A step by step repair and servicing guide for Minolta Rokkor PF 50mm f1.7 MC-X Series lens.
The MC-X Rokkor PF is actually part of a series of lenses loosely known as the MC-X series. These were a kind of halfway house between the earlier MC Series and what would be the last Manual Focus series from Minolta, the MD series.
The MC-X series were designed with the Minolta X1 in mind so lack the second lens tab required for full program mode on later Minolta cameras. The MC-X series produced some of the finest manual focus lenses ever made including the 50mm f1.4 PG but as always there were budget lenses also created like the 50mm PF 1.7 and the 50mm f2.
A note of caution here, the MC-X series quite often bear very little relationship to each other in regards of internal design. The 50mm f2 for instance is quite different..
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Many Minolta MC and MD series lenses are very simple to service and there are general guides for the 50mm versions on this site : –
This is not the case with the MC-X 50mm f1.7 which is a horror lens to work on. ALL lenses in this series are best left to people with lens repair experience as they often contain booby traps for the unwary. The 50mm f1.7 is no exception. Without adequate testing gear like a calibrated light source and a shutter tester capable of assessing accuracy of exposure this lens is very hard work indeed. Certainly NEVER tackle one of these without a working example to refer to.
You will need some small containers for screws. I use ice cube trays for my work and use a sharpie pen to scribble as to which screws are in which cell of the container.
This lens was a scrap item and the guide was built during examination as a preparation to doing a viable lens. On this lens the front element has been wrecked and the focus feel was over stiff.
Read through the whole guide before starting so you know what you are doing and always be mindful of what you are doing when you start work. The most common beginner mistake is to press down on the lens and forget that the opposite side is exposed, this can lead to snapped off aperture levers or wrecked front optics. Keep the lens capped on the opposite side to the side you are working. Work slowly and carefully and dont rush. There are no Olympic sports involving the fastest lens rebuild and a rushed job will lead to mistakes.
You will need the following tools and parts to carry out repair and servicing of the MC-X Rokkor 50mm f1.7
Minolta MC-X Rokkor 50mm f1.7 Repair Guide – Disassembly
First remove the rear lens group. You will need a good lens spanner to get this out as they often have a thread-lock/lacquer applied. If it doesn’t shift quite easily leave it alone. There will be an opportunity to remove it later but it’s easier to manage if it comes out at the start.
Use the spanner on the outermost spanner slots.
Cap the rear of the lens and turn it over. Remove the beauty ring. There are spanner slots on the beauty ring but a rubber cone is less harmful.
Note – on the F2 version of this lens the beauty ring is glued into place and not threaded.
The beauty ring will unscrew with even pressure applied using a rubber cone.
The filter ring needs to be removed next. This is retained by 4x JIS screws. Do not remove the screws visible in the cut outs at this stage. Extract only the filter ring retainer screws.
Set the focus ring to infinity. You need to make sure this stays there until you can get some measurements of the helicoils and also mark the helicoil positions.
On this lens getting the focus to stay in place was easy – the focus was very stiff – almost stuck in fact. On some copies the focus may be excessively loose.
The front lens group can now be removed. You will need a lens spanner for this. Use the spanner on the outermost slots only.
The front optical unit will now pull free. You will usually find at this point oil traces over much of the interior. Normally the inner optical surfaces will have oil haze on MC-X types although their diaphragms almost never suffer oil contamination.
And here is the big problem with the MC-X series. On every other Minolta lens the diaphragm is ‘keyed’ to a certain position which makes managing the lens quite easy. The MC-X series have floating diaphragms. In this case the diaphragm is held into its position by a circular retainer which is held by 4 screws. With the retainer removed the diaphragm is free to ‘float’ to any position. It gets worse – you can’t mark the position easily because the only adjacent parts are also able to rotate.
Your choices are either make markings of absolutely EVERYTHING (relationship of diaphragm position to focus helicoil and also mark the focus helicoil entry and exit points and their relative depths) OR manage the lens later using a calibrated light source and a shutter tester capable of assessing exposure errors and using a working copy to compare with. This is why the MC-X series is not a lens for beginners.
To remove the diaphragm remove the four JIS screws that secure the retainer ring.
Here’s the retainer ring being removed. The diaphragm is now free to turn in either direction without respect to the aperture control ring.
With the retainer removed the diaphragm will simply pull free from the lens body.
If you were unable to extract the rear optical group earlier it will come out with the diaphragm which will allow you to apply some Isoprop to it to break down any threadlock. Be careful because the back of the diaphragm contains aperture control stop down lever which is quite fragile.
Now it’s time to get the rest of the lens to pieces. Start by removing the rear trim ring/cover. This is retained by three tiny slotted head grub screws.
WARNING – these are VERY small.
Here’s the rear cover being pulled away after the three grubs screws are removed.
Set the aperture control ring to f1.7
With the lens set to f1.7 you will be able to see the lens rear assembly retainer screws through the cut outs on the rear of the aperture control ring.
Set the lens to infinity focus before going further and keep checking that it hasn’t shifted as you go forward.
Remove the four JIS screws. These hold the rear of the lens to the body.
The rear portion of the lens will now separate from the body.
MC-X lenses also have another difference to any other Minolta manual focus lens. This screw on the side of the aperture control ring is an access point for the small ball bearing which provides the click stops for the aperture.
You do not need to remove this screw unless you wish. You will need a very unique driver to extract it without damaging it. I use a driver designed for slotted Nikon lens screws.
With the screw removed you will find the standard 1.2mm bearing. The spring to hold the ball is also inside the screw body.
When reassembling this does provide an easier way to get the bearing back into position but it does require the use of a special screwdriver that has a vertical cut blade and is very thin but quite wide.
Looking inside the aperture control ring you will see this curved piece. This controls the opening of the diaphragm.
Unfortunately along with the diaphragm being free floating this part can also affect the diaphragm opening. If you wish to separate the rear portion of the lens getting this aligned on rebuilding will be very troublesome.
To remove the aperture control curve, turn the lens over and remove the two screws. Make sure you mark the position of the curve. It will be critical later.
Unless you really, REALLY need to remove this you are best advised to leave it alone. Getting the aperture right is hard enough without having to worry about getting this aligned as well.
The aperture control curve can now be manoeuvred out. Note its orientation.
The aperture control ring can now be removed from the lens body. When you do this take care as the small ball bearing can fly off. It’s best to do this in a large Tupperware box or a bucket to reduce the risk of losing the bearing.
If you did not remove the screw fitting earlier that retains the spring and the ball bearing, extract the spring now using a cocktail stick. The bearings are replaceable but the springs are not, so store the spring and the bearing safely. I use an old film container.
Remove the focus barrel by releasing the three JIS screws. Be careful as each screw has a very small washer underneath it.
With the screws removed the focus barrel will simply slip off.
It is assumed you set the lens to infinity focus at the start or at the checkpoint earlier
Measure the helicoil gaps and make a note. You need measurements between the inner (black/silver) helicoils and the brass helicoils plus the brass helicoil to the silver lens body.
You also need to mark their relative positions AND their exit points as they separate.
Remove the three screws that retain the focus helicoil lock tab.
The helicoils can now be unscrewed and cleaned.
With the lens almost completely stripped you can now clean all of the lens body work.
Unless you have issues with the diaphragm simply wipe its exterior surface clean and run a cotton bud around inside AVOIDING the aperture blades. Normally I would suggest stripping an oily diaphragm but the MC-X is such a nasty lens to work on I would most likely use a freon free spray degreaser and then soak in Isoprop.
The rest of the lens body parts can be cleaned with Isoprop or hot water and a mild detergent. When I service a lens EVERYTHING gets cleaned. For final finishing I flush all parts BUT NOT THE INNER HELICOIL AND DIAPHRAGM under cold water and then blow dry.
The helicoils and lens body thread for the outer helicoil are then chased out with cocktail sticks to ensure that all thread surfaces are free of dirt and contaminants. Finally make sure the helicoils are running smoothly without lubricants to assess if any threads are damaged. Very often people have seen You Tube videos and poured lighter fuel in the lens to get the focus moving. This is a fast fix but creates long term issues with excessive thread wear and as a result you may need extreme measures like a lapping compound to polish out any defects in the thread.
Minolta MC-X Rokkor PF 50mm f1.7 Repair Guide – Reassembly
When reassembling do not over-tighten any screws. This lens can be awkward and you may end up disassembling. The name of the game is lightly tighten stuff down. Test the lens and if all is well go back and retighten screws after you are sure all is working well.
First lubricate the helicoils. I usually use Helimax XP for most Minolta lenses as it provides a perfect damping while retaining a good feel. It’s also able to work at both high and low temperatures and resists migration.
The outer metal helicoil needs only a light coating while the inner helicoils needs a slightly thicker dose. Getting the feel right is a bit of witchcraft and comes with experience so you will need to experiment to get the feel right. The one good thing about the MCX series is they are not super fussy about dosing on the lubrication. An even thin coat on the brass outer helicoils and a slightly thicker coat on the aluminium inner helicoil will be fine.
When re-assembling the helicoils the name of the game is to get the helicoils at the same distances from each other and the body as you measured earlier and also aligned with the marks you made PLUS…
Getting a good alignment with the focus lock tab. Assuming you marked both helicoils with their infinity position and measured the distances correctly they should align at the locking tab position correctly. These screws can be tightened down if you are sure about the focus alignment
Refit the aperture control ring to the lens body. Note that there is a cut out in the aperture control ring that must align with a brass screw on the lens body. This acts as the end stop for the aperture control ring.
You will need to make sure this is aligned. You will need to replace the spring and ball bearing prior to this. Apply a little grease to the ball bearing to ‘glue’ it to the ring and then apply the ring at an angle to trap the bearing and the push the rest of the ring into place. This is the same procedure for most Minolta lenses. I didn’t get a picture of this but you can refer to any of the other Minolta lens guides on the site to get an idea of how to do this.
Refit the aperture control curve – notice how the silver stud on the aperture mechanism fits against the curve. You will need to move the sprung aperture part out of the way to get the curve in place.
The positioning of the curve is critical and will affect the lens aperture response.
The curves position is regulated by the two screws. If you made markings align as best as you can at this point…
Replace the screws that retain the curve. Don’t over-tighten these as the fixing point is plastic and it’s easy to strip the threads. You only need as much pressure as you can exert between thumb and forefinger on the screwdriver barrel.
Replace the focusing helicoils to the lens body. Note that the body has a cut out for the focus lock tab. These have to align.
With the aperture set to f1.7 and the lens helicoil focus tab correct orientated to the body cut out the fixing screw holes will be visible.
In the picture a cocktail stick is holding the hole position to the slot and blocking the aperture ring from turning out of position.
Replace the screws that retain the lens body. These can be done tightly but dont over tighten. Remember the screws holes are threaded into aluminium and it’s easy to strip threads out.
During the following phase keep your wits about you. You can’t have the rear of the lens capped so remember this because the fragile stop down lever is exposed and subject to breaking if you press down on the lens.
Note the cut out in the aperture lever (in the picture it’s just to the right side of the stop down lever on the rear of the lens).
The diaphragm control lever needs to be positioned into the cut out in the aperture lever.
Here is the diaphragm barrel being replaced. Unlike an MC series there is no flat on the barrel as an alignment point.
You can now see that by rotating the diaphragm barrel the diaphragm will open and close. You now need to tune this. At wide open (f1.7) the aperture blades should be just slightly out of sight behind the diaphragm mask. At minimum aperture (f16) the aperture should close down. Across the range the aperture should open/close evenly with each change of aperture stop.
This is a particularly tough job with the MC-X series because both the position of the diaphragm barrel AND the positioning of the aperture control curve will affect the aperture opening. Without a known good lens to compare with this is a very hard job. Even with a working lens it can still be a painful process requiring a lot of patience. It’s a very difficult lens to work with/on which is why I would advise people not to mess with the MC-X series.
I managed getting the aperture right by getting a close visual approximation and then loosely fitting the optical elements. By fitting the lens to a known good camera with reliable shutter speeds I am able to test exposure error at each setting and by seeing the results. I can fine tune the lens. Even with experience and test equipment the MC-X series is no fun to work on.
Assuming you are happy with the aperture/diaphragm performance you can now refit the diaphragm retainer…
FIT THE REAR CAP TO THE LENS NOW – YOU DONT WANT TO BUST THE STOP DOWN LEVER!
…and replace the 4x JIS screws to hold it in place.
Replace the rear trim ring with its grub screws. The rings fixing position is not specific but it’s best if you can to line it up with the original positioning. Normally there will be marks on the lens body where the grub screws were. If not don’t worry and just refit as is.
Replace the front optical unit after giving its inner surface a clean and blowing out the lens interior. Do not use harsh chemicals – the MC-X series can have very soft inner coatings. A very dilute Isoprop, breath and a fine lint free cloth gently applied is all that will be needed in most cases.
Unless there are issues like fungus it is best not to disassemble the optical units. There is always a small amount of dust from when they were manufactured and trying to get rid of it usually just means moving the dust around or adding some fresh dust. Tighten the unit down with a lens spanner – it only needs to be hand tight plus a small tweak.
Replace the rear optical unit after giving its inner surface a clean.
Mount the lens to a known good camera and bring the lens to infinity focus. Use a split image viewfinder and get infinity by focusing on something distant. I normally use two radio towers which are around 8 miles away. You can focus the lens using the brass outer helicoil.
DO NOT WIND THE LENS TOO FAR OUT or you will risk disconnecting the inner helicoil from the lock tab.
With the lens at infinity replace the focus barrel so that the infinity mark is aligned with the index mark.
Refit the focus barrel retainer screws with their washers. As ever do not over-tighten.
Replace the filter ring and refit the retaining screws.
Replace the beauty ring. Take care as the beauty ring is plastic and it’s easy to cross thread this and jam it. A rubber cone works best for this.
Assuming all has gone well – give yourself some congratulations, have a cup of tea (or the beverage of choice) and get ready to get some great pictures.
The MC-X Series are a tough challenge but I hope this guide will have at least eased some of the way.
This Minolta MC-X Series Lens Repair Guide article can always be improved. If you have any questions and or comments we would love to hear them in the comments below. Mel.
Mel is one of the driving forces behind High 5 Cameras and writes all our articles.
Starting serious photography back in 1972. Over the years she got to shoot film with most of the major brands in 35mm and large format as both a studio photographer and content provider for websites in the early life of the web. These days she is rediscovering photography and has become the GOTO person for knowledge on camera repair advice.
hey! is the aperture ring in this model plastic or metal?
Plastic – even later model SRTs started using plastic aperture rings.