Newbie Flash Question

After 3 months of waiting, my 10D turned up today. I haven't much idea on how to use it yet.

I have an old Cobra 700AF Flash gun. I thought I would see if it worked on the 10D. What I am finding is that my photos are almost a complete white out.

Can this flash gun be made to work with my 10D

It sounds like your flash is firing in manual at full output. The only way to use a flash in manual mode on the 10D is, if the flash has a Manual Mode, use a distance scale on the back of the flash for a given f stop or use a flash meter and set the f stop on the camera/flash using the meter. If you want a flash to act automatically with the camera to regulate the output of the flash, you need a flash that is ETTL compatible, like an EX series Canon flash or the Sigma 500 DG Super.

But more importantly, before using any third party flash on your new camera, you need to check the trigger voltage to be sure it is less then 6 volts or you will fry your shutter. If you don't know what the trigger voltage is, get someone to test it for you wityh a voltmeter or use a Wein Safe Sync between the flash and the camera. The PC socket and hot shoe on the 10D have a trigger voltage limit of 6v.
Scott

I hadn't even bothered looking at the flash settings on the back of the gun. I'm frightened to try again because of this voltage you mentioned.

This flash gun was used for years with my EOS 10. It is dedicated to Canon cameras. It should be OK with a 10D shouldn't it?

No. Each camera model can have a different max trigger voltage. The 1 series has a much higher max voltage. That flash may have been dedicated to your older camera, but it is obvious from its behavior, it is not dedicated to Canon digital cameras. Canon digital cameras will only work in dedicated fashion in ETTL, not TTL, or ATTL. That's why Canon EZ flashes will not work in dedicated fashion.
Scott