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Ezio
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, April 17, 2004 - 11:35 am: | 
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This is driving me crazy - not because I cannot repair it, but because I cannot understand what it is. A few months ago, I initiated a conversation on a Revue 400SE (Minolta 7 SII clone) with a "blind" meter. The meter responds promptly to light and is not erratic, but it sees far less light than it should; say, when it should be 1/125 f=11, it says 1/125 f=2. The needle is not blocked, because if I shift sensibility to the maximum (ASA 800) and point the camera towards the sun at 1/30, the needle goes beyond f=22. Despite some advice on this forum, I wasn't able to even start to understand what is wrong. Well, now I've got a 7SII which has exactly the same problem! What's more, the two "blind" meters are very much in accordance with each other, and produce consistently the same (low) readings across the entire range! The battery compartments are clean, and even after removing the top plates I cannot see any trace of corrosion. It is as if the circuits work, but the meters receive too little electricity??? |
Jim Brokaw
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, April 17, 2004 - 06:05 pm: |
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This could well be an 'electronic' problem. The CdS meter circuit works because the CdS cell resistance changes in proportion to the light hitting it. It sounds like that part works. However, the resistance sounds like it is at the wrong level relative to the light hitting for the proper exposure... when the cell resistance should be (for example only) 40,000 Ohms, it is actually 400,000 Ohms, so the meter thinks the amount of light is different than it actually is. I suppose if you can get at the leads for the CdS cell you could check the resistance relative to the light level, at least for 'dark' (cell covered) and light (with lamp close to cell). But I don't know what the levels -should- be.
Also, I have very hazy memory of seeing somewhere that CdS cells 'wear out' or 'fade' with time? too much light for too long? I can't remember any details, but it seems that they might fail with age, somewhat like the selenium meter cells in so many old cameras are now doing. The good thing is that you can still buy CdS cells, in a variety of sizes and resistance ranges.
Since it is a resistance, you can also adjust the sensitivity of the circuit by adding in a resistor or miniature potentiometer in series with the cell... but first you need a cell that is over-responding, in order to 'turn it down' by adding resistance. This probably can be figured out and the problems fixed, given the resources of this list! |
Donald Qualls
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, April 17, 2004 - 08:01 pm: |
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If you check your meter by pointing the camera at the sun and watching the needle go off the dial past f/22, that itself might be the cause of your meter reading wrong. The CdS metering cell might have been damaged by the extremely bright light or heat of the sun. However, first be sure it really is reading incorrectly. At an ISO setting of 100, an averaging meter viewing a scene with the sun behind you and not too much sky in the meter's field (which is typically smaller than the full viewfinder, likely about 1/2 that dimension in each direction) should give a reading of 1/100 at f/16, or 1/50 at f/22 (give or take the granularity of your shutter speeds; you might get 1/125 instead of 1/100, for instance). Or if shutter priority, ISO 100 with the shutter set to 1/125 should give an aperture of just wider than f/16 in the above conditions.
Assuming this isn't the case, and with two cameras giving the same error, I wonder if you haven't turned on exposure compensation of some kind, or mounted a lens that's incompatible with wide-open metering on both cameras -- or damaged the meter cells. Does this happen with the original lens, or only with an aftermarket lens you might be trying to use on both bodies? Is it even a possibilty you might have a 1.5 V alkaline battery installed, when the camera wants a 3 V lithium cell, or some similar voltage error? |
paul ron
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, April 17, 2004 - 08:53 pm: |
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Ah exactley my question... which battery is in it? Maybe you are using the wrong battery which posably has the wrong amperage and voltage for your equipment.
If you were to correct the meter readings to coencide with another known accurate meter, will it be linear throughout form dark to light situations?
In my basement here I have 8 foot double florecent lamps and use them as my standard light source for testing my meters. I find some meters are off but I can adjust the ASA to compensate to the correct readings and it seems to work although some meters such, as my old Westons and 30 year old cameras are just too old to be linear as the cells are shot.
Also another thing to remember is that old CDS cells have memory if exposed to very bright ligh, but with direct sun, you can blow the cell too. |
Ezio
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, April 18, 2004 - 08:09 am: |
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Both of these are RF cameras with fixed lens. The Minolta uses a D76 1.5V cell, while the Revue should in theory have the now discontinued PX675 1.35V mercury. I made my tests with a D76, which has the same size as the PX675. If anything, the higher voltage should cause the meter to see too much light and not too little. There is no questions that the meters are both active but way off the mark, well beyond the limits of what can normally be compensated with the ASA setting. If my Lunasix says it should be 1/125 f=11 with ASA 100 film, both meters say something like 1/30 f=2 or 2.8. By putting the ASA setting at the maximum (800) they say 1/125 f=4 or 5.6. Yes, dying CDS cells could be the answer, but I seem to remember a conversation suggesting that they just drop dead rather then getting blind. |
paul ron
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, April 18, 2004 - 08:27 am: |
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Sounds like you've been very thourough in your investigation and it is strange they would just start to go blind gradually. I have never noticed this before but I have never used one that fizzeled I just noticed it was completely dead after not using it for 30 years.
When you opened them to inspect the circuitry, did you notice any old solder joints that might be causing resistance? Lead does oxidize, so an old weak joint may be a suspect too. If you have access to the inards, freshen the sloder joints by touching a hot low wattage iron to them and see if this improves anything. |
lee crump
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, April 20, 2004 - 08:49 am: |
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I work with Minolta SRT series cameras. These cameras originally used mercury batteries that were prone to "exploding". Many of these batteries were replaced with batteries with varying voltages. (Are you using the same battery in both cameras?) When this is done an adjustment needs to be made to a potientiometer inside the bottom plate to compinsate for the differnt voltage. Unfortunately I cannot post the diagrams from the service manual I have. If you would like these diagrams I will e-mail them to you. To trace this potientiometer (on the SRT) i follow the wire from the battery compartment to a switch that closes either the battery check or the meter circut. Both of these curcuts begin with a potientiometer that can be adjusted with a flat blade screw driver. Good luck; I check this thread later |
Ezio
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, April 20, 2004 - 01:17 pm: |
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Thanks Lee, but unfortunately I don't think that adjusting the potentiometer will do. Based on my experience with other cameras, the variation is at the maximum 2 stops or so in each direction. Here we are talking something like ten times that! I'm testing both cameras with L44 batteries. These are 1.5V, but if anything the higher voltage should result in the meter "seeying" more light than real, not less. |
Werner J. Becker
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, June 10, 2004 - 05:54 am: |
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Hi, out there, I am faced with exactly the same problem as Ezio, the only difference is that one of the cameras with false meter readings is a Ricoh 500 GX (at least two f-stops less) and the other one a Revue 400 SE (= the above mentioned Hi-matic 7 S II clone), with more than 4 f stops less. I tried different batteries and CdS-cells with always almost identical results, when I measured resistance of the CdS-cells in the dark and in full light I discovered that resistance varied from approx. 1.5 kohms (light) to 5 to 8.5 kohms (less light to darkness). When I tested these cells in my Konica Auto S 2 they produced correct readings of the meter (compared to other Konicas) in different light. My theory, in accordance with Ezios findings, is that these cells in some circuits, as in that of the Revues and Mi-matics, are not linear, as in extreme positions they produce sensible readings.In between these positions the reading of the needle does not follow changes of the ASA or sec dial.BTW, does anybody know where can one buy suitable new LDRs in GER ? |
Werner J. Becker
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, October 28, 2004 - 06:27 am: |
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Have just discovered that the false readings of the exposure meter in a Ricoh 500 GX was caused by a defective soldering joint inside the meter itself. As in other cameras of that time (mid 60's to late 70's) the ends of the hair thin insulated copper wire of the galvanometer spool were possibly not prepared for soldering by scratching off the insulating coating mechanically but by removing it with the help of diluted acids ("soldering water", this causing havoc after so many years. The soldering joints still look perfect, but they are electrically faulty or even dead thus producing false (some f-stops off) or no meter readings at all! Finding out about all this took me quite a while because I didn't think in that direction and suspected the variable resistors of the meter calibration and the LDRs instead.The whole thing almost gave me the creeps. I own several "el cheapo" clones of the Cosina (Compact) 35 (E), like the Porst 135 L, or rebadged versions like the Revue electronic C,and also a largely identical Konica C 35 automatic etc. with meter problems of the above mentioned sort. It was either the spool or the two delicate retracting springs (part of the electric circuit !!) whose soldering turned out to be poor. Unfortunately there is nothing to repair inside the meter for the average camera tinker: even small soldering tips are far too big, and what is worse they sometimes are magnetic and attract those springs like mad which already bend just by carefully inspecting them! It adds to the problem that in some cases the glued-on tips of the galvanometer have come loose in their supporting bearings, this is another reason for erronous readings, and in the case of needle-trap mechanisms leads to over- or underexposures. Re-glueing is generally difficult. Installation of "new" meter units would solve all those problems, so if everybody out there knows a source, please let me /us know, otherwise one has to salvage them from junk cameras, if they are still ok. In most cases they are not. |
Ernesto
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, November 22, 2004 - 10:55 am: |
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Gentlemen: One source to obtain LDRs (light dependant resistors) Is Farnell in the UK. The 2004 catalog lists several Silonex types ranging from 3 mm in dia. and up, so I guess they could fit into a camera. For moving coil instruments, it is very diffcult to find small enough moving coil assemblies that would fit into a camera. There are some Taiwan or China based manuf. which would be able to supply them, but I had very scarce luck when trying to find one that could sell just one or two pcs. Good luck! |
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