I recently got a Beseler 45S dichroic head for my Beseler 45MCRX enlarger, which was previously fitted with a condenser head. I was already sold on the advantages of a dichroic head for B&W variable contrast printing and was happy to find this head for a good price on eBay. When the head arrived from the seller I began cleaning it up and noticed that the internal cooling fan had started to come loose from its mounting system.

The existing cooling fan is a small plastic unit about 3″ in diameter. It resides on the opposite side of the head from the 250 Watt halogen light bulb. Beseler has done a pretty good job of mounting it in a way that should reduce vibration. Theyíve used blue foam standoffs (rather like small, delicate versions of the motor mounts in some cars) to secure the fan motor and open-cell foam is also used to hold the fan in place.

While these vibration-reducing measures are probably effective, in my enlarger head I could feel vibration being transmitted from the enlarger head fan to the rest of the chassis when the fan was turned on. When I placed a finger lightly on the lens mount I could feel vibration from the enlarger head which was proof enough for me that the level of vibration this fan produced (in its deteriorated state) was excessive for my purposes.

A vibration-free enlarger chassis and head is important to getting sharp prints, and so I decided to deal with the broken internal cooling fan mount and the vibration it produced at the same time. Rather than repair the cooling fan mount, I decided to remove it completely and build an external cooling system for my 45S.

First, I measured the operating temperature of the light using the existing cooling system. I used my Chinese-made el-cheapo infrared thermometer from Harbor Freight Tools to measure the light bulb housing after 5 minutes of continuous operation, with the fan running during that time. It measured between 180 and 185 degrees F.

Then, I removed the existing fan blade and motor because I didnít want it to spin freely in the airstream being drawn in by the new cooling system. Here is the partslist for the new cooling system:

Hereís a picture of the plenum that holds the fans. Thereís a second fan on the other side of the plenum for a total of two fans:

You can see the (roughly cut) holes in the plenum box that allow the fans to draw air through the flex duct into the plenum box and out the sides. The box came from Ikea with three sides, and I added some 1/4″ plywood to serve as a fourth side and sealed it with bathroom caulk.

The plenum box currently sits on the floor of the darkroom near the enlarger desk. About 10 feet of flexible duct connects it to the enlarger head, on the side of the enlarger where the light bulb is mounted. It draws hot air out of the enlarger. The flex duct is mounted to a square piece of plywood with a 3″ hole cut into it, and this plate is sealed to the enlarger head with foam weatherstripping and attached with gaffer tape. It looks like this:

And the whole setup looks like this:

After installing the new cooling system, I tested it under the same conditions as the previous test of the built-in fan. The lamp housing temperature reads 5 to 10 degrees F cooler, which may be a within the range of a measurement error on my cheap-o infrared thermomether. But itís close enough to assure me that the new cooling system works at least as well as the old at removing hot air from the lamp housing.

The flex ducting has a tendency to gently oscillate back and forth a little, so I plan to secure it to the wall in 2 more places to help it stay put. Other than that, I am quite pleased with my external cooling system for my Beseler 45S!