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AO 120 Binocular prism repair

Posted: Tue Nov 12, 2019 2:29 am
by mnmyco
I just recently picked up a nice AO 120, complete with the external lamp housing, the lamp socket, filters, transformer, and binocular head. Oh, and the 180 eye pieces. For $80 I think it was a bit of a steal. Anyway, I am not sure if it was in the shipping during cold weather or the reason the scope was cheap, but the prisms in the binocular have detached from the frame. The prisms themselves seem in perfect condition with one or two minor chips that do not interfere with the optics. Does anyone have advices on the best method of ensuring that the alignment of the optics is correct when I epoxy in place? On of them seems it will be easy as I can tell where the epoxy was and belongs, but the smaller prism’s original positioning remains elusive. This is the newer, larger binocular than what is found on the series 10.

To apochromaut

The smudge on that eyepiece was actually tape residue. I was shocked when it arrived. The light housing case can be fixed with some plastic welding epoxy.


Re: AO 120 Binocular prism repair

Posted: Tue Nov 12, 2019 6:34 am
by PeteM
Apo will likely have the best advice for you.

I can note two things. First, this failure mode is not uncommon with the 110 and 120 - at least in my limited experience. Perhaps a caution for others? I've seen two or three failures out of about double that number of 110 series scopes -- and nothing similar in either in a dozen each of the earlier series 10/20 or later 410/420. That model (the 110) has also had more power supply failures than either earlier or later models; again in my experience. Your 120 with an external power supply (?) should be relatively bullet proof.

Second, in repairing AO Cycloptic prisms, I've had good luck using a thick instant glue, with a tiny circle of glue-soaked rag paper to cement the prism to its plate. The advantage over epoxy is that the cure is relatively quick AND that any goofs in alignment can be reversed by using acetone as a solvent. It's been years since the one time I attempted a repair on a 110 head (a trinocular model) with badly damaged prisms. I don't recall how the prisms were held -- but seem to recall there was a sort of mechanical cradle allowing adjustment and the prisms then cemented in place. Fiddly to fix without factory jigs?? Apo would know.

It's best to avoid a solid glued attachment of glass to metal (why the piece of rag paper). When gluing tiny DIC prisms I use a sort of rubbery contact cement. Without some freedom to move, as glass and metal heat and cool, one will often see the glass spall or a DIC prism delaminate. So if you decide on an epoxy, I'd pick one of the formulations noted for a bit of "give."

In my case repairing a 110, I ended up cannibalizing a binocular head to fix the trinocular, since the latter still had its tube lens and trinocular bits in good shape. The 120 should be a great scope once it's working -- good luck.