Akai S1100 DIY LED backlight

Quick summary

Akai S1100 EL backlight and noisy inverter can be replaced for cheap with iPhone 6 Plus backlight, a boost converter module and some cabling and connectors. This blog post is a walk-through of the process.

The problem this time

A while ago I got an Akai S1100 sampler. The most important repair was to change the broken disk drive to a USB disk emulator, as discussed in the previous blog post. This made the unit usable, but quite soon another issue surfaced.

Typical for the era, the LCD display uses electroluminescent (EL) backlight. This requires an inverter that produces the high voltage for the backlight. However, due to the aged components, the backlight inverter was producing a high-pitch noise, especially when the unit had warmed up. This is a typical issue with these inverters. Noise like this is naturally unacceptable for audio equipment.

The first aid fix was to put some rubber padding between the screws that attach the inverter to body of the sampler. This reduced the noise to some extent as it was not conducted to the body of the sampler anymore, but a better fix was still needed.

Options for the fix

The backlight system has a thin EL element that is relatively easy to replace. It just slides in behind the LCD panel. These elements tend to fade over time, and need to be replaced every few years. New elements are available at around 20 euros each. The EL element was quite dim already, so a replacement for that would have been necessary soon anyway.

Replacing just the EL element could have reduced the noise a bit, but the main source was still the inverter. A replacement inverter would have been around 50 euros, but there’s no guarantee about the noise. When these inverters occasionally pop up on eBay, they could be used ones that end up there just because they have become noisy. So, for about 70 euros – or actually closer to 100 euros if shipping is included – one could replace the backlight system with original parts and only hope for the noise to get fixed.

There are also new LED-lit LCD displays available. This is a whole new LCD module with integrated backlight. These cost above 100 euros + shipping. This would have been just too expensive for my budget, considering that I didn’t pay much more than that for the whole sampler.

So the next thing was to come up with a DIY solution.

Possible DIY approaches

There are various DIY backlighting projects for LCD displays online. Typically these use LEDs and some diffuser in front of them. The problem with this approach is that the space behind the LCD panel is really thin, just a millimeter or two. No way to fit a PCB with LEDs there.

Also a company called Lumitex offers a cut-to-size backlighting kit that could have been useful. They didn’t answer my inquiry though, so I don’t know about their pricing. But as they don’t list the prices on the web page, it’s probably an indicator that those are too high for an occasional hobbyist buyer…

Thin LED backlights are however almost universally available. People carry these around in their pockets all the time. It’s a common component in mobile phones.

The backlights of mobile phones are surprisingly simple. Usually there’s some LED illumination on just one edge, and various optical films distribute the light from the LEDs evenly over the whole surface of the backlight. This means that it’s relatively easy to modify these backlights. As long as the LEDs and their electrical connections are not destroyed, everything should work fine even if the films are partially cut off.

Below is an image of films used in an iPhone backlight. At the back, there’s fully reflective mirror film. Above that there’s a thicker clear plastic sheet. The LEDs are illuminating this sheet from one edge. Above this is a white diffuser film, and above it is a semi-reflective film. This film lets some of the light pass through, and reflects rest of the light back to the layers below. Last layer is yet another film that seems to act mostly as a diffuser.

Finding a suitable phone backlight for the project required some research. The first thing was the size. The original EL panel is relatively long, about 130 mm. Most phones don’t have displays this large. The brightness of the backlight panel should also be adjustable by voltage, not by PWM, as that would simplify the electronics required. I believe usually phones do it this way, as PWM could produce flickering that’s tiresome for the eyes. Preferably the voltage should be below 12 V as well, as it would be possible to get this easily from the sampler. There are unused connectors with 12 V power, e.g. for the optional hard disk.

After some searching on the internet, the backlight panel for iPhone 6 Plus seemed to be a good candidate. Most backlights have just a single LED array, but iPhone 6 Plus backlight has two; one illuminates about 2/5 of the surface and the other illuminates about 3/5. The smaller portion requires around 10-12V depending on the desired brightness and the larger portion 15-18V (although full 12V / 18V is definitely too much, it will burn out the LEDs quickly). Provided that there are no additional wires circling around the edge of the backlight, it should be possible to cut a strip of the backlight and remove the other half.

Best of all: iPhone 6 Plus backlight is available from the usual Chinese sources for around 3 euros per piece. So this is definitely the cheapest option for fixing the backlight issue.

In the best case, this would have been the biggest expense, with just some additional cabling and connectors to attach to the 12V power. However, after I received the backlight and tested it, it was obvious that the side with 12V power was not large enough to cover the whole LCD area, and I had to use 18V power and the larger half. Since there is no 18V power available in the sampler, a DC boost converter was required. There’s a popular design with adjustable output voltage of 5V-35V. These can be purchased for as low as 2 euros per piece, but I didn’t want to wait for the delivery from China, and got one from a local electronics store for around 8 euros.

Rest of the hardware was just some cabling and connectors, and some screws and raisers to attach the boost converter module in place of the original inverter.

Disassembly

For the replacement operation, the sampler had to be opened. This was pretty simple; there are two screws on each side (and possibly one on the back, in my sampler it’s not there anymore). Opening these screws allowed the top cover to be removed.

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Always make sure the mains cord is unplugged when opening the device, and if you need to operate the device while it’s open for testing something, unplug it after the testing is done.

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After the removal of the top cover, the front panel had to be opened. There are three screws in the bottom of the sampler and five screws on the top. Also volume knobs should be pulled off, and connectors for the microphone inputs released. As I was going to remove and add cables, I also snipped off the zip tie holding together the various cables at this point. This is not necessary if you are not modifying the cabling, but it allows the front panel to open further and makes it easier to access the LCD module.

The LCD panel is attached with four screws to the body of the sampler behind the front cover. First the cable at the back of the LCD panel had to be removed, and then the four screws. When the LCD panel had been detached, it was possible to slide out the old EL strip.

The inverter is attached to the sampler with two screws, between the left side of the body and the disk drive. These can be accessed from the outside. The power cable was released from the inverter, but the cable to the EL backlight could stay attached, and I just slid the old backlight through an opening in the front part of the body of the sampler.

Building the replacement parts

The first step was to modify the iPhone backlight. This was also the scariest part, as I didn’t know whether the backlight would still work after the modification. So I tested the backlight continuously during the process.

The connector of the backlight has six contacts. The top 2 and bottom 2 contacts in the image are ground, and in the middle are contacts for +12V (upper) and +18V power (lower). The upper three contacts are not used here, as that portion of the LED strip will be cut off.

After a practice cut in the area that was to be removed, I cut out a section of the backlight so that the remaining part was 42 mm wide (the same width as the original EL panel). Cutting out the film layers didn’t affect the light distribution of the remaining part at all. I also cut off the unnecessary LEDs just above the connector, and was relieved to see that the remaining part of the light still worked after this. I closed the edges of the film layers with a strip of tape to prevent any dirt from getting into the backlight. The layers are separate from each other, there are no adhesives keeping them together, so they need to be somehow attached after a cut is made. Originally there’s a thin plastic rim around the whole backlight and some tape attaches the top and bottom films to this rim.

The next step was to attach the power cable. There has to be a connector on this cable, because the cable needs to be routed through rather tight spaces in the sampler. I used a ready-made battery extension cable with JST connectors, but basically any small connectors that fit together can be used. The hardest part was soldering the power cable to the backlight. The ground wire was easier, as that gets connected to two of the lowest contacts. The +18V wire needs to connect to a single contact. This requires a small tip for the soldering iron and a steady hand.

After soldering the connector, I applied some hot glue over the solder joints in order to prevent the ribbon from twisting and accidentally shorting the connection. Over this I applied some heat shrink tube. This was special tube covered with hot glue from the inside, the original purpose is to make watertight joints, but here I could use it for protecting a flat connector. I heated it only moderately, and when it had shrunk enough, I pressed it flat over the connection. Because of the hot glue inside, it stayed in the flattened form after cooling down.

As I was using a step-up converter, there was no need to tap into some 12V power source inside the sampler, but I was able to utilize the same 5V power connection that was powering the original inverter. This used a connector with 0.1″ pitch, and first I was going to just use a regular header with two pins for the connection. However, in order to make sure the connector doesn’t get attached the wrong way around, I decided I needed something better than this. Since I didn’t have exact matching connectors for the ones used in the sampler, I just modified a 2-pin Molex KK 254 header to fit the existing power connector. As the shape of the connector is asymmetric, it fits only one way. I soldered the connector to the end of a cable and protected the joints with heat shrink tubing.

This was soldered to the input side of the DC boost converter. The power cable for the backlight was soldered to the output side.

Reassembly

Before attaching the electronics together, I tested every individual piece separately. For the back light this meant testing with 16 volts from the bench top power supply. The DC boost converter was tested with 5V from the power supply, and I adjusted the voltage so that the output was set to 16V. This step is critical – just blindly connecting to the boost converter will most probably result in overvoltage that will destroy the backlight.

After this I connected the backlight to the boost converter, still powered from the separate power supply. Finally the boost converter was connected to 5V power from the sampler, and the sampler was powered on. No surprises here, everything worked.

After the test, the components were disconnected and each attached separately to the sampler. The backlight was slid into the LCD panel. Next the LCD panel was screwed in place. The best way to do this is to first attach the two screws on the right, then slide the LCD panel in place and make sure the top screw goes behind the backlight, attach the screws on the left side to secure the LCD in place, and finally tighten everything. Remember to connect also the LCD cable.

In order for the boost converter to fit in place of the original inverter, I cut a piece of scrap PCB board. The positions for the holes in the inverter and in the boost converter were marked there and drilled with a 3 mm drill bit. After this, the boost converter was attached to the PCB with some raisers and screws, and the raisers of the original inverter board were attached to the other side of the scrap PCB piece.

At this point I did some final adjustments for the backlight brightness. Once the DC boost converter has been attached to the sampler, it’s not possible to access the screw that controls the output voltage, so this has to be done first. I ended up to use about 16.5 V. This gives brightness level that’s quite bright but still comfortable for the eyes.

There’s still room to adjust the backlight to be brighter. Normally I don’t think it’s necessary to have it much brighter than this, but it’s possible to change the color of the display by installing a colored film over the backlight. This might require higher level of brightness, but I was OK with the original blueish color.

The cable for the backlight needs to take a slightly different route than the original EL backlight cable. There is a tiny part of flexible ribbon cable between the connector and the backlight. In order to avoid stressing this part, the cable was attached to a post in the front part of the sampler body with a zip tie. The cable has to be routed carefully through the opening in order to avoid damaging the weak spot!

When the LCD was in place, the front cover was closed and the five screws on the top were installed back in place. At this point it’s important to remember to reattach the cables of the microphone connectors also.

Next the boost converter was attached with the original screws in the same place where the inverter used to be, and the power cables were connected to it. As all cables were now back in place, they were tied together with a zip tie like they used to be. Then the top cover was closed, three screws at the bottom of the front panel were put back in place, and the volume knobs pushed in their shafts.

Conclusions

A quick test drive revealed no problems. The backlight was bright and even, and there was no disturbing noise anymore. The added electronics didn’t cause any interference with the audio. This was not only a repair but an upgrade!

Below are before / after pictures showing the difference in brightness. I had to do some post-processing and adjust the overall brightness of the images to match each other. The new backlight is so bright that it’s difficult to get a decent photo without underexposing everything else than the screen.

I prefer to do modifications to the vintage equipment so that they are fully reversible, and this is such upgrade as well. Apart from a zip tie that had to be cut, no original parts were damaged. It’s still possible to remove the upgrade and put the old inverter and EL strip back (and suffer from the high pitch noise).

All parts for this upgrade could be obtained for as little as 6-7 euros. I did spent slightly more as I didn’t want to wait for the delivery, and used a ready-made cable instead of buying separate connectors.

This repair is not limited to just this specific sampler model, but any equipment that uses EL film for LCD backlight. Cutting a mobile phone backlight to required shape is easy, and as long as the LED strip is not damaged, wide variety of display sizes can be targeted. I would suppose some tablet backlight could be used for even larger displays than this.

There could be opportunities for utilising these backlights for wearable electronics or cosplay as well. The backlight is slightly flexible, but the thickest film will break if bent too much, so some reinforcement may be needed in those use cases.

(After I had already finished the project, I did see an LED-based backlight replacement kit for Akai S1100 on eBay. It seems that these are occasionally available. The cost was around 30 euros, and with 15 euros for shipping it’s still more expensive than DIY.)