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GanjaTron
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, August 08, 2005 - 02:48 pm: | 
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Hi folks,
I'm new here (and to camera repair), so please bear with me. This is a rather long one for the SL35 gurus out there. (Are there any? Does anyone else actually still *use* this camera?)
I've got two Rolleiflex SL35's here (Singapore and Germany). I've mostly used the German model these last few years and noticed no systematic exposure errors (predominantly with slide film), while the Singaporean sat on the shelf. Recently I compared their light meter readings, and they differed substantially, so I figured the Singaporean needed adjustment.
Armed with Prochnow's "Rollei Technical Report", I decided to try my hand at calibrating the meter. After carefully removing the cap, I found 3 pots for meter adjustment above the lens mount, corresponding to the low, mid, and high ranges (located respectively at the left, centre, and right as viewed from the front).
I then followed the procedure listed on page 224 in Prochnow's book. It involves setting the meter to 50 ASA and stopping the lens down to f/4 (I used the standard f/1.8 50mm Planar), then nulling the meter for the following EVs from a light source (I used a lamp behind a diffusor hooked up to a lab PSU and adjusted the lamp's EV with a Gossen Lunasix):
EV 8 (mid range): set SL35 to 1/30s
EV 3 (low range): set SL35 to 1s
EV 13 (high range): set SL35 to 1/1000s
Those familiar with EVs will notice that these are *not* valid exposure times for f/4, but rather f/2.8! I checked my German SL35 with this procedure, and it was absolutely dead on! Can someone please enlighten me why these cameras are supposed to be calibrated for underexposure by 1 stop? After all, the camera does expose correctly on film. Is this an SLR quirk I've missed in all these years?
Somewhat stupefied, I assumed the EVs in the book are *not* typos and proceeded to calibrate the Singaropean model. Nulling the meter in the low and high ranges with their respective pots was a cinch, but there's no way in hell I can null the mid range -- the meter's off by at least 2 stops, despite turning the centre pot all the way. Does the pot need replacement? Is there an easy way to fix this? I'm not reeeaaally inclined to remove the entire meter and finder assembly...
Any hints greatly appreciated!
--GanjaTron |
Robert
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, August 12, 2005 - 04:05 pm: |
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Yes, this is a quirk you've missed! If you were to habitually photograph illuminated screens, then you should deviate from the service manuals instructions.
As for that pot, it's probably needs cleaning, or more likely, based on your description, the center contact is not make good contact with the resistance band. |
Robert
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, August 16, 2005 - 09:52 pm: |
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After digging out the meter schematic and studying the manual a bit, I think your problem is just (very) old CdS cells. They frequently age from a linear response to a non-linear response, the line on the graph goes from a straight(ish) line to a U-shape. Then you can get the low and high ranges in spec easily, but the center will be way out. At least with the Rollei's 3 pots you can fit the curve to some degree. |
Roland Schregle
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, August 17, 2005 - 09:36 am: |
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Thanks a lot for that resonse, Rob!
Yeah, I figured the cells' aging might have something to do with it (altough I assumed the response would simply level off like a logarithmic curve instead of sagging in the middle like a U-shape).
Replacing the cells isn't exactly a cinch, though from the diagrams in Prochnow's book it doens't look like I need to remove the entire finder assembly. If I remove the retaining clip over the finder eyepiece I might be able to just pull them out.
Any idea how many kOhms they have? Can I get a rough figure by measuring the old cells in darkness?
Thanks a lot for the hint! |
Winfried
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, August 18, 2005 - 12:21 am: |
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CdS photo resistors typically have very high resistance (more than 1 MOhm) in darkness - that's why many cameras do not have a meter switch, they are virtually switched off as soon as you block the light with a lens cap.
Usually the resistance at 100 lux is indicated in CdS data. However, it is just a game of luck to find a suitable one. RS, Farnell and other electronic components suppliers still have CdS cells, mostly in a case design identical to that used on many cameras.
BTW cadmium (one ingredient of these cells) will be banned in the near future, so probably noone will make CdS resistors any more. Hamamatsu has designed a photodiode-amplifier arrangement which is supposed to work as a replacement. |
Roland Schregle
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, August 19, 2005 - 07:37 am: |
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Hi Rob,
thanks for the info. So I need to specifically look for CsS photoresistors, right? Do your schematics list any specific type?
I see a bunch of photoresistors in Reichelt's catalog, though CdS isn't mentioned anywhere. The resistance at 10 lx is indicated and ranges between 16 and 340 kOhms, depending on type. I'll try RS components, since I've managed to get some exotic stuff from them in the past.
Btw, still haven't managed to extract the old ones. I've removed the retaining clip, but the eyepiece-and-cell assembly doesn't detach so easily (yet).
Regards,
--Roland |
Roland Schregle
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, August 23, 2005 - 10:33 am: |
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Howdy folks,
an update on the CdS thingies: I've managed to remove them after lifting off the finder prism and unsoldering them.
The photoresistors come in a gold-coloured metal casing and bear the label CL1323, which I assume to be the type, although I found nothing in a net search. Anyone know these or where I can get 'em?
I checked them out under a loupe, and *one* (not both) had corrosion on the traces. That pretty much explains everything, tho I do wonder why only one is affected.
I checked the resistance under daylight (overcast day, ca. 500lx), and got ca. 6 kOhms on the good one, and only 1.2 kOhms on the corroded one! Figures...
Now to get a replacement... hmmm.
Regards,
--Roland |
Roland Schregle
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, August 25, 2005 - 10:32 am: |
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Well, I've been looking round for replacements and I'm having a *very* hard time.
Reichelt's catalog simply lists three LDR types without any specs whatsoever, and the guy on the phone couldn't tell me more.
RS Components lists a bare CdS without the metal/glass packaging which doesn't come close to the response of the original cell.
The only cell I've found so far that comes close in packaging (TO-46) and response is the VT200 series (particularly the VT20N2) made by Perkin Elmer. I have no idea if these are CdS cells, as Perkin Elmer simply refers to the material as 'type 0' or 'type 3'. Farnell has these cells in their catalog, but doesn't sell to consumers, as I found out much to my chagrin. Anyone know where I might get these components?
I noticed another thread in which Winfried mentions a replacement cell he got from Conrad Electronic. This might be a viable replacement, although its packaging is somewhat larger (TO-18). (Btw Winfried, according to the data sheet this cell *is* manufactured by Heimann).
--Roland |
K.Sivasankaram
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, August 29, 2005 - 05:38 pm: |
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Hi Folks,
I have had similar experience with my Rollei SL-35. When new the exposure meter was reading correctly but as time went on setting the aperture by the exposure meter indication resulted in under exposure. I now open the aperture a couple of stops more till the needle in the meter goes above the notch and click. This has solved my problem. |
GanjaTron
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, April 27, 2006 - 02:43 pm: |
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Well, thought I'd get back to this thread after several months for the benefit of those searching the archives.
I experimented with several replacement photocells from Perkin Elmer (some of which were bloody difficult to get), and the upshot of it all is: *none* worked in the circuit. Apparently the tolerances are so tight that it's simply impossible to find a decent replacement without modifying the whole light meter (i.e. swapping all the potentiometers as well). Sorry for the sobering news, but I simply gave up.
I ended up using a light meter from a junk SL35, which turned out to be the *revised* meter using silver-cased photocells with a much lower resistance curve. The cells that went bad in the original meter are the older gold-cased variant found in earlier cameras. Rollei must have had some reason to change the photocells. Perhaps they were aware of problems with the gold cells.
I've put up a webpage with photos documenting the... uh... ordeal and hope it'll be useful to those with similar problems (also of interest to those who've never seen the inside of an SL35):
http://www.ganjatron.net/photo/35/sl35-lightmeter/sl35-lightmeter.html
As I said, I simply swapped light meters. If the light meter in your early vintage SL35 (particularly those made in Germany) can't be calibrated anymore, chances are it's also got gold photocells that have gone bad. Sorry, but from my experience your only recourse is to simply grab a junk SL35 off eBay and scavenge its light meter (preferably a Singaporean one with the revised circuit).
If anyone actually manages to find a decent replacement photoresistor, I'd love to hear about it. Drop me a line and I'll include the info in my webpage.
Best regards,
--Roland |
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