Minolta HI-Matic G/CDS Cell Spare Part

[ccouasnon]

Hello everyvone,

I’ve undertaken the restoration of a small Minolta Hi-Matic G from 1976. It’s a model that operates with automatic exposure using a CdS photoresistor cell located in the lens and connected to the light meter.

My problem is that this component is defective, and even after consulting the service manual, I’ve been unable to find any information about it (for example, the expected resistance in darkness).

Has anyone, by any chance, already dealt with this kind of replacement on this model or on a similar camera, and would they have a method for identifying this component? Thanks in advance.

 

[Paul Howell]

At least here in the U.S there are no modern replacements, many CD cells have stopped working. I had thought about taking a Silicon Blue cell from current point and shoot for a Konica S2, but seems parts are not compatible at least with my limited knowledge. Perhaps someone with a better understanding of the engineering can figure it out.

 

[4season]

It ought to be possible to determine the range of resistances needed by temporarily installing a potentiometer, but I have not tried this. Instead, I've simply taken my best guess. In the case of a Konica S3, my first attempt was wrong (20 megohm dark resistance was much too high for that camera). But once we eliminate the parts which are too large, or which operate outside the spectrum of visible light, the selection process becomes easier.

https://eu.mouser.com/c/optoelectronics/optical-detectors-sensors/photoresistors/

 

[ic-racer]

I was able to get a near perfect linear response of my Nikon FTn meter with replacement cds cells from my parts box.

Nikon F Photomic FTN Service

Back to the camera body and lenses. Infinity focus checks perfect:

www.photrio.com

 

[ccouasnon]

I was able to get a near perfect linear response of my Nikon FTn meter with replacement cds cells from my parts box.

Nikon F Photomic FTN Service

Back to the camera body and lenses. Infinity focus checks perfect:

www.photrio.com

Thanks for your reply — your article is really interesting! Unfortunately, I don’t have a light box to test the exposure.

However, I had an idea. The only indication in the manual is that at EV11 the meter needle is in the standard position. So if I gradually power the meter using a test power supply, I should be able to determine at what voltage the needle reaches this standard position.

Since lux ≈ 2.5 × 2^EV, then 2.5 × 2^11 = 2560 lux. It should therefore be enough to find a CdS cell of the right size with a resistance curve corresponding to about 2560 lux. Then I could subtract this CdS resistance from the 1.3V battery voltage and adjust the remaining voltage with a resistor to reach the standard needle position at 2560 lux.

So the idea would be:
standard position voltage = 1.3V − CdS resistance at 2560 lux − adjustment resistor.

From an electronics perspective this seems simple enough, but I don’t know whether this reasoning actually applies to photographic exposure systems. Since you have more experience in this field, what do you think?

 

[xkaes]

Good luck. Keep in mind that the Hi-Matic G is pretty easy to find and at great prices. I have three G2 models that I've found on EBAY. I don't need three, but they were so cheap -- under $10 each -- that I couldn't refuse. Two came with cases and one came with the user manual.

So if all else fails..... You can also look for the G2 or GF

http://www.subclub.org/minman/minfind.htm

 

[julio1fer]

Thanks for your reply — your article is really interesting! Unfortunately, I don’t have a light box to test the exposure.

However, I had an idea. The only indication in the manual is that at EV11 the meter needle is in the standard position. So if I gradually power the meter using a test power supply, I should be able to determine at what voltage the needle reaches this standard position.

Since lux ≈ 2.5 × 2^EV, then 2.5 × 2^11 = 2560 lux. It should therefore be enough to find a CdS cell of the right size with a resistance curve corresponding to about 2560 lux. Then I could subtract this CdS resistance from the 1.3V battery voltage and adjust the remaining voltage with a resistor to reach the standard needle position at 2560 lux.

So the idea would be:
standard position voltage = 1.3V − CdS resistance at 2560 lux − adjustment resistor.

From an electronics perspective this seems simple enough, but I don’t know whether this reasoning actually applies to photographic exposure systems. Since you have more experience in this field, what do you

Safe path would be to insert a potentiometer instead of the CdS, adjust to get the reading for EV11 in the meter, and use that resistance to look for a CdS cell matching it at 2560 lux.

 

[4season]

Don't forget that when it's installed in the camera, that CdS cell sits behind a variable mask or slit which reduces the amount of light reaching it. That's why I felt some trial-and-error would be involved.

 

[Paul Howell]

If you work it out, you can make money replacing CdS cells on Nikon F and early Nikon F2 metered heads, not to mention all the other 60s to 80 vintage cameras with failing meters.

 

[ccouasnon]

Hi guys, thanks for all your replies — I really feel well supported on the forum!


I’ve carried out the first power supply tests on the galvanometer. Based on the needle reading, at an exposure of EV11 the galvanometer should be supplied with a voltage of 0.3V, so I decided to follow your advice.


I bought a box with various resistors and a CDS cell. To run my tests, I’m planning to sit in a dark room and use a Raleno LED panel. I’ll calibrate the distance to obtain EV11 using my phone’s light sensor. Once the EV11 area is validated, I’ll just need to position the new CDS inside the lens to account for the attenuation from the ISO setting mask. With the output value measured, I’ll then fine-tune it by adding a resistor to obtain 0.3V at the output.


I’m also planning to switch to a 1.5V supply since it’s much easier to find, for example with zinc-air hearing aid batteries.


Do you think this is a viable solution? I’m expecting the components to arrive sometime next week!