Backlight Repair / Custom Led-strip

An oscilloscope is a indispensable piece of equipment in electronics; unfortunately it isn't very useful with a broken backlight.

In this instructable I will show you how I repaired my oscilloscope's backlight & how to make a custom led-strip.

An LCD screen consists of several parts: the LCD panel, the backlight and the diffuser (the layer behind the lcd panel that, you guessed it, diffuses the light). The backlight was a small TL light bulb, with an inverter (this converts the power for the bulb). I could have bought a replacement, but opted to make my own backlight instead. This also gave me the opportunity to experiment with SMD soldering.

Let's begin!

Step 1: Design Your Custom Led-strip

Since the led-strip needs specific dimensions, start by measuring the space where the TL bulb was located. Mine was 160 x 4 mm, and the maximum thickness was about 4 mm.

The other parameter is the voltage that goes to the inverter. My scope had the voltage and maximum current written near the connector: 8.4 V, 2.5 A respectively. Also verify the voltage with a multimeter.

Based on these measurements, we can choose the amount of leds we will put in series, and the total amount of leds. White leds need about 3 V each, so I chose to put 2 in series with a resistor. The resistor can then be calculated to have a current of about 15 mA, so I chose 150 Ohms: (8.4 V- 6 V)/15 mA = 150 Ohm. They are placed as seen in my very nice paint drawing (the black part is the resistor, the orange the led) :p

The amount of leds depends on the space available, the size of the leds, and the resistors you want to use. I went with 0406 size resistors, that's about the smallest size I would personally go with.

Step 2: Build the Led-strip

Before we can solder anything at all, we need to make the custom pcb. I started by cutting my pcb to size, and printed out the template. I used the toner transfer method to transfer the design and etched the board with ferric chloride. There are plenty of guides available here on instructables on how to do this.

Once our pcb is ready, it's time to solder the resistors and leds. This is tedious work, so take your time and be patient. Also, make sure to test the polarity of the leds before soldering. I used some blu-tack to hold the small pieces in place while soldering.

As you can see in the photo, the led-strip is very bright. Now that our led-strip is complete, we can install it in the oscilloscope.

Step 3: Install the Led-strip & Test

For the install, begin by disconnecting the power from the inverter and checking the polarity of its wires. Proceed by connecting these wires to your led-strip.

Place the led-strip in the display assembly and make sure it stays in place. I added some pieces of paper behind the strip to mount it snug against the diffuser. Route the cable through the holes and close up the display assembly. Connect all other cables you might have disconnected and give it a test!

If everything went fine, your display should light right up. Thanks to the high amount of leds, the screen is very evenly lit and plenty bright.