Dynax/Alpha 7 Aperture Repair: Easy Mode

The problem:

Minolta chose a very poor material/manufacturing process for the aperture drive gear which ultimately drives the lens aperture. These were press fit and over time they crack and will spin freely on the motor shaft, producing a momentary high pitch whine as the motor shaft spins freely, leaving the camera non-functional.





Every single a7 I’ve encountered is broken and the “working” ones are just hanging on for dear life until you mount a challenging lens. A lens with a very large set of aperture blades requiring slightly more force to actuate like the 85mm f/1.4 will invariably finish what time has started and break the drive gear free from the shaft.

This is all quite a shame as the a7 is, in my opinion, the very best A-Mount film camera ever produced in terms of performance and features.

Repair history:

Some years ago neotil1 published a fix on reddit that requires complete disassembly of the camera. Due to the mounting, every single major component needs to be removed. It seems the a7 was initially designed quite competently with many solderless flex-flex interfaces throughout, but there are maybe 2 dozen random point-to-point wire connections running across the design, likely as the design matured and it was too late to add additional signals across the many flex PCBs.

The initial repair takes something close to 12 hours with the massive amount of disassembly, soldering, and reassembly. Manipulating so many aging wires, and flexing old solder joints also introduces many many opportunities for damage. Too many to make the repair economical or comfortable to me.



The fix:

In disassembling the a7, I noticed the broken pinion gear is only made inaccessible by a single wall of plastic with a partial edge of a metal cover underneath. Given the small size of the pinion, a ~3mm access hole can be drilled to directly access the pinion gear without removing any major components.



The repair itself only requires the removal of one side plate, the front plastic cover, and loosening the bottom cover. About a dozen screws in total with no soldering, major disassembly, etc. Is is best to perform this after the drive gear has fully broken as it will simply slide off the shaft during repair.

Tools required:

Phillips Screwdriver #0/3.0mm
Phillips Screwdriver #000/1.5mm
Curved tweezers
3.5mm flat bottom drill bit or appropriate endmill(s)
1.0mm drill bit (recommended if freehand drilling)
1.000mm reamer (McMaster item 30545A61)
Compressed air for clearing debris while drilling

Materials required:

Loctite 609 (McMaster item 91458A230)
9T 0.25m 1.0mm shaft pinion gear (AliExpress item 2255801169194980)

Procedure:

1. Prepare the pinion gear

The listed pinion gear has an approximate 0.97mm shaft diameter while the motor has a 0.997mm drive shaft. This would be a good case for heat assisted press-fitting, but that could damage the plastic driven gear and the motor drive shaft has an unknown (probably extremely low) axial loading tolerance.

Instead we ream out the pinion gear to 1.000mm, providing a <10um fit that is very snug but can be pressed on with minimal force. Loctite 609 is a permanent bonding agent intended for this type of press-fitting and can comfortably join gaps up to 10x what we have here. We will use the loctite at a later step.
Because the reamer and gear are so small, I just grabbed the reamer with a rubber glove and manually drilled by rotating the gear.



2. Remove screws

Unscrew the two door-latch side plate screws, the two visible front plate screws, two front plate screws under the flash, and most of the bottom plate screws.









I recommend leaving the screw nearest the door latch side unscrewed but in the hole, and only loosening the screw on the opposite side nearest the flash mode selector.

3. Remove Side and Front plates

Pivot the bottom plate and free both the side and front plates. They are loose at this point but may require some wiggling. It’s fine to fully remove the bottom plate, but you have to be much more mindful not to tilt the camera as the film advance gears underneath are unsecured.

Re-tighten the two bottom plate screws, or reattach partially to ensure the gears underneath don’t escape.

4. Drill the hole

Make sure you put the body cap on to reduce the amount of debris entering the mirror-box. The point you need to access is approximately under here:



I highly recommend something like a drill-press or mill for this operation as performing it freehand is somewhat tedious and easy to do wrong. If using a mill or drill-press, secure the camera and setup oil-free compressed air blowing into the gap above the drill point. This blows out any drilling debris as its generated and keeps the inside of the camera clean.

If using a freehand drill, it’s best to start with a pilot hole to get a feel for the thickness of the plastic and provide a guide for the larger drill bit



Now drill the hole with the larger bit. If freehand drilling, be exceptionally careful with the amount of pressure applied. It is very easy to over-penetrate and smash into/bend the driveshaft of the motor when you finally break through. You should have a hole that looks like this (but hopefully cleaner):



5. Remove the old gear

Ideally you started this only after your drive gear broke, so even if you can’t see the crack in the gear, it should simply lift off the shaft. Using curved tweezers in the area shown, either get underneath or just push the side of the pinion gear upward. It should slide off the drive shaft.





6. Prepare the new gear

Apply the smallest amount of loctite 609 possible to the bore of the new pinion gear. Capillary action will draw in the compound, so however much you think you need, use less. Be sure to remove any excess 609 from the outer gear surface.

7. Drop/push the new gear on the motor

Using tweezers, line up and push the pinion onto the motor shaft. If you bottom out the pinion against the motor, just use the tweezers from the side again to pull it up slightly so it’s not rubbing against the motor body. You have about 10 minutes to move it about before curing begins.

8. Wait

Wait 1 hour.



9. Check operation

At this point assuming you don’t live in a freezer, the bond should be well over 50% of final strength, enough to do a quick test. Put some batteries in and fire the shutter without a lens. You should see the pinion rotate and the camera should be free from any errors.

Startup Test

10. Reassemble camera

Put the side and front covers on the inverse way you removed them. When done, it’s worth removing and taking a quick look under the bottom cover to make sure the film advance gears are all still meshed together and moving freely. They can be moved around and settled by hand to fix any misplacement.



11. Wait

Wait 24 hours for bond to reach final strength

12. Enjoy

Hopefully you’ve just permanently fixed your a7. I’ve only done this repair once myself (although have three other 7s I need to repair), but so far so good. Doing it on the mill makes this a super quick repair in the neighborhood of 15 minutes minus the cure times and pretty repeatable.

Future Work/Concerns:

I’ve done some napkin math and there is a risk of the bonding agent failing at higher temperatures as the brass thermally expands more than the steel shaft. My numbers may be too conservative, but this could happen as low as 35C/95F. Summer is winding down here, but I might be able to give the camera a hot car treatment to see what happens.

The good news is a future repair will be extremely easy as the access hole is already present, and if I can source/make some steel gears in the future, the CTE mismatch won’t be a problem and temperature difference would become irrelevant.

While there may be better bonding compounds than can tolerate the gap changing over temperature, if mine does break due to the thermal mismatch, I’ll probably go the new pinion gear route instead.



UPDATE 1:

I let the camera soak in direct sun in a hot car with the mirror-box reaching over 120F/50C and did a bunch of testing with the 85 to no ill effect. I've also put a roll through at various apertures afterwards and everything seems fine, so the bonding agent might be working better than I feared.

UPDATE 2:

I'm a few rolls in and took off the front cover again to inspect the gear. There's no discernible signs of wear, and things seem stable. At this point it's just seeing how it holds up over time. I'm going to do the same to my two other bodies and call it a success.

ORIGINAL POST:

So Backstory, my last 7 finally developed the cracked aperture drive gear issue (I knew that 85/1.4 was a bad idea 😅) and took apart the camera to see how awful the repair was going to be. I design small electronics for a living, so the internals aren't that frightening, but I was not impressed by the number of small jumper wires and sheer amount of disassembly required to access the gear. While documenting the disassembly, it occurred to me:

The slipped gear is not normally accessible without removing the mirror box, but drilling a 3mm hole through the face of the plastic underneath the AF assist light will allow immediate access to the broken gear with only minimal disassembly and no soldering.

This wouldn't change any of the cosmetics as it's all underneath the front plastic cover. I've ordered the requisite pinion gears, and the one thing about the previously published repair I'm going to do differently is ream out the gear instead of modify the motor shaft. Shaft measures 0.997mm, so a 1.000mm reamer with loctite 609 should make an excellent press fit.

I'll update with how it goes, but it should turn a 8 hour tedious damage-prone repair into a 20 minute one and hopefully breathe some life back into my favorite Minolta. Maybe I'll dump the H8's firmware while I wait and poke around...

Edited by ChrisPVille - 08 September 2024 at 22:44

I’ll watch from here

Photos and part numbers/vendors please

Thanks.

You can put a sticker to cover the hole, it will help reduce the dust entering that hole.
Congratulations on the repair.