Earlier this year I did some work on my Kawai K5, including (ofcourse) replacement of the LCD display with a new white-on-black(ish) LED backlight type. Additionally updated the firmware ROM to the latest version, and I cleaned all the tactile buttons which became almost non-responsive.
Display
The old display had an EL-foil backlight which was barely emitting light, just like all synths with the 240×64 dot matrix display from that era.
I bought a couple of 240×64 displays white on black with LED backlight (from the polish company artronic, but similar to the Newhaven’s white on blue which seem to be popular) which are pin-compatible with the original display.
The flat cable is to be desoldered from the old display, and soldered onto the new display. 
One addition needs to be made: Font Select pin needs to be pulled up to VDD, this is done using a small wire soldered directly on the display board between pin’s 19 and 3. On the PSU board the inverter can be desolderded, and replaced by a 100-Ohm resistor (between pins IN and OUT). See schematic below:
The contrast-control gives a large enough range for the new display, so no modifications in resistor values needed here! See below the result:
Buttons
Now the synth is opened up, we can take a look at the tactile buttons. You need to press pretty hard to get some response. To clean them, you need to take them apart.
With long pliers you gently pull the inner part out of the switch.
Inside the casing you will find small copper discs. These need to be taken out.
By soaking a q-tip in alcohol, and pushing it in the casing, you can pull the disc out.
Placing them a piece of foam, you can thoroughly clean the ‘hollow’ side of the dish. Also you need to thoroughly clean the small metal contacts inside of the plastic casing.
This way I’ve cleaned all buttons, and the results are remarkable! Buttons all became very responsive, as new! (Presumably, as I have never owned a brand new K5 🙂 )
Firmware upgrade
As I do with all my synths which I open up, I check if they are running the latest (available) firmware, and if not.. I replace the ROM with a freshly burned EEPROM. I purchased a couple of cheap 27SF512 DIP-28 package EEPROM’s of ebay, which are roughly pin-compatible with the original 27C512, and.. most importantly: programmable using an EEPROM programmer 🙂
The latest 1.3 firmware can be found on the K5synth file section on yahoo groups.
TODO: Output modification
The last upgrade/fix I was planning to perform was an output modification to lift the output level further above noise-floor, an issue which is inherent to the K5.
The mod consists of replacing R33, R34, R35, R36, R37, R38, R39, R40 by 15kOhm-resistors.
This is an updated mod, as the original mod seemed to filter the high end frequencies on the output single, making it sound less ‘brilliant’. The mod is described in this groups.yahoo.com post and more thoroughly in this (German) thread: http://www.sequencer.de/synthesizer/viewtopic.php?f=3&t=108837
This new method obsoletes the old modification described here: http://herbert-janssen.de/k5.html
However I could not find any reports of this modification apart from the author itself.
As I found the low s/n ratio not particulary disturbing i decided to hold the modification off for now.
Hey Synth man, I’ve just done this upgrade, didn’t use the resistor just took the inverter out and bridged the in to the out, works fine.
I bought the screen with the header presoldered and a new ribon cable, we just cut off pins 20/1 from the header and did the patch (19 – 3) to set the font size,.
The issue I’ve got now (I don’t think it’s related to the screen but it’s probably something to do with the fact that the I had it open) is that it thinks the ‘protect’ switch is on when it’s not (so I can’t write patches from my librarian) has anyone else come across this? (and more importantly does anyone know a fix?)
Synth man: Probably too late for you. You put the resistor from the top left hole to the bottom right hole (as you’re looking at the PSU with with LCD output on the right).