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Wayne
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, July 22, 2005 - 05:29 am: | 
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I've recently acquired a 7sII which has an active meter but it indicates about 5 stops below what it should. I've searched the archives and have found that others have seen the same phenomenon in 7sII's (and clones), but has anybody ever resolved this problem successfully? I've got a fresh battery (admittedly 1.5V instead of 1.35) and I've resoldered all of the joins in the circuit but the problem remains the same. The disc that covers more or less of the CDS cell according to ASA or shutter speed setting is correctly positioned and moving as it should. Would it most likely be the CDS cell or the galvanometer at fault here (or something else)? Can the CDS cells lose some of their sensitivity or would they just stop working altogether? I believe you can get replacement cells from electronics suppliers, so maybe my next step would be to try a replacement. The only source of a replacement galvanometer would be a parts camera I imagine? I already have a 60's model 7s which is a fine camera with a very sharp lens, but it is quite bulky. I have plenty of classic cameras which require me to use a seperate meter but would love to have this little rangefinder to carry around with it's own working meter. Any help would be much appreciated.
Cheers, Wayne |
Ezio
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, July 25, 2005 - 09:06 am: |
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I'm one of those who initiated very much the same tread a while ago, and unfortunately the answer is no - I went through your very same routine to clean/resolder all contacts, but the defect is still there. I had never heard previously of CDS cells getting "old" this way, but I had since encountered the very same problem in an old Minolta clip-on meter. The innards of this latter are much simpler, and there is no sign of anything wrong but possibly the cell. |
wayne
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, July 25, 2005 - 07:49 pm: |
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Hi Enzio, thanks for your reply.
I checked the CDS cell using a multimeter and it seemed OK according to the resistance ranges I've seen mentioned in the archives. However I found that a local electronics chain store here in Australia ("Dick Smith Electronics") stocks a CDS cell (part # Z-4801) that is exactly the same size and shape as the Hi-matic part. It was only $2.50 so I bought one to try. Unfortunately it didn't make any difference to the readings.
So it looks like my problem lies in the galvanometer. I adjusted the linearity using the variable resistor at the back and the error is now about 4 stops or less. The resistance I've measured across the galvanometer is about 3.2kohm. In previous posts Winfried suggests that 0.5 - 2kohms is the normal range for camera galvanometers, so I wonder if mine is has too much resistance and this is causing the problem. I'm not much of an electronics person, so any suggestions would be much appreciated.
Previous posts suggest that the absolute reading of galvanometers in some cameras such as the Canonet can be adjusted by loosening a screw and turning the meter. I can't see any provision for this in the Hi-matic - the only adjustment screw I can see looks to be for the bearings in the meter. Does anybody else know of any adjustments that are possible?
Cheers, Wayne |
Winfried
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, July 26, 2005 - 04:07 am: |
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The value I indicated is just a rough guideline, and it is possible that some galvanometers are designed with a higher coil resistance. However, the coil resistance will not change over time (except when the meter is broken but then it will be inf.).
You are right that the method of turning the whole galvanometer is more efficient - it was used in a few cameras only. Most others use series and/or parallel resistors to adjust the meter. This method works in most cases, but sometimes it is not possible to get both a linear AND a correct reading.
I don't know whether I ever opened my 7SII but in the german clone Revue400SE there is an adjustable resistor behind the meter. However, with a single resistor you only can adjust linearity but this adjustment will affect the absolute values, too.
Concerning replacing CdS cells this is not easy sometimes. I found that most japanese cameras of the late 60s and 70s use CdS resistors with very similar characteristics. I know that many CdS cells available off-the-shelf today have much higher 'bright light' resistance than those used in old cameras. |
Wayne
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, July 26, 2005 - 06:30 pm: |
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Thanks for that Winfried. Your contributions are always appreciated.
I've now found something that seems to adjust the absolute readings in the galvanometer. At the 'front' of the meter (the end where the indicator needle is attached) there's a forked lever which is soldered to the end of the spiral spring. The lever was glued in position at the factory, but I've removed the glue and moved it to change the meter readings. I wonder if this is the same effect as turning the whole meter in cameras such as the canonets?
Moving the lever to loosen the spring produces lower readings and tensioning the spring increases the readings. A small movement makes a big difference. By moving this lever to increase the readings and adjusting the resistor at the 'back' of the meter I've managed to achieve readings that are just about correct (and linear). I just need to do a little more fine tuning to get it spot on.
I realise that I haven't found the original cause of the low readings, and that this adjustment is just a way around it, but if it stays accurate then I'm happy.
Cheers
Wayne |
Winfried
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, July 27, 2005 - 12:22 am: |
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The lever you found is the same lever which is used to adjust the zero position of the meter needle in (old) analog instruments. It will not work the same way as rotating the whole galvanometer in the QL17. However, I am glad you found a way to get correct readings from your camera.
I have no experience with the 7SII besides my own item but I have heard from others that the meter on some items of the german Revue400SE clone is up to 2 f-stops off. Over the years I have bought and sold several 400SE's (and their sister, Vivitar35ES). Fortunately all their meters were correct, except for a very ugly 400SE I cannibalized. |
Wayne
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, July 28, 2005 - 06:54 pm: |
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I've now got a meter that seems accurate at all but the very lowest light values, but maybe these meters aren't normally that accurate under those conditions anyhow.
Winfried, forgive my ignorance, but does moving the zero position lever have any electrical influence or is purely the physical effect of tightening the spring and making the needle more 'reactive'? If it's just a phyical effect I'm wondering if there was some binding in the meters bearings which is being overcome by tensioning the spring? Perhaps I should have tried adjusting the screw behind the variable resistor which (I think) controls the tightness off the bearings?
I don't think I'll play with it any more because it's working very well now, but I'm still curious about what the original problem may have been.
Cheers, Wayne |
Winfried
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, July 29, 2005 - 12:12 am: |
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Your mechanical assumptions are correct. The zero adjustment lever just controls the start position of the spiral hairspring. Of course, there will be limits for mechanical travel of the pointer. If you set the lever to a position which will push the pointer against the mechanical stop the tension of the spring will increase. But moving the zero position in the 'normal range' will not tension the spring.
The screws you found are for adjustment of mechanical play of the bearings. BTW sometimes even galvanometers get gummed up, and a tiny droplet of lighter fluid on the bearings will help. More often particles get into the gap between the moving coil and the ring magnet. If the particles are magnetic they will stick to the magnet. You can remove them by carefully blowing 'canned air' into the gap, but beware, don't blow away the spiral spring etc.
Also, check whether the spiral is tangled. If so, it's time to replace the meter, it's almost impossible (at least I never managed to) to re-shape the spring(s). The windings should never touch each other when the coil is moving. |
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