by ʇolɐǝz ǝɥʇ qoq

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My 30D + 340EX has started acting up. About half the time the flash doesn't synch properly, leaving some portion of the frame incorrectly exposed (sometimes the top doesn't get flash, sometimes the bottom doesn't get flash, sometimes none of the frame gets flash).

This happens in both manual and ETTL mode, at different shutter speeds (even at 1/30th), with second-curtain enabled and disabled, and with high speed synch enabled and disabled.

The contacts look fine on both the flash and the camera.

The 30D's built-in flash appears to work without issue.

Edit: I've switched to my 20D, and the flash appears to be working fine there.

Any ideas what could be causing this?

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It probably wouldn't hurt to post some symptomatic pictures, showing everyone precisely what you mean by "doesn't get flash" and "incorrectly exposed." – Lyman Enders Knowles Apr 2 '11 at 16:53
I don't have any examples right now, but I'll take some later and post them. – David Wolever Apr 2 '11 at 17:54
The pictures look a bit like the one shown on this page: – David Wolever Apr 2 '11 at 17:59

Sounds like you've gone through a variety of troubleshooting steps - if you can't discern any sort of pattern or consistency in the errant behavior, it might be a problem with the flash. If you have a friend with a Canon camera, see how the flash behaves on his unit - or perhaps bring it down to your favorite camera store and see if they'd let you test it out on another body.

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Thanks for the suggestion. Since I posted, I've switched to my 20D and the flash has been synching without any apparent issues. – David Wolever Apr 2 '11 at 17:53

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