is not the same as the LCD voltage (+5V) and is often around +4.2V (check your module datasheet if it is available) so it needs a current-limiting resistor. Assuming a (typical) LED current of 140mA a 6.8 ohm resistor (1/2W or bigger) is fine. If the specs of your module differ check an online calculator. Finally, solder the resistor to pin 2 (VDD) and connect it to A (red wire on the picture). Note! In case of a modern 16-pin LCD module the current-limiting resistor is sometimes already on board, in that case find out the purpose of pins 15 and 16 (one of them is +5V, the other one GND, but the order may vary) and connect them to pins 2 and 1 in proper order.
The bias voltage between VDD and VO is divided by a resistor network on the module and used for contrast control. The system board of MDF2 supplies 0.5V contrast (bias) supply at pin 3 (VO). This is fine for a typical LCD module with bias voltage VDD-VO around 4.0..4.5V but would probably not work with an LCD module designed for negative supply voltage so check the datasheet.
Most of the LCD modules are very similar in geometry but the ones with backlight LED are usually about 5mm thicker than the ones without. So the new module is a tight fit but not a mission impossible. Try to be kind and not break it by pressing too hard. The LCD display should coincide with the plastic window on the front without problems. Fasten the four screws using some imagination with the aluminium brace as it does not fit exactly the same way (I turned it over and bent slightly). Solder back the black grounding wire. Reconnect the ribbon cables and put the lid back. Fasten the rest of the screws.
You're done. If you did everything properly you should now own a rare and unique Yamaha MDF2 with backlit display perfectly usable on stage (drawback: it would probably be impractical with batteries as the backlight LED draws more power).
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Finally, some useful links: