I have a couple of old photo strobe flash units which I'd love to be able to use with my new DSLR. It's well known that DLSRs will not tolerate the high voltages which appear on the hotshoe pins of these old units. Several circuits have been published which will convert the 200-400V triggers to something below 6V, which is a voltage easily tolerated by DSLRs. Of course, every flash unit is different; there's a good listing here of the various models and some comments about their compatibility.
The only sure way to know is to measure it yourself. I would suggest doing this, even if the list shows your flash is compatible. There are often different revisions of the same model, and you don't want to take any chances! Put in a fresh set of batteries and turn your strobe on. As it charges, take the probes of a DMM (capable of measuring a DC voltage up to 600V) and carefully probe the hotshoe contacts. If you see a gradually increasing voltage in the 200+V region, you have a candidate for conversion to a low-voltage trigger. Make sure to turn off your strobe and discharge it before proceeding further, either by pressing the manual trigger button or by shorting the trigger contacts on the hotshoe. I measured my Minolta Auto 128 trigger voltage at around 300V.
There are two ways to deal with the problem. You can purchase adapters that fit between your flash and the camera. One of these is the Wein Safe Sync, but there are many similar devices. These have the drawback of making your flash taller, increasing the stress on your camera's hotshoe. I thought it would be more fun to build the circuit into my flash, and besides the fact that I'm cheap, it's always more fun to build something yourself. I laid out the converter circuit and had some boards made at ExpressPCB. The circuit is very simple and I found all the parts at Digikey. The only change I made was to use surface mount (SMT) parts and to raise the voltage spec on the triac from the original 400V to 600V by using an NXP Z013MN,135.
Here's what the Minolta Auto 128 looks like when you open it up. The round object at the left is the high voltage capacitor, to its right is the battery compartment and hanging by the wires is the power supply circuit board (which I thought I might need to modify, but later discovered that I didn't). At the far right is the hotshoe. I could have done the whole project simply by removing the hotshoe, but I didn't know this at the time!
Here's a close-up of the hotshoe and the small circuit board inside. On the left side of the circuit board is the manual flash trigger switch. It connects the red wire (+) to the white wire (-) and there's a big "POP" and a bright flash of light. On the right side of the board is some kind of slave trigger connector. I think it takes a 3.5mm mono headphone plug. Two screws hold the board down against two springs, which connect the board to the hotshoe contacts.
You can see that the original board and my adapter circuit board are roughly the same size. Turns out, that's a Good Thing. You can see that I have removed the slave trigger connector parts as well. This gives me a bit more room.
Here is the adapter board connected between the high voltage trigger wires and the hotshoe PCB. The circuit is powered by the high voltage trigger - no external power needed. Notice how the adapter board fits right on top of the original hotshoe PCB? Not planned at all, just serendipity! Murphy doesn't always win...although the yellow wire on the adapter PCB shows where I got the triac pinout wrong (fixed in the layout above).
Here's the hotshoe, with adapter (note the yellow kapton tape, which keeps the output wires from shorting to the manual trigger switch), ready to be reassembled to the body of the flash unit. The SMT components on the adapter are just low enough to fit into a small space between the hotshoe and the flash body. Trigger voltage was measured at 5V.
Every flash is different, and you'll need to find a place to put your adapter circuit. These old flashes are pretty low tech and there's usually a good amount of space inside. On my other flash, the power supply board didn't even come close to filling the space allotted for it in the housing, and I was able to slide the adapter board into a piece of heatshrink tubing and snuggle it in with the power supply.
