I have a 50D and have acquired an old studio flash for free. See pics.
It will not fore unless the 50D pop up is up and enabled. I looked through the settings but cannot figure it out. Is it because the unit is an older technology device?
You probably just have a defective Sync Cable going to the main strobe, replace it and all should be fine. This strobe unit also is designed to work as a "Slave", meaning when it sees another strobe go off it will fire also. That is the reason it fires when the pop up flash goes off.
The smaller strobe mounted on the inside of the larger strobe is designed only as a slave unit, you would be better off using it elsewhere and replacing it with a regular incandesent light bulb for the intended use of a Modeling Lamp on the larger strobe.
The only other issue I can think of that might not allow triggering of the strobe is if there is a setting on your camera that turns off the flash sync to an external strobe. Go through the user manual and and camera menus to see if there is a menu item that controls the external flash sync on/off.
Most likely the sync requirement is working at a different voltage, too much power coming from the 50d. The indication is that it has a 6V battery option, probably best not to use the sync to avoid feedback which could damage the 50d
Thanks for posting the photos. What you have there is a studio flash with another Morris slave flash where the modeling light should be.
The Morris unit is a low end slave designed to be screwed into a standard "Edison" lamp fixture. They can be useful in interior shooting with flash to match the color temp of table lamps, recessed fixtures, etc. and for hobbyists trying to do flash lighting on the cheap. It should not be used as a modeling lamp in that flash. Remove it and use a regular 60W tungsten bulb.
The Spiratone studio flash appears to have an optical slave cell above the power cord, but your camera's pop-up flash always fires a pre-flash and will not trigger it in sync. The pre-flash will fire the Spiratone before the shutter opens. The presence of the 6V battery is something I haven't seen before but I'd assume its required for the control circuits on the flash. The required battery may not still be available.
Actually the 50D is only closing the circuit and not supplying any voltage, the strobe supplys the voltage to the trigger circuit. I have a pair of these strobes under a different name, the 6V battery compartment is empty as I use only AC power in a studio setup. Have no trouble triggering them using a 1Ds3 or a 5D, so my guess is still a bad Sync Cable or a Menu Item on the 50D that turns off the external flash signal.
FYI: he said it will only fire with the pop up flash so the sync cord advice, while accurate isn't addressing the problem.
I suspect what the pop-up flash in the 50D may be doing is just triggering the Morris flash slave screwed into the modeling lamp socket, probably by the previous owner so he wouldn't loose it.
Capacitors become "deformed" over time and if the big flash hasn't been used for several years it may be DOA.
