AX70 Field Diaphragm Repair


Last year, I purchased an Olympus AX70 ProVis Microscope locally, I deemed this to be a significant upgrade from my previous BX53. The transit company “toll” ensured that I paid mine by barbarically treating the parcel. The microscope frame was damaged, and I had to source another frame from an online friend in the USA to source parts for repair. Nevertheless, the repair went well and the microscope was up in full working order. The bottom plate of the microscope’s fluorescence arm was cracked and bent, which misaligned the optical axis.

I soon realised another issue, the field diaphragm seemed oddly crooked. I did not pay much attention because I wanted to use the instrument and rarely bothered to adequately utilise the field diaphragm.

This all changed after I acquired a 40x water immersion objective, which I now believe to be heavily decentred. Without adjusting the field diaphragm correct, I would observe quite a lot of glare. With the field diaphragm closed, I would get an oblique-like effect despite the microscope being set up for ordinary brightfield. I deliberately decentred the condenser and looked into a centring telescope and observed something rather horrifying.

With the diaphragm closed, it was supposed to look like some even-sided polygon, not the crooked nonsense above.

Disassembly and Repair

I used the tools below to perform this repair.

  • A couple of tweezers
  • Krytox grease to lubricate the mechanics
  • Metric size 3 balldriver
  • Metric sizes 1.5, 2.5 and 3 hex wrenches
  • Cheap metric size 1.5 hex wrench
  • Lint-free q-tips
  • Vessel “Japanese Industrial Standard (JIS)” screwdriver
  • Philips Screwdriver
  • (Optional) Mitutoyo gauge block
  • A small bug that found its way onto the bench

Firstly, everything attached to the microscope frame should be removed. This includes the nosepiece, fluorescence cube turret, microscope stage, illumination lamphouse units, observation/photomicrography head and preferably the substage. I was lazy, so I did not bother removing the substage and the motorised focus drive. Follow up by unplugging all the cables.

Carefully set the microscope frame on its back, and remove the JIS screws from the bottom plate. I would recommend a JIS screwdriver. JIS screws feature a small dimple near the cross, and low-quality Philips screwdrivers risk stripping the screw. There are a total of 7 JIS screws highlighted in red below.

The internal Köhler illumination optical train is revealed, very complex when compared to “built down to a cost” versions in modern microscopes. The field diaphragm on the BX53 (right) is actuated with plastic gears and directly accessed from the transillumination light port. The over-engineered AX70 features some gears and some more gears, allowing more ergonomic access from the side. The first surface mirror on the BX53 is not easily dismounted, unlike the AX70.

Using the size 1.5 hex wrench, carefully loosen the 2 setscrews at the knob, and pull the knob out. One protrudes, and the other is recessed deeper in the knob.

With the size 3 hex wrench, dismount the three screws and carefully pull the diaphragm module out.

The diaphragm iris blades are crooked. Using the size 2.5 hex wrench, remove the condensing optic.

Somehow, one of the blades was dismounted and displaced. I have no idea how this happened and with no damage to the blade at all, this was truly a miracle. This also illustrates the complex mechanism of this diaphragm unit, extremely precise settings are possible and accurate.

With the aid of 2 tweezers, I lifted the aperture blades and nudged the displaced one back into place.

With the diaphragm completely opened, rotate the shaft to one extreme end and carefully drop the large ring gear back in place. The limiters are simply two screws. Once the ring gear is installed, play with the diaphragm to ensure it opens and closes all the way, adequately within the two limiters. If not, pull the gear out, manually open the aperture, and try again. You can simply push the brass block until it hits one of the limiters.

I used the Mitutoyo gauge block as a platform to ensure perfect flatness when I mounted the condensing optic back onto the block. I also opened the diaphragm completely. Reverse these steps to reassemble the frame.

To remount the dial, I aligned the white line with the icon which indicates that the diaphragm is completely open. This is not exactly necessary since I never care enough to look at the dial.


Despite the repair issues and difficulty in sourcing all the AX specific parts, I do not regret selling the BX53. It has found a good home as well!

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