Nikon DP-20 finder for the F4

Recently, as part of a beautiful, practically unused, late model Nikon F4s which (ahem) came my way, I immediately noticed a problem with the DP-20 finder. There was quite a heavy infestation of fungus on at least one of the internal eyepiece surfaces which was so bad it made viewing the lower meter readout difficult, blurring the LCD. I’d already knocked the seller down quite a bit (without knowing about this issue, of course) and so my initial reaction was not to demand a refund. I’ve fixed this kind of thing before – in lenses and more simple finder pieces – so how difficult would it be to eliminate the fungus from this otherwise pristine DP-20 and restore it to minty condition?

Well, as it turns out, quite difficult. But feasible, given care and patience.

This project required some careful planning, given the condition and obvious complexity of the finder. I got hold of the service manual for this finder which, unfortunately, is an x-generation photocopy scanned by an ancient scanner (probably) and so the image quality gives rise to a fair amount of guesswork. I was so determined to get this right that I bought a well-used finder for a cheap price, so that I could practice and note any caveats before working on the “real thing”. At worst, I’d spam it up completely and learn something in the process; at best I’d have a working spare finder that I could use for parts if need be. The practice run went very well in the end; a very grubby, fungus-infested DP-20 (though not nearly as badly affected as the minty one) with decomposing foam seals is now very clean, fungus-free, re-sealed and functioning perfectly, with just a few external scratch marks. I’m keeping it as a spare.

Now much more confident that I could do this, I set to work on the minty one. I’ll describe the steps I took in case this is of use to anyone else who wants to have a go at restoring their own finder. Note that this doesn’t require a complete disassembly but it’s worth mentioning that the finder optics and areas around are quite complex and fragile. With care there should be no need to de-solder any wires, but some patience is required.

The finder optics comprise three separate lenses in front of the prism (not including the screw-in eyepiece on the outside of the casing). The first lens is held in place by two small screws, whereas the second, which is able to travel forwards and backwards, is held in only at the top by the dioptre adjustment mechanism. The third and final lens, before we reach the viewing edge of the prism, is attached to the meter cell unit and is the most difficult to work on. The worst possible area for an infestation of fungus, if we were to try to remove it, would be the back-side of the third lens. Guess where it was in my finder.

Step one, a challenge in itself, is to remove the centre parts of the dioptre dial and meter selector dial without causing too much damage. These plastic discs cover the screws that need to be removed in order to remove the dials. They are held in with glue and are, frankly, a pig to remove because it’s likely that there will be some degree of marking to the outer parts of the dials during removal. To minimise this I covered the outer dials with insulation tape and then gently levered the covers out using a very small screwdriver. Once the screws are exposed both dials can be removed. Take care with the two washers for the meter selector dial: the white plastic one goes against the inside of the dial and the black foam one goes against the case.

After removing the four black screws of the outer case, the top can be lifted away from the body. A little bit of wiggling may be required here. The wires attaching the flash shoe to the main body are quite long and I didn’t need to detach them at all. With the main cover off the first lens can be seen directly, with the two screws holding it in place. 

Take the first lens off and inspect both surfaces; with luck any fungus will be here (this is where it was, mostly, in the “scruffy” finder with a tiny bit also on the front surface of the second lens). If there’s some on the back surface of the second lens then this can be taken out by first loosening the dioptre adjustment unit which sits on top. Notice the two small screws holding this part down (not the larger one; that can stay in place) and loosen them a a bit. Be careful here as the two mercury switches are held by this unit and they are glued to the unit underneath, yet there’s some play to take advantage of. The top adjustment unit may be lifted just enough to wiggle out the second lens, which is held there by the pin of the swinging arm. Again, inspect both surfaces.

If there’s fungus on the front surface of the third lens, it can be treated in-situ. If it’s on the back side, though, then the meter cell housing needs to be taken out. There are two screws holding this in place – a small one and a larger one, but this unit is the most difficult to get out so take extra care here. Remove both screws. Facing the lens, first lift the left side and then lift the right side a little. It may not appear to want to move much but persevere here, gently lifting either side and twisting slightly, firstly at the top, outwards, and then push up, swinging out at the base. Watch for wires snagging and gently lift them out of the way so that the lens/cell unit can swing outwards. Don’t pull it too far out. To clean the back side of this lens it will be best to turn the whole lot upside-down and move the unit out far enough to get at the surface, but no more. 

DP-20 insides
Lens no. 3 of the eyepiece group – clean and clear. Nearly half of the inside surface was covered in fungus.
DP-20 eyepiece lens group holder- side view
Side view. Notice one of the mercury switches and wires going into one of the meter cells in the lower left of the picture.

Once clean, re-assemble everything in reverse and look for any wires that may have become detached. I had just one such casualty – the black wire of one of the mercury switches – which was a simple fix with a soldering iron.

Neither the fungus in scruffy nor that in minty had etched the glass coatings at all and so I was able to remove it completely, using my usual method. There’s no secret to this, although people tend to have their preferences as to which tools and compounds to use. I have cleaned fungus from photographic optics for a few years and I’ve never had any return, using simple, easily obtainable isopropanol, cotton buds, cocktail sticks and certain brands of optical cleaning wipes which are, specifically, impregnated with alcohol. It’s the cocktail stick wrapped in an optical wipe which really does the trick, with gentle rubbing over the affected area. If necessary, the area can be finished off with an optical-grade microfibre cloth. Perhaps stronger chemicals, such as hydrogen peroxide, may be required for very well-established fungus but I’ve yet to encounter any.

Saving Pentax-M

I’ve just spent pretty much the entire day restoring a Pentax-M 50mm f/1.7.

img_0201
Fungus-free!
I not able to do much moving about at the moment and so, rather than mope around and feel sorry for myself all weekend, I thought I’d spend an hour or two doing this. Lesson number one: estimate the time required to clean and restore a lens as bad as this was and then multiply it by three.

The lens arrived this morning from a well-known auction site and my expectations were fairly low from the start; it was at a buy-now price which was about half that of most others, with no real description to speak of and no way to see close detail from pictures. This was a cheap gamble and I was expecting to do some work on it, but I wasn’t quite prepared for the state of the thing when I examined it. Not that it needed close examination to see that it was completely infested with fungus – the worst I’d ever seen.

The outside barrel wasn’t too bad; a little grubby but easily cleaned. The elements, though, looked completely stuffed. Just about every glass surface inside and out was covered in masses of threads and patches of mould. Nasty! I’ve had success at cleaning fungus from elements before but I have to say I was close to giving up on this as it was so badly affected throughout. Pretty much unusable, optically.

OK, so it may take more than an hour, I thought. More than two, even. All in all, it took about six hours! Was it worth all that time, considering price difference between this one and a good one? Well, yes and no but I favour the “yes” because it gave me a project to do whilst unable to go out and, more importantly, I have saved a now-decent lens from the scrapheap. Make do and mend!

Often, it’s said that fungal infestation of lens elements is the end for them: the fungus etches into the coatings and, sometimes, the glass itself, causing irreparable damage. Up to now, though, I’ve been lucky as I’ve been able eliminate fungus on a few old lenses without leaving much permanent damage, if any at all. This lens turned out to be no exception but it’s been the most laborious lens-fix I’ve ever done, without any mechanical fixes, even, as everything mechanical worked perfectly. It was necessary to separate all of the individual elements and clean all surfaces which is very, very difficult to do because, even with fungus all gone, getting truly crystal-clear cleaning results on internal surfaces, without any smears whatsoever, takes a lot of time and patience. Still, time is what I’ve had and it did pay off in the end. Lesson number two: do not use a blower bulb with a metal end too close to an internal surface because one small slip could result in a permanent coating mark. Doh! Thankfully tiny and won’t affect anything but still, a swear-inducing moment.

I wish I’d taken a “before” picture but I honesly didn’t think I could salvage it so didn’t bother. Believe me, it was horrendous! Here it is again, in its fully restored state (I restored the filter, too, which was also in a very sorry, mouldy state but it’s a nice Hoya one so worth saving. I had to take the glass out of the frame to clean everything but all’s well and clear with that now, too):

img_0202-1

When elements and groups are taken apart and re-assembled it’s always a good idea to check for any misplacement or decentring as a tiny amount of misplacement of any individual element can have profound optical consequences. I’m glad to say all is well, wide-open and at high magnification (digital test). I replaced the front name ring and cleaned the barrel exterior thoroughly.

One clean and clear lens to add to the collection, and a very good one at that. I could now sell it for twice the amount I paid but it’s definitely a keeper.