I could buy both the shutter and the aperture control units, but I don’t want to buy something that I don’t need. I don’t think the camera is worth what it would cost to take it to a repair shop.
I honestly don’t think it’s worth the time or expense to try repairing a 14-year old camera, unless you want to do it just for the fun and experience of it. If you’ve tried it with multiple lenses and it still doesn’t work, it’s definitely the camera’s problem. I don’t have any experience with Nikon DSLRs, but I rather doubt that the shutter and aperture control units are connected. (That’s my very uneducated guess, so don’t attach too much importance to it. 🙂 ) If you do take a shot at it, here’s a video for fixing the aperture on a Nikon D80 that might be helpful, presuming that not much changed between the D80 and the D90. Oh, and here’s another video showing the D90. I’m assuming you have a copy of the Nikon D90 service manual?
Let us know if you make any progress! I’m not brave enough to attempt to disassemble a camera, but I applaud those who have the skills to make them work again!
If you need a cheap DSLR, and you’re OK with an older one, check out KEH.com for a reputable dealer that sells used cameras for good prices. A friend of mine had an old Nikon DSLR that didn’t work any more, and when he listed it on Ebay “for parts” for a small sum he sold it within an hour. So you might be able to get a bit of cash for your old one to help with the cost of one that actually works. 🙂