Minolta MD 75-200/4.5 (III): The so far unsuccessful fight against lame aperture blades

[Andreas Thaler]

This zoom has slow aperture blades, which I was able to make move again for a short time using different techniques. I'm still looking for a sustainable solution.

Maybe someone has ideas?

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A long journey that has no end

I bought the zoom for EUR 10 (USD 11) which is not only pretty but also has very good reviews on the web.

The aperture blades were oily and stuck, and there was a stain on the lens below the aperture register.

The oil problem is mentioned a lot on the internet, so at least I wasn't alone

I found a report online that also had a sluggish aperture on the topic. The lens was dismantled to resolve the problem.

I didn't want to do that because it would be too complicated for me and I prefer the shorter, and therfore safer, way. So I just skimmed through the report briefly.

More information was not available so I started the long journey without any preparation.

---

The rear optical unit can be unscrewed and the aperture is then accessible.

The condition of the aperture blades and the lens underneath.

Clearly someone was already active in the matter of cleaning and, counterproductively, soiled the lens.

With this „dipstick“ I check the iridescent reflex patterns of the coating to make sure that this side of the lens is actually dirty and not the other. A small scratched line through the stain provides certainty.

A first attempt with Zippo lighter fluid does not make the aperture blades move much more smoothly.

So I need to go deeper and see if I can open and clean the aperture register from that side.

Dismantling begins from the rear.

Here, on the top of the aperture register, you can't go any further.

In order to remove the register, I would have to disassemble it from the front and probably touch the zoom mechanism.

I don't want to do that.

So I have to start from here and flood the aperture register via openings and the aperture blades with Zippo.

I will clean the lens underneath afterwards. Hopefully it holds back the fuel so it doesn't contaminate the front lenses.

In total I make four passes with lighter fluid, which I use the syringe. Then I move the aperture blades through and dry them, which I do with lint-free Kimwipes and the Dremel with hot air nozzle.

The heating causes the lens to fog up, but the haze disappears by blowing it off with the bellows.

Be careful not to let the lens get hot. Especially since it is likely to be a composite lens and the cementing should not be damaged.

The aperture blades now run with almost no friction.

 

[Andreas Thaler]

I clean the lens with isopropyl alcohol.

The consumption of cleaning material is considerable.

Overview


It's time to assemble

I clean all parts with isopropyl alcohol.

The structure is held in place by a locking ring.

There is a thin ring underneath.

The spring is attached.

It works against the open aperture lever and closes the aperture.

At least it should.


The aperture doesn't close completely

Apparently there is still a bit too much friction in the aperture mechanism.

But I don't think it makes sense to carry out further cleaning cycles.

Four times should be enough and the aperture moves smoothly overall.

So I swap the spring for one with a little more pulling power, which I'm lucky enough to find in my spare parts supply.

That worked.

The aperture now closes completely.

Here, in the picture, it doesn't have to do that because it is currently at the widest opening.

The rear optics block can be easily screwed in using the lens sucker.

The end ring is fixed.


The test on the Minolta XD-7 shows ...

... that the aperture does not open completely after the shutter is released.

So I'm studying the aperture mechanism ...

... and notice that the spring on the left should apply a little more pressure.

I make sure of this by bending the two ends together.


Now everything works as it should.

However, springs resent such treatment and can lose the additional tension after a while.

But for now the matter is settled, the lens works

 

[Andreas Thaler]

Excursus: Bending springs - safety glasses

Since I repeatedly have to deal with springs that no longer work in their once calculated environment, for example in aperture mechanisms that have become stiff. I looked around on the topic.

As a rule, you probably won't have a suitable replacement because the small springs are tailored to the specific requirements of the device. So you have to make do.

Either with a well-stocked supply of spare parts (the reward of a device section) or by trying to bend a spring and thus restore the force ratio.

However, this can only be a makeshift solution because there is no guarantee that the spring will maintain its new shape in the long term.

Here is a helpful introduction:

„Instructions: Bend spring steel“

Anleitung: Federstahl biegen - Frag-den-heimwerker.com

Im Vergleich zu Kupfer, Aluminium oder Stahl ist das Federstahl Biegen wesentlich schwieriger. Dieses Material hat die Eigenschaft, nach einer Verformung bis zu einem gewissen Grad, wieder in die ursprüngliche Form zurückzukehren. D.h. beim Biegen muss man den Federstahl mit sehr viel Kraft...

www-frag--den--heimwerker-com.translate.goog

(Google Translate, German > English)

Anleitung: Federstahl biegen - Frag-den-heimwerker.com

Im Vergleich zu Kupfer, Aluminium oder Stahl ist das Federstahl Biegen wesentlich schwieriger. Dieses Material hat die Eigenschaft, nach einer Verformung bis zu einem gewissen Grad, wieder in die ursprüngliche Form zurückzukehren. D.h. beim Biegen muss man den Federstahl mit sehr viel Kraft...

www.frag-den-heimwerker.com

Original German Version


I wear safety glasses

for all projects, not just when bending springs.

You never know what will suddenly come your way when you are working on a device.

Small springs, parts that are stressed by them, the tip of the screwdriver slipping and moving towards the closely observing eye, etc.

Liquids picked up with elastic cleaning sticks can also be catapulted towards the face at an awkward angle. Lighter fluid, acetone, alcohol…

Safety glasses are mandatory when soldering anyway.

All of this shouldn't get in the eye or damage the expensive prescription glasses.

 

[Andreas Thaler]

Stubborn springs

Here, in my MD 75-200/4.5, I bent one of these springs so that it fights against a resistant aperture register.

After two days the aperture became tired again.

I bent it once, let's see.

Now I have found a new replacement in my stash:

The spring on the bottom left is from a Nikkor zoom and is extra powerful compared to those from Minolta.

I put my hope in it.

Otherwise all I can do is flush the aperture register again with lighter fluid.

The way of the cross, namely, removing the register and cleaning it in detail (here I would have to dismantle it from the front and also partially disassemble the zoom mechanism) I hope this saves me.

Or perhaps a second, weak spring can also be installed for reinforcement ...


Double spring force

And so I implemented it and created this double spring:

On the first spring, which was bent two times, and weakened again, there is now a second spring that is in its original shape.

Here they are both waiting to be installed:

Fortunately, the space on the retaining screw is just fine:

The left end of the upper spring is held by the left end of the lower spring:

There is enough space on the black retaining screw on the right (center of the picture):

The spring game

Both springs act on the aperture lever via translation in the direction of the arrow:

When the lens is screwed in, the aperture lever moves, against the double spring pressure, to the right and opens the aperture. It works against the shutter lever in the mirror box.

When the shutter is released, the aperture lever in the mirror box releases the aperture lever on the lens. This is now moved by the double spring and closes the aperture blades.

After exposure, the aperture lever in the mirror box returns to its previous position and the aperture opens.

---

Hopefully the spring pressure should now remain the same and the aperture closes quickly.

Let's see what it's like tomorrow

No matter what there is to do, the table is always full ...

 

[Andreas Thaler]

Advanced flushing the aperture register

Despite increasing the spring force (replacing or inserting a second spring), the aperture fails again.

This means that the aperture register has stored oil again

Since I don't want to disassemble the zoom any further and can't remove the aperture register from the back for cleaning, I thought about flooding the register again with Zippo lighter fluid.

But this time as a rinse by attaching the cannula of a syringe to the marked opening and sealing it with kneading glue. This should create more pressure when rinsing:

I hold the lens with the front facing upwards so that the fuel doesn't run into the front lenses and let it drip downwards. Alternating with the largest and smallest aperture opening and moving in between.

This should remove the oil/lighter fluid mixture more effectively than just flooding, moving the aperture blades and blotting. I've already done that four times - with unsustainable success.

 

[Andreas Thaler]

I become more and more careful when handling solvents

especially when a lot of it is used over time, such as when cleaning apertures and helicoids.

Open windows and a protection mask are mandatory.

Also a cloth to cover flames.

As stated I always wear safety glasses whenever I work.

At least you should be aware that you are not working with water, but with harmful and flammable solutions.

 

[Andreas Thaler]

It goes on

I had applied a lot of Zippo lighter fluid to the aperture blades of the MD 74-200/4.5 because they only closed with a delay due to oil contamination.

Meanwhile the aperture blades were back in place and I tried a pressure flush with Zippo lighter fluid to get the oil out of the aperture register.

To do this, after dismantling the lens from the back, I injected lighter fluid into the aperture register with the syringe. And do it quickly with pressure on the piston.

I placed the needle at this opening ...

... which I use with UHU patafix, which are kneadable adhesive pads ...

... seal it as much as possible so that the fuel runs nicely downwards out of the aperture register when the tube is turned.

This worked and I did two passes with about 5 ml of lighter fluid each.

The lens in front of the aperture became dirty, but this was completely remedied with isopropyl alcohol and a microfiber cloth.

Move the panel, wipe off the lighter fluid/oil emulsion until only residues are visible, then dry the panel register with the hot air device and constantly open and close until the aperture runs easily.

Now the lens will remain open for a few days and I will observe whether the aperture remains free of oil.


Interim conclusion

• Some work probably has to be done several times, as in this case.
• Patafix dissolved some in lighter fluid and left small residues that I was able to easily remove with isopropyl alcohol. Nevertheless, I'll stick with it because it's a simple application, but nothing should get into the aperture register.
• It remains to be seen whether the aperture register has now finally been deoiled. If not, I repeat the process. Dismantling it from the front doesn't bode well, as the aperture register is installed deep and I don't want to disassemble it any further.

 

[Andreas Thaler]

The lens is reassembled but problems remain

Since the aperture blades continued to snap together perfectly and there were no traces of oil on them, I set about assembling it.

After putting on the bayonet ring and engaging the aperture mechanism, I saw that the aperture closed again slowly or got stuck. But it could no longer be oil, because without the bayonet ring the aperture jumped jaggedly and cheerfully.

So there had to be a problem transmitting the set aperture, namely in the area of the bayonet ring.

I suspected this lever with a stud ...

... which presses horizontally on its spring-loaded counterpart in the tube to close the aperture.

The stud was obviously „a bit too long“ and pressed the mechanism on which it sits against the bayonet ring. Therefore there was friction which caused the problems with opening and closing the shutter.

Those were my thoughts.

I noted that it was strange that this part of the mechanics, which I had not changed, should suddenly no longer be in the right dimensions.

Some things just happen.

The solution was the Dremel with a corundum grinding tip.

I used this to grind off some of the stud to shorten it.

Now the aperture snapped

I was hesitant when assembling it.

A brass ring would not fit on the bayonet ring.

Which wasn't surprising since its place is under the bayonet ring.

In the photo the ring is sitting correctly.

And suddenly I realized my mistake.

This ring creates the right distance for the lever, which is now shorter!

If I had used it correctly, everything would have been fine and the Dremel wouldn't have had anything to do

Now the lever has more leeway, but this doesn't affect the correct aperture function.

I recognized the problem correctly, but solved it incorrectly

Quickly insert the massive rear optical unit into the tube ...

... and put the tools away.

Project completed - it kept me busy enough

 

[Andreas Thaler]

This zoom is really finishing me off!

I don't know how many times I've dismantled it from the rear.

Although I was able to successfully remove oil from the aperture register, the aperture didn't want to close properly again.

De-oiling was a success because the aperture mechanism worked perfectly without the coupled aperture mechanism in the bayonet ring.

But as soon as the ring with the aperture lever, which docks to its counterpart in the camera's mirror box, was attached, there were problems.

So I put everything back in the rear of the tube and did a check:

• Aperture blades - ok
• Aperture mechanics around the aperture register - ok
• Bayonet ring with aperture mechanism - some contamination under a lever that runs just above the inside of the bayonet ring.

So I dismantled the aperture mechanism in the bayonet ring to clean it with surgical spirit (which was at hand, Zippo lighter fluid does the same).

Axles and bearings each received a drop of Nyoil, a light oil.

Then reassemble everything.

The mechanism now ran smoothly, everything should now fit.

Hopefully!

This bearing also received a drop of oil.

Because friction points in a row probably add up in the mechanics, similar to the ohmic resistances in electronics.

Where things are easier, there is therefore less resistance. And the aperture should ultimately thank me for that

But the aperture didn't do that.


It still didn't want to work properly

and didn't close completely.

And on the XD-7 the aperture blades didn't completely disappear once the lens was attached, as they should when the aperture is wide open.

What else to do?

Here should be the solution.

This end of the spring must be behind its counterpart!

If the end is in front of it, under the black stud that I shortened in the last session, the aperture does crazy things.

It should have occurred to me that things weren't harmonious there.

The stud worked against the end of the spring and was not on the plate, everything was wobbly ...

I hope that the problems are now finally resolved.

With all my love, it has to end at some point.

And it's interesting that until the solution is reached, all paths have to be taken until the last one, the obviously right path, is found.

A repair manual that shows the correct positions of the components in relation to each other would be helpful here.

Lights out - dinner!

 

[Andreas Thaler]

After a few days, the aperture became slow again

As a final measure, I have added again a second spring to the bayonet ring, which is responsible for closing the aperture.

Here are the photos again from one of the previous sessions.


But now it's finally over.

If this doesn't fix it, the MD goes into the spare parts box


+++

All information provided without guarantee and use at your own risk.

 

[Andreas Thaler]

And since the problem is still not resolved, I ask you if you have any other ideas.

 

[Andreas Thaler]

I could still lubricate all moving parts of the aperture and bayonet mechanism with graphite powder. But that's a mess and as long as I don't know where the error is, it would be a pretty helpless action.

I will therefore disassemble everything again and check from the point where the aperture no longer jumps jaggedly.

 

[Andreas Thaler]

This case is still unresolved …

… and is highlighted in red in my repair log.

The latest status is that the aperture is still lame, despite repeatedly rinsing the aperture register with Zippo lighter fluid and adding a second spring to the aperture mechanism to increase the spring pressure. The latter is not a solution, but an act of desperation.

I assume that the aperture register is still oily and therefore needs to be cleaned thoroughly.

In this case, that means dismantling from the front, removing the aperture register, cleaning and re-greasing the helicoids.

A lot of work, but I can't avoid it if I want to get rid of the red entry in my repair log.

So forward to attack!

 

[Kino]

Any time I have tried to repair sticky aperture blades, I had to totally disassemble the mechanism, polish the blades with Brasso or another metal polish and then clean every bit of metal spotless of grease, oil and dirt with a solvent.

Reassemble with absolutely no lubricant and then I had a happy aperture.

 

[Andreas Thaler]

Any time I have tried to repair sticky aperture blades, I had to totally disassemble the mechanism, polish the blades with Brasso or another metal polish and then clean every bit of metal spotless of grease, oil and dirt with a solvent.

Reassemble with absolutely no lubricant and then I had a happy aperture.

I found a repair report for this zoom. The problem of a sluggish aperture, which my zoom also has, is discussed and solved.

According to this, it has to be completely disassembled to get to the aperture register.

This will be challenging, even more so when reassembling.

SR-Bajonett-Objektive » Minolta MD 75-200mm F/4.5 - träge Blende

Guten Abend! vor mir liegt ein 75-200mm in einem insgesamt sehr schönen Zustand mit einem ärgerlichen Fehler: Die Blende schleßt sehr tr&

www.so-fo.de

(German, pictures only visible after login.)

 

[dxqcanada]

Yes, you really should start trying to take these lenses down to the point where you are taking out the blades for cleaning.
I do not tend to like more "modern" zoom lenses, but I have taken down a Sony zoom lens down to the blades ... it was quiet the adventure.

 

[Andreas Thaler]

Yes, you really should start trying to take these lenses down to the point where you are taking out the blades for cleaning.
I do not tend to like more "modern" zoom lenses, but I have taken down a Sony zoom lens down to the blades ... it was quiet the adventure.

I hope all the screws can be removed this time ...

 

[Andreas Thaler]

Section and how to get to the aperture register

Since a second, flawless MD 75-200 arrived today, I changed my plan.

Instead of disassembling the zoom in a fully documented manner and then trying to reassemble it, I made a section.

This was also because I got an idea of what to expect from the repair report above.

Several tubes built into one another had to be taken apart and I simply didn't want to do the work including documentation.

Since the report mentioned is not a tutorial, I looked for my own way of getting to the aperture register. I wanted to see the reason why the aperture was jamming again.

Following the report, I disassembled the zoom from the front.

I didn't need to cut through the rubber grip ring because it was already attached separately at the factory.

But it is not a disadvantage, as the cut edges of the rubber close precisely.

This zoom is made entirely of metal parts.

The plain bearings are therefore also made of metal and not plastic.

This means that no wear and tear or blockages due to broken plastic bearings are to be expected.

Top Minolta quality.

I worked my way from screw to screw.

I couldn't loosen some of the plain bearings with the screwdriver. I got further with the Boa Micro Grabit Extractors.

Boa Tool - Micro Grabit Extractors

New page

www.boatoolcorp.com

To do this, the screw head is drilled using the milling head on the other end of the bit or a metal drill. The screw can then be unscrewed using the extractor on the cordless screwdriver in anti-clockwise direction.

This means that the borehole and its surroundings remain undamaged.

In this case, the screw can even continue to be used because the slot has been retained.

Better than milling off screw heads. Provided that you manage to drill the screw head properly and the screw is not too tight, e.g. because it is rusted or fixed with glue.


The only way to the aperture register

Only later did I realise that the route to the aperture register indicated in the German report could have ended here.

Instead of removing the lenses - and thus having the usual problem of separating and later adjusting the helicoids - here only the rear part has to be unscrewed. This removes the aperture register.

This is also the only way to remove, disassemble and clean the aperture register.

Here the dismounted aperture register after I had previously removed the lenses which would not have been necessary.

 

[Andreas Thaler]

I see traces of the last attempt to clean the aperture blades with Zippo lighter fluid, but no new oil stains. It is therefore unclear to me why the aperture has become stiff again.

If I had decided to reassemble the zoom, the problem would have remained unsolved.


Dismantling the rear parts

(See also further earlier in this post.)

Disassembly of aperture ring and aperture mechanism.

The tube with the small lenses can be unscrewed by hand.


The not recommended path to the aperture register that leads to a dead end

The front lenses are guided in helicoids for focusing.

The surrounding tube is heavily oiled.

After removing one lens, another one can be seen on the bottom of the tube, screwed directly above the aperture register.

With this tool I cannot reach the two holes for inserting the pointed bits.

That's how it works.

The lens is removed, the aperture register is exposed.

But the aperture register cannot be removed or further disassembled from this side. It is secured by three small, pointed grub screws, which are not accessible from the outside.

Only when the aperture register is unscrewed, as shown previously, can it be disassembled.

Since I had already decided at the beginning of the session not to reassemble the zoom, I tried out a few techniques that were not always non-violent.

I used the pipe wrench on the bottom left to unscrew the aperture register.

I also tried to drill or mill out the stuck plain bearings.

Only the method with the screw extractor (see above) was successful.

 

[Andreas Thaler]

Conclusion

• This zoom tends to oil the aperture register, as described on the web.
• Cleaning the aperture blades from the back several times with Zippo lighter fluid had no lasting effect. The aperture became weak again after a while.
• Even installing a second spring in the aperture mechanism (without solving the underlying problem) did not help in the long term.
• In order to clean the aperture register completely, it must be removed.
• To do this, the front tubes must be dismantled.
• In addition, the inside of the tubes should be cleaned and the helicoids should be re-greased after cleaning to prevent oil from re-applying.

+++

All information provided without guarantee and use at your own risk.