Nikon D90’s Achilles’ heel: The Main Drive Gear


Recently I repaired my dad’s Nikon D90. It has been broken for a while, as the mirror refuse to move when needed(it isn’t stuck, however) and the LCD shows a generic “ERR” message. According to my dad, he was quite gentle with this 14 year old camera, and it broke itself when he was taking a picture of the sky. He got a ¥1000+ quote when he took it to a Nikon authorized shop, who told him that the main drive gear most likely cracked. A bit of my own research confirms this: If the mirror moves external force(such as a light push), but the camera could not function and report that the mirror is stuck, then the main drive gear is most likely the culprit.

Nikon D90 make use of a single main drive motor to power most of its in-body moving parts, such as the shutter and the mirror. This motor is connected to the rest of the gearing system via a tiny plastic gear, which could become cracked through normal usage. Surprisingly, the gear itself only cost ¥4 on Taobao(though with ¥7 shipping), so I decided to repair the camera myself.

Screenshot of the gear listing

I followed a Youtube tutorial to disassemble the camera. Due to the complexity of the repair, a video guide is definitely helpful. The Nikon official repair manual skips many important steps and is still hundreds of pages long. If you wish to attempt this repair, please read this post in its entirety to get a better understanding of the complexity of the repair. I will also attach the resources I used at the end. During the repair, I gladly noticed that there is no glue holding the camera together, just a few clips and a lot of screws. I used a iFixit screwdriver kit lid to keep track of the screws, and the used all of the available slots. It was a very long process, but I ended up almost completely disassembling it in around an hour:

And that is where I encounter a major problem: As it turns out, the main drive gear is hidden deep inside the core of the camera and a complete seperation of the front/rear body is required to access the gear. Therefore, the de-soldering of a solder bridge and several cables is required. I did not have a soldering iron at home, so I had to order one off JD:

After waiting almost a whole day, I got my iron and started working again. Long story short, I successfully de-soldered the connections and wiggled the front/rear bodies apart:

If you are attempting this repair and have trouble separating the bodies, I would recommend pushing the large capacitor toward the top of the camera, which would free it and allow more the camera bodies to separate. I didn’t take many pictures after this, but if you are curious about what the main drive gear/motor unit look like, here’s a picture from the official repair manual(its a little gear under the relatively large motor):

After replacing the gear, all I had to do is putting the camera back together. I still need more practice soldering as I had some trouble creating the solder bridge and even accidentally hit a ribbon cable. Luckily, the pin that I hit was not used and the camera remain fully functional. Putting all the 100+ screws back was quite a pain as well, especially since Nikon used a handful of different screw sizes.

The ribbon cable connector I accidentally hit

Overall, the repair was a success and my dad’s trusty old Nikon D90 was back in action. I use the word trusty in a very loose sense as the camera broke before reaching 5000 shutter count. It is quite unfortunate that a classic model such as this D90 has this fatal design flaw.

After completing the repair, I can say that I understand why repair shops would charge ¥1000+ for the labor. If you ever want to attempt this repair, I have several resources and warnings. Resource wise, I have attached a link to the official Nikon repair manual as well as the Youtube video I followed. Warning wise, first off, please wear an electrically insulating glove as the main capacitor in the camera might still be charged and could give you a nasty shock. I got my fingertip burned by a contact point on the front of the camera while putting it back together. Secondly, if you follow the video guide, please make sure to keep all the screws organized and follow the disassembly part of the video backwards during the reassembly. The video(and many other video I saw) was very casual with the screws and nobody want extra screws as a souvenir. Here are some final photos to conclude this post:

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