I just bought a used Sunpak 344D Thyristor flash super cheap. Is it safe to use with my Canon Digital Rebel XTi? Also, I can hear the capacitor charge up when I turn it on, but no ready/test light comes on. Any suggestions?

According to the Digital Photography Answers database, the 344D has a sync voltage of 6 volts, which is safe for the Rebel.

You may need to re-form the capacitor to get it to come up to a full charge if it's not lighting the ready light. Either that, or the ready light could be broken. To test it, charge it for a longer-than-usual time and then take a test shot of a gray card. You should then be able to read the shot with Photoshop or similar software to see if you get a middle gray reading.

Of course, if you can't get it to fire at all, that's another matter.

Hope this helps.

Thanks for the info Brian!