Upgrading to LED-backlit LCD

The goal of this little project was to upgrade the LCD on a Roland D-550 Linear Synthesizer with a recent, LED-backlit model. The D-550 comes with a HD44780-controlled, EL-backlit LCD. EL has a relatively "short" lifespan, and will usually die out after a dozen years or so, which is what happened on this unit. Replacing with a modern LCD was not entirely straightforward, and since I couldn't find a HOWTO on the internet, I decided to publish my method, which is probably as good as any. Enjoy! It's all documented after the usual:

Disclaimer

The information and methods described herein are provided "AS-IS" WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED OR IMPLIED. Use the concepts, examples and information at your own risk. There may be errors and inaccuracies, that could be damaging to your devices. Proceed with caution, and although it is highly unlikely that accidents will happen because of following advice or procedures described in this document, the author does not take any responsibility for any damage claimed to be caused by doing so.

Tools required

You'll need the following tools to complete this project:

Procedure

All operations must be conducted on an unplugged unit. First, open up the D-550:

Next up, you'll want to remove the front bezel:

LCD removal

Here comes the removal of the old LCD:

Once this is done, you've freed the old LCD.

Preparing the new LCD

First, the old LCD data connection from CN11 must be removed and transferred to the new LCD. You have two options:

Note: If you choose the second option, you must cut the wires off as close as possible to the LCD: there isn't much slack, and you'll want to preserve the plastic soft holder that binds them together. Either way, make absolutely sure not to lose track of which wire goes where. The two black wires mark pin 1 and 2 respectively, but you must not swap the odd and even sides. Finally, if the new LCD has a 16-pin header, don't fret: insert the old cable starting from pin 1 up to pin 14, as if the last two pins didn't exist. They are left unused (they provide another way to power the backlight, we won't use them).

Next, we have to prepare the backlight power feed:

You're done, you can now get the LCD out of the way, the next steps will happen on the unit's front panel.

Preparing the front panel

The LED-backlit LCD is thicker than the original, EL-backlit one (typically 4mm thicker). To make room, we're going to replace the original 10mm standoffs with 5mm ones. But then, there's another issue: the LCD's PCB overlaps with the panel board. Here's what needs to be done to get it all together:

These two screws provided support for the switch matrix to prevent it from bending too much when the switches are pressed. We need to provide alternative support points. In order to do that, we need to move the two 5mm spacers further down. Ideally, they should fall right between the two rows of switch, that way they won't be facing any pin on the PCB.

Do some measuring and figure out the best spot, then drill two 3mm holes in the chassis. The spacers were previously screwed into the chassis. Unless you have the right tool to do that (a tap), this is no longer an option. Instead, use two M3 nuts to fasten them to the chassis. Make sure to keep the locking washers on the front side so that the spacers are exactly the same height as the ones left in place. See:

spacers

On the back of the switch matrix PCB, on the spots facing the spacers, stick a bit of insulating tape. Put the panel board back in place. You'll have two spare screws that we won't use anymore.

Next, place the four new 5mm M3 spacers on the spots previously occupied by the 10mm ones. Note: one of the screws that held the 10mm spacers is longer to accomodate for the ferrite bead plastic support. You'll get this:

lcdsupport

VoilĂ , the front panel is ready to accomodate the new LCD!

Connecting the new LCD

We're almost done, we must now fasten and connect the new LCD.

At this point, you can test the new LCD by turning on the D-550, the synth should boot and the LCD should display the usual information, only without any backlight.

We must now deal with powering the LED backlight. The old LCD uses EL blacklighting, which requires high voltage provided by the C2 connector we removed initially. We obviously can't connect the new LCD there. Instead, we must find a place to tap into the 5V output of the power board. There are two good places to do just that, and a lot of bad ones.

The first good place is to solder the white wire onto the "+5V" wire jumper on the power board. But, if you used the old black/white cable as I suggested earlier, you'll notice it's too short to reach there. No big deal, here's the second good place: the "+" lead of capacitor C50 (a fairly noticeable electrolytic cap in the bottom right corner of the main board when the D-550 front panel is facing you). The + lead is the one closest to the CN8 connector (also the opposite side of the "-" sign on the cap's body).

The reason other places are not good choices is because C50 is located right at the beginning of a filter chain of caps and coils that's used to smooth the voltage provided by the power board before it's used to feed important parts of the circuit. The D-550 service manual clearly shows this in the following diagram excerpt:
circuitdiagram
Tapping anywhere else in the +5V flow would entirely defeat the purpose of that filter. LED backlighting clearly doesn't require super smooth voltage. That's also why I didn't add jumpers to pins 15 and 16 on the LCD header, these would tap in the filtered +5V supply.

The ground plane can also be found nearby: either by soldering from the "-" lead of the same capacitor, or, if you want to be extra cautious and avoid potential short circuits, it can also be found on a small jumper wire further down the board. See the following picture illustrating the above:

backlightpower

Note: I highly recommend you remove the nearby connectors before bringing your soldering iron in this cramped spot, otherwise you risk melting some of those wires and that would be a Very Bad Thing™ ;-P

The piece of heatshrink tubing that can be seen in the right side of the picture is actually there to protect the current-limiting resistor I had to add, as the LCD I had in stock draws too much current for backlighting. If you face the same situation, the symptom is simple: the D-550 won't boot, nothing will be displayed on the LCD (though the backlight may actually be working). What happens is, the +5V supplied by a L78MR05 voltage regulator (which has a maximum rated output of 500mA) falls below 4.5V as the regulator goes into over-current protection mode, at which point the "brains" of the machine can't operate anymore.
Note: the contrast adjustement should not need any modification, it'll work out of the box.

So there you have it: all is left is to reassemble everything (reverse the initial disassembly steps), plug the D-550 back, turn it on, and enjoy the result:

workingresult