If you wish to use a generic non-dedicated flash unit on the Canon EOS camera, you will first need to check the triggering voltage of the flash unit. The Canon Speedlite flash units triggering circuits is not more than 6 volts. Furthermore, the Canon EOS 1, A2/A2E, 10s, 630; Elan, RT and Rebel also operate with 6 volts as the maximum voltage going through their triggering circuits. As such, you must make sure the generic flash unit also uses no more than 6 volts for their trigger.
Triggering circuits using more than 6 volts can damage the X-Sync contact point of the camera, the connecting wires and shutter units. You might not be aware of the damages as it could be happening gradually. In addition, the high voltage that is conducted through the Camera connecting wire will also generate electro magnetic noises which can create undesirable effects on the Camera functions. Problems can arise from the metering system, shutter speed, aperture settings etc. The effects will depend on which part of the camera circuitry system is being interfered with.
If you wish to test the Voltage of the triggering circuit of the flash, simply use a voltmeter. These can be purchased at any Radio Shack stores. Connect the positive lead to the positive point of the Camera Sync cord and the Negative lead to the ground of the Sync cord. For Shoe mounted Flash units, the positive lead must be connected to the center of the mount. The negative lead should be connected to the ground contact which is located on the side of the Lock nut. All the voltmeters have varying settings. If the voltage of the Flash unit is unknown, just use the highest settings and work your way downwards.
To fire a Studio flash safely without a sync cord, use a Speedlite 420EZ or 430EZ with 1/32 power under manual flash mode. Ensure that the flash’s optical slave trigger is activated before firing. You are actually using the bounce feature of the Speedlite 420EZ or 430EZ to direct the light to optical slave trigger. Alternatively, you can use radio slave triggers to fire the studio flash. Companies like Lindahl and Quantum Instruments sell radio transmitters that connect to the PC contact of the camera and a receiver to connect to studio flash power pack.
If the Studio flash does not work with the Canon EOS Camera, you have to check on the following listed below:
(1) That the third party PC socket adapter do not have a metal shoes. This will result in a short circuit in the trigger circuit. The remedy is use Canon PC socket adapter which is made from plastic.
2. Make sure that the flash units polarities are in the correct order with the X-Sync of the camera contact point. The reversed polarities are normally caused by a diode which is coupled to the camera’s X-Sync contact for the protection of the camera internal flash. To correct this problem, simply reverse the polarity of the Sync Cord.
3. If the two troubleshooting tips do not work, just use the method described above on how to fire a studio flash safely.