I have an older Quantaray QTB-7500A flash with the Canon TTL module. I used this with my old Canon EOS Elan 35mm which died a few years back. I recently bought a Canon Rebel XSi, because I could use my old EF lenses, and figured the Flash would also work. However, when I mount the flash on the camera, it does not acknowledge that the flash is there. The flash powers up, and if I hit the test button, the flash fires, so I know the bulb/charging system is OK. The TTL connectors on the flash look like they match the connectors on the Camera's Hot shoe. Am I missing something? Is the Hot shoe bad? Has Canon changed the TTL connector over the years?
There is an in-depth post about Canon TTL here.
The short version is that there is TTL (film/old style), E-TTL (early digital), and E-TTL II (recent digital).
Some new flashes will work with older systems, but flashes designed for TTL are not fully compatible with either version of E-TTL. According to canon-eos.webuda.com, the Quantaray QTB-7500A is only compatible with TTL.
Canon hasn't changed the hotshoe connector, but they have updated the communication specs, and trigger voltages over time. As Canon's communication algorithms are proprietary, 3rd party manufacturers are stuck having to reverse engineer them to work with bodies. As canon updates their bodies and changes how the body communicates to accessories like flashes and lenses, 3rd party accessories may not work.
I have an E-TTL compatible Sigma DG Super 500 that works awesome with my Canon 40D, but does not work with my 5D Mark II.
The Quantaray 7500A flash is a Promaster / Promatic FTD 5600 (also sold as the "Black's DM460TZ"). They never made a Module for any Digital camera. The Promaster MF (Manual Focus) Module non dedicated (part #2673) with 'PC cord' flash cable (available for about $20 + postage) would work in full manual mode with a camera that has a PC Sync socket (eg Canon 40D).
So long story short: it won't work with the Rebel or any digital camera without a PC Sync socket ('converters' do exist that attach to your Camera's 'Hot Shoe' to provide a PC Sync socket, however these are problematic, often requiring you to turn off the camera between shots).