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Dmax
Tinkerer
Username: Dmax
Post Number: 1
Registered: 11-2007
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, November 09, 2007 - 06:26 pm: | 
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Anyone out there who has experience re-calibrating meters of Ricoh or Vivitar manual-only bodies (circa late 1980's to early '90s)? There are four trimmers/potentiometers sitting on a flexcircuit board atop the camera prism. I have done some preliminary work to see how each one affects readings, but hesitate to proceed further. Any info, general or detailed, will be appreciated. |
Hovaness
Tinkerer
Username: Hovaness
Post Number: 50
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, November 17, 2007 - 09:57 am: |
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In this type of circuit, there is usually one trimmer that controls the sensitivity of the circuit and a second one that controls the linearity. Once you have determined which is which, you set the sensitivity trimmer to give you the right exposure with a known light source, then adjust the linearity trimmer until the deviation from correct exposure is linear with different combinations of aperture and shutter speed. If the meter response is linear, each stop of aperture or shutter change will result in a one stop change in the meter reading.
The main precaution in doing this is to carefully mark the starting positions of all trimmers. Some of them are factory calibrated, and not meant to be disturbed. You always want to be able to get back to the original trimmer settings. |
Donnie_strickland
Tinkerer
Username: Donnie_strickland
Post Number: 38
Registered: 09-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, November 17, 2007 - 07:21 pm: |
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I haven't done a Ricoh or Vivitar, but I have done a Fujica AX-3 which had 4 pots and adjusted exactly as Hovaness describes. I traced the wires coming from the metering cells at the prism in order to find out which two pots to adjust. |
Dmax
Tinkerer
Username: Dmax
Post Number: 4
Registered: 11-2007
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, November 20, 2007 - 04:43 pm: |
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Hi Hovaness and Donnie,
Thanks for the input. Since the time I last posted, I had succeeded in identifying what pot controlled overall sensitivity, and what controlled linearity. I do have some informal background in camera repairs/modifications as well as basic electronics, and got the whole thing sorted out with the usual setup: constant, even light source, a good reference meter (Gossen Luna Star in this case), and a reliable multimeter that gave me the correct kilo ohm settings of the separate pots once I had nailed the correct aperture and shutter speed settings. Yes, I did record the resistance values of all 4 pots even before I began tinkering. Still wondering what the two other pots do, though. Needless to say, I won't touch them. Practical field testing of sensitivity and linearity (accurate to plus or minus 1/3 stop) have been conducted, and the results are good. Again, thanks for the information. I hope this helps guide others working on cameras with similar schematics. As a side note: Vivitar V335s have 4 pots, while the Ricoh KR 5 Super II has only two. The circuit layout and components are identical though. |
Hovaness
Tinkerer
Username: Hovaness
Post Number: 53
Registered: 07-2006
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, November 30, 2007 - 08:57 pm: |
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My guess is that the other potentiometers are there to compensate for component tolerances. When in doubt, don't touch it. |
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