A photographer's zoom spot creates a hard edged light that can be shaped by internal shutters and focused with an adjustable barrel. They are generally quite expensive, so this is an attempt to build one for around $100.
Step 1: Materials:
A used theatre ellipsoidal spotlight $20-$60. Search Ebay for "Ellipsoidal" or "Leko"
A small studio strobe 100-160 watts $50 new/$30 used on Ebay
(2) 4 way trailer light connectors or (1) 5 way connector $10
Bolts and lock washers
Small clay flower pot saucer - (Dollar Store)
Copper plumbing bracket
Old aluminum tray or pan
Dry wall compound or plaster of paris
3/4" copper pipe with cap
3 ring connectors
1 bullet connector male/female
2 banana plugs and sockets
Step 2: Light Repair
Used theatre lights often need repairs. If the lenses and reflector are unbroken, most problems can be fixed. Shutters may have to be hammered flat or new ones cut from an old aluminum pan. Lubricate the focusing barrel and make sure it can slide and lock into position. To support the light , bolt a 3/4" copper pipe with a cap to the yoke. Use two washers to prevent the bolt hole from deforming. The pipe can then be fitted over the top of a strong light stand.
Step 3: Opening the Strobe
Warning: Being shocked by high voltage is unpleasant. Unplug the strobe while you are working on it. Wear gloves and use insulated tools. Get in and get out with the minimum touching of components.
Turn on the strobe and fire it at the lowest power setting by hitting the test button. With the strobe still turned on, quickly unplug it and touch the test button. The strobe should fire again even though it is unplugged. This drains the capacitors and hopefully prevents them from recharging. Turn off the strobe switch.
Wearing gloves, pull the modelling light out of its socket and set it aside. Pry out the two plastic pins in the side of the strobe and their locking collars with a knife blade. Very carefully slide the two halves of the strobe slightly apart. Reach from the side with needle nose pliers and gently wiggle the banana plugs of the flash tube out of the sockets. Wiggle one and then the other. Use a strong light so you can see what you are doing. Only the plugs are metal, the rest of the tube is glass. Slow pulling of the tube from the top can help, but should be done very cautiously. Remove the strobe's metal reflector when the tube is out..
Step 4: Connectors
To allow the possibility of taking apart the zoom spot for storage, trailer light connectors were used. A five way connector would have handled all the wiring in one cable, but I made do with two used 4 way.
Cut the cable of a 4 way connector in half. Cut and remove one wire. To one half solder 2 banana plugs and a bullet plug. To the other half solder 3 ring connectors. This cable will connect the strobe and the trigger wire. Solder a female bullet connector to the trigger wire in the strobe.
Cut the other 4 way connector cable in half. Cut and remove 2 wires. Trim 2 ring connectors to make prongs and solder them to the cable. This cable will connect the modelling light.
Step 5: Flower Pot Saucer
Flatten the copper plumbing bracket in a vice and make two fasteners to hold the saucer against the ellipsoidal reflector. Drill holes in the clay flower pot saucer for the banana sockets, trigger wire bolt, fasteners and modelling light. The 50 watt modelling light is very hot, so there can't be any plastic near it. Unscrew the plastic covers from the banana sockets. The wire that attaches the trigger bolt to the flash tube trigger wire should be bare copper or enamel coated.
Step 6: Testing
Use drywall compound or plaster of paris to cement the modeling light in place and leave it overnight to dry. The modeling light's leads will most likely not take solder. Crimp a small leftover piece of copper bracket around each lead with pliers, and solder to that. Hook up the other connectors to the saucer and the strobe, making sure there are no cross connections. Zip the cables to the case of the strobe to stop strain on the plugs. Turn on the strobe and test for arching and heat damage.
Step 7: Install Flash Tube
Use the copper brackets to attach the saucer to the hole in the reflector. Make sure they do not touch the metal base of the banana sockets or the trigger wire bolt.
Step 8: Aluminum Cap
Cut a thick aluminum tray in the same shape as the old light bulb housing. Make a hole in the tray for cables to thread through and drill bolt holes. Bolt the strobe to the tray. Unscrew the old bulb housing latch and use the bolt hole to attach the tray to the light.
Step 9: Finish
Don't trigger the strobe with a sync cord, use the built in slave or a wireless trigger. You don't want to be physically connected to the light if there is a short. Even with commercial zoom spots, a blue line can appear around the edges of the shutters. Sometimes the light looks best with the barrel slid so that the beam is slightly out of focus.The zoom spot is heavy, so sandbags are needed to secure the light stand.
Unfortunately the theatre light did not have a gobo slot for projecting patterns, so gobos are held in place against the shutters with a magnet. A gobo can be made by cutting an aluminum pie pan with a razor or spray painting a piece of dollar store glass and scraping a design. There is some ghosting to the patterns. Using an ellipsoidal spotlight that is designed for gobos might prevent this.