Nov 23, 2014

We had a party last weekend and one of the RCA cables went out on a turntable. The pop up light has also been broken forever. Since the deck must be disassembled anyway, why not give it some TLC?

This weekend, we will be replacing the old yellow neon pop up light with a white LED. Thanks to Viperfrank on Youtube for providing a base with his tutorial videos.

To convert the pop up light to a LED you will need the following items:

  • A 3mm LED of your favorite color
  • A 7508 voltage regulator
  • A resistor, 200-1.5k ohms, depending on how bright you want your LED to be
  • Black and red stranded wire
  • Small and large screwdrivers
  • A soldering iron, solder, and braid
  • Heatshrink tubing or electricians tape
  • A towel or pillow
  • Some perfboard or busboard
  • Helping hands (nice to have)

Here is the deck in question. It’s a Technics SL-1200M3D. As you can see the deck needs some love. The record on the platter is Chris Vareland – Universal Language EP.

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The very first step is to remove your cart and secure the tonearm, and carefully remove the platter. Heed the warning sticker’s suggestions. After removing the platter, carefully remove the plastic shield. Take care not to lose or misplace the screws.

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Removing the shield will expose the main PCB, motor, spindle, power supply, and other components. At this point you can use compressed air to remove any detritus.

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Flip the deck over onto the towel or pillow. Take extreme care to position the towel so that the weight of the deck is not resting on any components of the tonearm assembly. Remove the feet.

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There are 21 screws, all of various length. Make note of their positions and remove them. You can then sort of peel the rubber base away from the plastic case. The pop up light assembly is held in place by 2 screws.

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+5 and ground wires go directly to the neon bulb from the main PCB. The pop up light assembly is pretty neat. It is activated by means of a bolt that pushes a leaf switch. It works in a similar fashion to a clicky pen.

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The resistor measured to be 1.46k ohm. Its placement suggests it may be a pulldown resistor.

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The voltage regulator is necessary because the original neon lamp uses about 20v. 20v will immediately fry a typical LED. We could put a larger resistor inline with the LED, but I think this is a better solution because we can be sure we will always get clean 5v to the LED. The LEDs are typical super bright 3mm LEDs. You could also use 5mm LEDs of any color. They are rated for around 3-4v, so we must use a resistor. Take note of the tiny screw that attaches the top of the assembly to the base. Do not lose this screw.

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Remove the top of the assembly and clip any wires as close to the switch as possible, so we can preserve the length going to the connector. Pull the old neon light straight out. Save it for a future project if you would like. Take note of how this is disassembled these parts. You will need to reassemble it in exactly the same order.

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Desolder the resistor and use some solder braid to clean up the switch. I decided to save this resistor for reuse since when paired with the LED, the brightness was sufficient.

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Here is a rough schematic of the circuit. I had some scrap prototype PCB in my parts box, so I decided to make a small board for this project. I cut the piece with a Dremel. You can also simply solder the wire directly to the voltage regulator. As you can see, the resistor is inline with the LED, the LED is connected to the output voltage and ground on the regulator, and the main PCB is connected to the input voltage and ground on the voltage regulator. The wires are soldered directly to the LED, and I used some extra housing and electrician’s tape in lieu of shrink tubing to insulate the wires. You can also use shrink tubing, and it is probably a better option.

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The most difficult part of this project is snaking the LED wires through the pop up light housing. It is necessary to remove the small bolt so you can access the whole housing, and then use whatever technique you like to carefully thread the LED and wires, and reassemble the pop up light. You remember how you took it apart, right?

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Now that we have the whole thing assembled, let’s give the voltage regulator some voltage and see what happens.

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If it works, the next step is to include the switch. Clip one of the wires near the switch terminals, and solder so that the switch now interrupts the wire. Test again to make sure the switch works. If the LED is not centered, you can remove the tiny screw, rotate the pop up light housing until the LED points in the right direction, and then replace the tiny screw.

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Finally, replace the pop up light housing. Snake the cable assembly back near the main PCB connector as needed. My wires were a little too short, so I routed them underneath the main PCB. There is a little room available on a heatsink near the top of the main PCB where you can conveniently mount the voltage regulator.

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Put your deck back together.

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Looks pretty good in the dark. I may adjust the angle of the LED up a little more, but the color and brightness are spot on for my tastes.

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In the future, I may revisit this mod and replace the long wires since they are subject to some mechanical stress from the movement of the switch, adding some heatshrink and giving them some more slack. Adding connectors instead of directly soldering wires to the components might also be better in case the part needs to be replaced again someday.

Next time, we will replace the built in RCA cables with jacks, and possibly add an internal ground. If you know someone who can cut 14g sheet metal into discs, please send me an email at fred at fake computer music dot com.