Following is the installation manual and some pictures of the fan retrofit. Thanks to David Clarke [21030085++] for winning an eBay auction for the fan and providing the information!
Chroma Fan Retrofit Instruction Manual 30-9004801
Refer to Figure 1. Remove the top cover by removing the brass screws (1 through 4) on the wood rail above the control panel, and the black screws (1 through 8) on the rear panel. Do not remove screw 9 yet. Screw 9 secures a green ground strap with a nut on the inside of the cover. Gently remove the cover until you can each and remove this nut. Keep track of these screws for later reinstallation in their proper place.
Mount the fan to the studs on the top cover using the 2 brackets, 4 machine screws, 8 lock washers, and 8 nuts supplied. Orient the fan so that the AC lugs face the center of the instrument and the air flow is up and out of the Chroma. A small arrow on the fan indicates the direction of the air flow.
Refer to Figure 2. Remove the Chroma Power Supply by allowing that corner of the Chroma to overhang a table edge so you can remove the screws from the bottom of the instrument. Solder the fan wires securely to E18 and E19 as shown.
WARNING: AC VOLTAGES WILL BE PRESENT WHEN THIS UNIT IS POWERED UP. BE SURE NOT TO MELT INSULATION ON THE WIRING OR LEAVE SOLDER SPLASHES WHICH MIGHT CAUSE SHORTS.
Refer to Figure 3. Route the fan wires as shown at B, under the brace. Use plastic electrical tape to keep the wire in place as shown at A. Do not use Tie Wraps or other material with any thickness, as this may bow the top cover.
Reconnect the Ground Wire to the Top Cover with screw 9. Position the cover over the instrument and plug in the fan connector. Be sure the connector is straight and fully seated. Lower the Top Cover straight down into place.
Power the Chroma up and check that the fan is functioning. Air should be gently moving up and out of the vent. Unplug the Chroma.
Reinstall the remaining Top Cover hardware. Recheck the fan for proper operation.
Thanks to David Clarke for these shots.