Medical Nikkor 120mm f4

[Macro_Cosmos]

I figured it out, and this method I am using does not strictly require destructive removal of parts to decouple.
With the conversion, I decided to remove all the electronics anyway, it is a useless nuisance for my work.  I will mount a LED ring light in place instead.  A controller board can definitely be tucked within that housing. 

I am going to reverse the disassembly to verify its function.  If this works, it is unnecessary to enter from the rear element.

Edit:
Here it is, I got in from the front and removed all the ring light stuff.  I thought the ring light was plugged in, so I tried pulling and got a handful of glass shards.  How do they fix these if the ring light goes bust?  A typical user would not be able to perform the repair.
Anyhow, there is a white gear that engages a much larger internal gear which is a part of the "focus ring" with scales on.  I simply rotated the gear to close down the aperture fully, and then removed the gear.
Doing so disables the coupling, which allows me to control the aperture using the BR-4 ring.  I purchased an expensive ai/ais to Z-mount "PK cine style" adaptor from Kipon/XPimage.  This adaptor allows external clickless aperture coupling which will make the lens much easier to use.

Does this affect chipping?  I think it still works!  The aperture is set to minimum already, a chip should allow my camera to set the appropriate aperture from the body.  I do need to get the rear ring chipped first, hopefully I can pay a more experienced member to do this for me, I will mail just the rear mount.  I think this conversion is better.  Accessing the rear mount requires the removal of some kind of rear element group which may cause dust to enter.  For mine, all you have to do is snip all the wires, remove all the electronics and then remove one gear, much less disassembly involved.  It is possible to carefully remove all electronics using a soldering iron but I see no value in that tungsten ring flash which requires some stupid bulky dated external box.   

I will try and get another to make a video.
Here is the XPimage adaptor being demonstrated: https://youtube.com/shorts/gL3lPh0woSQ?si=V3PlVz8oJd6JKQeG
I took it from their Taobao listing.  It should arrive next week.

[Wannabebetter]

It's been a great while since I last visited here and I'm heartened to see that this lens is still being discussed. Only moments ago I was looking at a Nikon extension ring posted on an auction site, finding myself once again wondering if there isn't something I could do to modify one to facilitate manual control over the aperture lever. Clearly, I'm not the only one thinking these things, moreover implementing these solutions. Now, the fun stuff! Shortly after receiving my own 120mm Medical Nikkor I devised my own crude but effective solution to controlling the aperture, precluding any "surgery on the lens". In effect I found I could use dental rubber bands between the bayonet mount and aperture lever allowing for a friction-hold of the lever at an approximate preset f-stop value. It's a pain and often requires dismounting the lens to reposition the lever on it - but it works! And should a rubber band come off in use, it bounces harmlessly off the mirror. At which point I can shake it out of the body and put it back on the lens.

[Wannabebetter]

Reading further along in this discussion, I see the LED ring-light solution is also being explored! Frustrated by my own inability to find any cords for this lens, never mind an OEM battery pack costing nearly as much as a plane ticket to Japan, I resigned myself to building my own portable power supply. Only, I couldn't source a schematic and/or pin-out diagram for this lens and didn't want to risk blowing any number of leaking capacitors or shock myself with an indeterminate number of joules; my heart couldn't take it. So, I thought, why not go the LED solution? Simply put: I don't believe it's the same as real "flash", moreover provide enough output for some night time UV experiments I wish to conduct. But I might be wrong.

[Patrick Blancheton]

Hello,

I have the repair manual for the the "AC Power Unit LA-2" at home.
Of course there is the schematic and the pin-out diagram inside.
As soon as I have access to a scanner I post them.

[Patrick Blancheton]

Here you go 

I have the repair manual for the lens as well (thanks to Tim Becraft).
I can post the schematic of the flash unit, but it's harder to read because you have to dig into the parts list to figure out what are the components.