Modifying an SB-25 for 3.5mm Sync
I've been using Radiopopper JrX Triggers for a while now, and one of their more useful features is remote power control. They can remotely control the power level of up to three groups of flashes, provided the flashes support the older style of TTL: this works with Nikon flashes up through the SB-800, Canons up through the 580EX, and probably others I'm not aware of. The catch is that to make it work, you have to attach your flash to their $30 RPCube. With Nikon flashes, you can make it work by building custom cables with 3.5mm on one end and Nikon's TTL connector on the other, but those are expensive to build and easy to lose. My solution is to build a 3.5mm jack right into the flash so you can connect it directly to the Radiopopper with cheap cables. I've done this with both SB-25 and SB-26 speedlights before. In this post I'll be showing the modification of an SB-25, but the technique is not difficult to apply to an SB-26.
The mod is relatively simple, but you'll need basic soldering skills. In addition, my method removes the auto flash sensor to fit in the 3.5mm jack, so you won't be able to use the auto mode afterwards. Of course, messing around inside a speedlight can be dangerous, so I'll start out with the obligatory disclaimer.
WARNING: The inside of your flash contains a very large, very dangerous capacitor. Take caution opening and modifying the flash. You are solely responsible for any damage you cause to your flash or yourself.