Intro #

Basic monolight. Sadly, it doesn’t have a built-in optical sensor, but two 1/4" sync jacks. Caution, old-school sync voltage around 220V, as expected. Standard threaded light socket.

Disassembly #

To state the obvious: HIGH VOLTAGE inside! Don’t open if you don’t know what you’re doing. To discharge most of the energy:

• disconnect
• immediately pop flash one last time
• leave it alone a few hours or overnight

Unfortunately there is no access to the flash tube terminals to discharge the capacitor bank manually. The service manual says to remove the flash tube, but this is not possible on my unit as its terminals are held by screws. The rest of the instructions are fairly reasonable. The outer sleeve mostly slides off, after desoldering a grounding wire near one of the sync jacks. The inside seems very well built, including laced wiring harnesses that is a rare sight. Almost looks like the inside of some avionics instruments, if it wasn’t for the weight.

Switch repair #

Seems like this is a fairly common failure. Massive current pulse of a flash discharge (see measurements below) is far beyond the published ratings of all these small consumer-grade rotary switches. They work until they don’t, and then the failure is total:

As replacement switch, I’m trying a C&K, # A20503RNZQK, 2-pole, 5-position rotary switch with 1/4" D-shaft and almost-standard 3/8-32 threaded bushing. This bushing ends with an idiotic shoulder only slightly larger in diameter than 3/8", which requires the panel hole to be drilled up to the next size (around 10mm). It also features a special bent washer that needs to stay in the correct position during installation (it sets the number of stops), which is easier said than done. Just an awful, awful design.

The wiring layout is almost identical, but the two ‘common’ wires had to be extended to reach the inner terminals

Current pulse measurement #

Out of curiosity I measured the current and voltage during a discharge. White trace is the current (clamp probe, 1mV/A), showing a peak value of around 630A. Green and blue traces are both sides of the capacitor bank. These have a superimposed 60Hz signal since they are ground-referenced; the actual voltage would be the subtraction of green minus blue. Orange is the trigger signal; peak flash current is reached within approximately 30us. Running the numbers, I get a discharge energy of around 240J, which is near enough the 250 W*s figure in the specifications.