The Olympus FL-LM3 on camera strobe is used in an underwater housing to trigger external strobes (e.g. Sea&Sea YS-D2) via fiber optic cables. The recycle time of the FL-LM3 can be as great as three seconds if the flash dumps all of the charge on its photoflash capacitor into the Xeon tube. On the other hand the YS-D2 has a recycle time at least 50% shorter than the Olympus strobe.
Given that the FL-LM3 is only used as a trigger for the external strobe, there is no need for a Xeon tube with its high output power. This modification substitutes a pair of high brightness red LEDs for the Xeon tube. The net result is that recycle time is limited by the communication protocol between the camera and strobe, or about 10 fps. Otherwise full TTL capability is preserved given the LEDs emit light pulses of the same duration which the native Xeon tube emits, including pre-flashes necessary for TTL operation.
Step 1: Disassembly
Remove the four torx screws from the base of the strobe.
Between the rotating / tilting strobe head and main body are four screw covered by black stickers. Remove the stickers and screws. A knife at the edge of the sticker works well for removing the sticker without damage, or you could just pierce the center of the sticker if you plan on disposing of them. Store the sticks for reuse on a sheet of wax paper.
On the strobe head is a sticker that runs almost the width of the head. Remove this sticker and two screws that are hidden.
Remove the foot of the flash, side of the main strobe housing and separate the two half of the head just enough to remove the diffuser / reflector assembly with Xeon tube attached.
Step 2: Remove Resistor
A 750K ohm surface mount resistor must be removed to disconnect the photoflash high voltage from the IGBT.
A hot air rework tool can be used, or a soldering iron with a little more work.
Step 3: Remove Connections to Flash Tube and Trigger Transformer From Main Board
Disconnect the wires that run from the main strobe circuit card to the strobe head. These connections are to the Xeon tube and trigger transformer within the head. They are reused for connection to LEDs which will go into the strobe head.
Step 4: Replace the Photoflash Capacitor With a 1 UF 350V Standard Capacitor
This step is optional. Given that the IGBT / Xeon tube is never allowed to discharge the photoflash capacitor, recycle time is not an issue thus the size of the photoflash capacitor is not relevant. Though the initial charge time with a 1 uF capacitor is almost instantaneous.
Step 5: Attach the Base of the 2N2222 Transistor (LED Driver) to the Gate of the IGBT
The heat shrink assembly to the left of the image contains a 2N2222 transistor and resistor to set base current. One side of the resistor is connected to the gate of the IGBT device using 30 gauge wire.
Step 6: Connect to +3.3V and Ground
Connect the 3.3V and ground connections to the main card using 30 gauge wire wrap wire.
Step 7: Remove Xeon Tube and Trigger Transformer
From the strobe head remove the Xeon tube and trigger transformer. The reflector is not used and can be discarded.
Step 8: Attached LEDs
Form the leads of the LEDs and attach to wires that route between the main strobe body and head.
Step 9: Drill Two 5mm Holes for the LEDs and Epoxy in Place
In the diffuser assembly drill two 5mm holes for the LEDs. Note that the edge of each LED coincides with edge of the opening to the diffuser. The LEDs are centered top to bottom on the diffuser.
Positioning of the LEDs in the diffuser is important given they need to be positioned behind the fibers when the strobe is mounted to the camera body and the body is in the underwater housing. Try to be as accurate as possible.
Use a small bead of epoxy between the diffuser and LED, on the inside of the assembly.
Step 10: Reassembly
Reassemble the strobe. If a smaller capacitor is used in place of the large photoflash capacitor, use foam or another similar material to fill in the void such that the smaller capacitor is not left to rattle around inside of the strobe body.
Step 11: Test
Step 12: LED Driver
The LED driver and LEDs is a very simple circuit as shown. Note that there are no series resistors inline with the LEDs. The pulse duration is very short thus peak current is within what the LED can dissipate. We need a lot a photons to drive the fibers, thus this approach was adopted.
Step 13: Olympus Reference Schematic
For reference this schematic shows the portion of the Olympus strobe pertaining to driving the Xeon tube. A traditional IGBT based design is used by Olympus.
Step 14: Bill of Material
1 - 2N2222 transistor
1 - 10K ohm resistor
2- High brightness red LEDs
High brightness LED specification: 12000mcd, 8 degree viewing angle. Multiple sources exist, where one possible supplier is SuperBrightLEDS.com part number RL5-R12008.