Chapter tree: A disk size sensor goose chase

Now that the tonearm is able to move, let's try to play a 17cm single to see if the automatic size-detection works.

// The original Blog-Post has a video here. Here's the alt-text instead: A video of the tonearm moving towards the platter and lowering at the 30cm position despite the 17cm disk inside the device. //

I guess that's why it felt like it was too easy.

The first thing I tried was to clean the sensor board under the platter, which obviously did nothing. So I went ahead and measured the test points on the main system control board just to find out that the PH101 photo transistors and all the supporting electronics work just fine.

A screenshot of the schematics from the service manual. The two photo transistors that detect the disk size connect trough a couple of resistors and transistors directly to the chip than controls everything.

A picture of me measuring some solder-points on a PCB inside of the machine. The Ground-connection of the multimeter is connected to another PCB using a crocodile clamp.

At this point I feared the worst: The LM6405A that controls everything is fried. This microcontroller has 8 Kbit of ROM on board which I don't have a dump for. If that's the case, this project would be over.

After googling a bit, I found out that a common failure point on record players with automatic size detection is a burnt out lamp. I used the flashlight of my phone to test if the sensors are working but for some reason it didn't cross my mind that there needs to be a light source somewhere inside the case as well.

And here it is, hidden in plain sight:

A picture of the record player with the top cover removed and the tray ejected. At the far back is a plastic-piece which is highlighted in red.

A screenshot from the service manual, showing how the plastic piece is mounted from the other side using two screws.

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So I removed the bulb holder from the other side by removing the two brass screws holding it in place. A quick glance at it under the microscope confirmed it: the bulb just burned out which makes the sensors "blind".

Replacement bulbs are still being sold, but shipping takes time and I want to fix it NOW ?. So I opted for the second option: replace the bulb with an LED I already have lying around. Luckily the original light source was mounted in the case with a small plastic holder, so I just had to design and 3D Print a new holder that would fit one of the 5mm LEDs I had on hand.

After a few iterations I came up with a 3D model that holds the LED I wanted to use. I just needed to file the same angle into it that the plastic light guide on the original part had. I'm not sure if this was necessary from a technical perspective, but it made it look more stock.

A photo of the screen of my digital microscope. On it is a zoomed-in view of a burnt out miniature light bulb.

A photo of two small, gray and rectangular plastic pieces. One has cables sticking out from the bottom.

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In parallel to that I looked trough the service manual for the voltage the bulb was running at. It's -11.4V. Why? Ask Sony. Having roughly 12V available was actually pretty handy because I already had some pre-soldered 5mm LEDs meant for 12V sitting around from a previous project. So I could just use one of those. I just needed to make sure to connect it the wrong way around because of the negative voltage.

A screenshot from the service manual showing the path electricity takes trough a transistor and a fused resistor to the bulb.

A photo of a multimeter displaying -11.75V. The two alligator clips connected to it clip to two white wires inside the record player.

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Since both of the conductors leading to the bulb where white, I had to use my multimeter to detect the correct polarity. For a classical light bulb that doesn't matter, but for an LED it does. After that I marked GND black and connected the new LED by twisting together the wires to check If it works like that and is bright enough.

I was OK with the brightness so I decided to shorten the wires to fit, soldered them together and used some heat shrink tubing to insulate them. Then I screwed the holder with the LED inserted on and secured tha cables with some tape. I probably should've tested the functionality of the sensor with that LED beforehand, but I felt pretty good about it working.

A photo of the wires for the new LED sticking trough a hole into the underside of the record player. There's another set of wires to the left of them.

a photo of the wires from the last picture after they've been soldered together and taped down with tape.

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So I turned it on to see what'll happen. And it just worked.

// Boooring!, I want to see explosions! //

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just like I planned all along. The only thing left to try was playing a 17cm record to see if the LED is bright enough for the sensors. And it was.

// The original Blog-Post has a video here. Here's the alt-text instead: A top-down video of the record player showing how the newly installed light turns on. //

// The original Blog-Post has a video here. Here's the alt-text instead: I video from the front of the record player showing a record being spun up and the tonearm moving into the correct place before lowering. //

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I didn't take any pictures of that, but at that point I connected a phono preamp to have a listen at the output and it sounds completely as expected. Which means the electronic side of the record player is now fully fixed.

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