Repair - Field Iris - Wild MDG4 Illuminator
Here's what the MDG4 looks like with an M20 microscope installed. It has a 100 watt quartz-halogen illuminator and a 200 watt mercury burner. The field iris adjustments are controlled by the knob and concentric ring at the very front. It is an absolute joy to use. All adjustments can be made with one hand without looking away from the oculars. The mechanism is amazingly simple and elegant - a tube with the iris in one end and a mirror and knobs at the other:
It is also very reliable - except . . . . .
I had been working with some experimental condensers and started getting very unexpected images. After eliminating the usual variables, I put the Wild achro-aplan condenser in and imaged the field iris. Here it is - wide open:
YIKES!!! Several of the leaves had come loose.
Here's the bottom panel:
Remove these four small screws and beneath the cover you will find the two larger screws which hold the main tube in place. This is a good point to mention tools and screwdrivers in particular. All Wild screws are black and are quick to show damage. As a rule, the screws on the M20 have much narrower slots than any screwdriver you can buy. Every time I work on a Wild instrument I grind new blades.
With the above cover removed, the two mounting screws are easily accessed:
With these removed, out comes the tube. Here's the base sans tube:
You can see the mirror for the mercury burner and beamsplitter for combined sources. The center position allows the halogen source to pass uninterrupted. Notice the switch which turns the halogen source off when only the mercury burner is engaged.
Now to the problem. My guess is that the end of the iris assembly somehow came loose and allowed the iris to exceed its normal excursion. Several leaves were loose and many were sharply wrinkled. To get to the leaves, you must first loosen or remove three set screws. The end piece with the collector lens also IS the upper part of the iris assembly. When you lift this, the leaves are free to fall out.
What I found was frightening. The leaves were a mess:
If the leaves are a good quality steel, there is an ironing technique which will get most of the wrinkles out:
Drawing the leaf across a smooth, rounded surface will smooth things out, but leave a curve. Massaging the opposite side should leave things more-or-less straight. Any remaining damage can be removed by working at right angles to the first procedure. This requires a degree of skill and caution. The steel is hardened at the start. This process causes further work hardening. If done to excess, the leaf may crack.
At first, reassembly looks impossible - the assembly sits in the bottom of a tube! Fortunately, this can be removed by taking out one screw. I'll show that in the reassembly. Here are the two halves of the iris assembly:
The leaves were coated with dried grease, so they got a good bath in addition to the massage - the full spa treatment:
I've rebuilt more iris assemblies than I can count - freehand. Between the warped leaves and insecure mechanism, I needed a very reliable technique. What worked was a combination of inserting the leaves backwards (sliding them underneath instead laying them down from above) and . . . . . . . gaffer's tape! This is like duct tape, but leaves no residue:
A strip of tape every two or three leaves made the process easy. The weak adhesive makes it easy to press down on adjacent leaves and pull out the strips of tape between.
The scariest part is dropping this assembly down into the main tube. There are a few distinct steps:
1) Find the slot in the main (black) tube for the location screw. This slot is quite wide, but almost impossible to see. A screwdriver locates the opening with a degree of confidance:
2) Align the screw hole in the iris assembly with this slot and GENTLY lower the assembly (with all those loose leaves) into place. There's nothing to grip, so I used a small screwdriver to gently control the descent.
3) Find the hole for the locating screw inside that dark slot in the main housing. A cheap jeweler's screwdriver serves as a guide:
4) Insert the screw and tighten firmly:
Now, the second scariest part. Lower the end piece (with the collector lens) onto all those loose leaves. It will feel as though the piece has hit bottom, but a gentle twist counter-clockwise will align the pivots with the slots and the end piece will drop nicely into place.
Now, gently rotate the main assembly back and forth to open and close the iris. This not only serves to reassure that all is well, it also sets the limits of excursion. After this procedure, reinsert the three headless set-screws and tighten FIRMLY to lock things permanently down. A magnetic screwdriver is very helpful:
Having gotten this far, work the mechanism back and forth many times to be certain that all is well:
All that remains is to reinsert the tube into the base, insert the two screws, and reattach the cover with four small screws.