disassemble & clean element of Sears zoom 28-70m f/3.5-4

fstop · Joined: 11 Apr 2010 Posts: 29

This is a Sears fd 28-70mm f:3.5-4.5 one touch macro zoom lens that I got as an incidental purchase with a camera a little while ago. I wanted to give it a try when I noticed that the second element from front seems to have some hazy or oily marks on it. Apparently somebody left it uncleaned after touching it.

How do I go about opening the lens to reach that (it is right behind the front element)?

I have taken out the decoration ring around the front lens (the plastic ring with the lens specs on it), but the front element did not fall out as I was hoping.

Next, I took off the rubber grip from the zoom ring and I see four small screws under the clear tape and one screw in a slot that appears to guide the zoom motion of the lens.

fstop · Joined: 11 Apr 2010 Posts: 29

Some progress. By removing the set screw in the slot in the focus ring, I am able to unscrew the front element.

With the front element off, the inner element is accessible. I was able to clean the front surface of the inner element.

However, the oily marks are also behind this element, i.e. on it face on the rear. I am suspecting reaching that has something to do with the four screws on the focusing ring. If I try to unscrew these (by first removing the clear tape that covers them), what should I take care about? Any springs or loose parts to surprise me?

unclemack · Joined: 21 Jun 2009 Posts: 51 Location: UK frozen north

Hi, I don't know this particular lens but it appears to be of conventional design so I hope some general information may be of help.
First though, just so we're on the same page: a single piece of glass is referred to as an element - from your description I'm guessing you've removed the front (focussing) group of elements by unscrewing the multistart focus threads, sometimes called the helicoid.
Usually behind the decorator bezel there's a locking ring which is unscrewed by using a lens wrench in its 2 notches.
You should be able to see how the front group comes apart if it does come apart. Sometimes an element is held in by the edge of the housing being pressed over the edge of the glass ("peened") and if this is the case you may want to stop here.
Be very gentle with the multistart threads when you replace the focussing group - slowly "click" it backwards, then try it forwards at different start points - if you force it you can cause burrs on the alloy threads and it won't focus smoothly.
There's usually only one correct position for the multistart threads which allows that screw from the slot to align properly with the slot. You'll have to use trial and error unless you marked the housing before removing it.
The tape usually just holds the focus ring to the distance scale so that you can adjust infinity setting.
The 4 screws probably hold the outer zoom/focus grip to an internal sliding group and are best left alone unless you want to dismantle the complete lens.
Good luck!

fstop · Joined: 11 Apr 2010 Posts: 29

unclemack · Joined: 21 Jun 2009 Posts: 51 Location: UK frozen north

Hi, sorry I wasn't clear which element you needed to clean.
Yes, you need to remove the screws after marking the distance scale/focus & zoom ring & removing the tape, then remove the ring.
You should then see the sliding assembly (or assemblies).
Usually with zooms of this vintage there are nylon/ptfe bushes sliding in slots.
Easy enough usually to see which bushes need to be removed to withdraw the group you need to access. Make notes, scribe adjacent parts, take pics.
Often though these elements are peened into the housing rather than being held by locking rings - not an easy job to remove & replace them.
Good luck with it, I'll check back tomorrow.

fstop · Joined: 11 Apr 2010 Posts: 29

unclemack · Joined: 21 Jun 2009 Posts: 51 Location: UK frozen north

Hi, should have added this last night but was too tired. It was a long day.
Sometimes in addition to the screws holding the focus/zoom ring there's a locking ring which can be hard to see and harder still to remove.
And, just to make life awkward, the focus/zoom ring sometimes has to be withdrawn rearwards rather than forwards - in which case it looks as if the aperture & mount lock rings would need to be removed first.
The FD with the lock ring is easier to work on than the newer FD mount if you're doing it for the first time.
Hope it turns out to be easy after all! Very Happy

fstop · Joined: 11 Apr 2010 Posts: 29

Well, I took out the focus ring by removing the four screws. It slid off the front. But I still don't how to go further.

I also removed the ring that has the aperture numbers on it. It was secure by three grub screws. It also did not reveal how to go further.

Looks like you are right. Something else needs to removed to get access to the inner elements. BTW, upon further close examination, the oily stuff is not finger marks, but looks like haze. Needs to be cleaned anyway (softens the image). And this lens is not the new fd mount, i.e. it does not have the button to press to unlock it, just a ring to turn.

Just remove the rear element might be enough to reach the middle one. So I need to remove either of these two, which ever is the easiest.

Do you have any online resources on your hands which might show with pictures how to tackle this problem further? Perhaps on similar lenses?

BTW, once I take off the front element, the next inner element has aperture blades on top. Basically, if I ever wanted to clean the blades, I just need to take off the front element. Is this order of construction usual?

Also, the mount locking ring has only one screw. I tried unscrewing it, but I wasn't sure if there are any things under spring tension underneath it? Any hints?

Thanks.

fstop · Joined: 11 Apr 2010 Posts: 29

fstop · Joined: 11 Apr 2010 Posts: 29

Well, so as I mentioned in my earlier post, all I had to do to get the rear part of the lens out was to just take out the four cross head screws on the mount. Their location is shown below (screws have been removed).

Basically, you need to rotate the locking ring a bit to get access so couple of screws. It can be rotated by pressing the locking pin and unlocking the lock ring.

Removing those screws allowed me to remove the mount part. Note: be very careful when you remove it. The part that comes off engages the lens at at least two places: one is the fork on the mount part that engages a pin on the lens. And the other is the stop down lever I think.

NB: Do not pull out the ring that has the aperture numbers written over it. My lens is not expensive so I am experimenting and learning. I removed and two ball bearing fell out. The larger one goes below the button that alows one to put the camera in "A" mode. This bearing is pressed to the lens body by a spring under that button. I am not sure what the smaller bearing is for. Haven't yet found out where to put it back. The aperture ring, however, seems to be a bit soft now.

And, lastly, here is the order of the group of lenses in the rear element.