Minolta AL selenium meter ISO control

[thevdm]

Hello all, I have acquired a Minolta AL which appears to be in good working order other than perished light seals and a non working light meter.

At first I thought the selenium light meter was completely dead, but if I set the camera to 1 second at f2 and hold the cell close to a lightbulb then the needle moves fully into overexposed. Stepping it down a couple of stops the needle moves to the middle which obviously isn't right (f2 1/4 second).

Something I have noticed is that when held to a lightbulb the meter reacts to changes of shutter speed and aperture, but it does not react to changes on the ISO selector. This implies to me that the meter might work but has lost connection to the ISO selector.

I'm assuming the camera hasn't been used for some time as the slower shutter speeds were very slow, after repeatedly firing at 1 second the shutter has now freed up nicely. Assuming that the ISO selector is a similar issue I have been working the ISO control back and forth in an attempt to remove any corrosion from the connections but so far to no joy.

Does anybody know the process of accessing the ISO selector from within the lens to clean the contacts and check that the wire hasn't come loose?

Alternatively are there any other points of failure which could cause results like this?

 

[thevdm]

I found out how to access the ISO control dial. If it's any use to anybody else the steps are:

1) Remove the 2 screws on the "1000" plate and lift plate away
2) Remove 3 screws that are hidden under the "1000" plate and carefully live away the shutter speed dial, metal shim and plastic piece.

The contacts for the ISO control connect via a very fine set of metal pins near the ISO selector, 1 set of very fine pins at the bottom of the lens using a strip around the inside of the plastic piece.

After cleaning away a fair bit of dirt from inside the plastic piece it made no difference, I picked up a cheap dead Minolta AL for parts and switched over the selenium cells and needle assemblies in the top housing and the meter now works but is under exposing by around 3 stops (nice and easy to adjust by selecting an ISO 3 stops lower, or selecting the correct ISO and adjusting 3 stops after matching the needle).

For those that are concerned for the camera I'm stripping, it came with a cracked rangefinder screen which is very faint, the aperture is jammed at F2 and it's clearly had somebody in it before as there's a few screws stripped/missing. I'll harvest it for parts and then store the non working shell with the other "display" cameras.

 

[Sarcophilus Harrisii]

Welcome to Rangefinder Forum.
From your description of the meter behaviour it suggests a loss of linearity across its measuring range so it's possible the output of the cell is being affected by excess resistance. It's not uncommon in selenium cell powered meter systems. It's obviously important to ensure that the whole circuit is in good repair, but frequently a factor in the loss of effective cell output may be corrosion around the wire connections to the cell, or deterioration of the outer protective coating that seals the cell against atmospheric intrusion and resulting deterioration. In particular, the transparent conductive layer which passes the output produced by the selenium to the cell contacts can be compromised, leading to exactly the sort of symptoms you've outlined.

It's not possible to rejuvenate cells with defective coatings at home. The conductive layer was originally applied by vacuum sputtering a silver layer which conducts effectively but is so thin it's transparent. So if the cell itself is low in output you can only attempt adjustment, or resistor deletion or substitution (if used in circuit). If after ensuring the cell connections are clean and the rest of the circuit is optimal it's no more accurate, cell replacement is warranted.

As you now apparently have two cells producing output to some extent I'd suggest measuring their output across a range of light levels. If I was trying to get your meter working I would be measuring the cell output at various EV and would probably select the cell which yields the best output at lower EV even if it's not necessarily the most powerful at high EV levels. You'll likely find it easier to correct slight inaccuracies at the higher EV levels than at lower EV. The latter involves lower power output in the first place, hence they will be inherently more sensitive to voltage inconsistencies making them harder to correct.

 

[thevdm]

Thank you for the welcome.

In my research I have read numerous comments which mention cleaning the contacts. Unfortunately the cell on the Minolta AL is inside a bent metal case so the only way to get to the contacts is to unfold the metal case (which will never look good again afterwards).

I have installed the best of the two cells into the working camera and have checked it against my Fuji X-T1 in various situations, across the board it is consistently 2 stops under exposed (error in my last post saying 3 stops).

On the outside of the metal housing containing the cell is an in-line resistor, when I get some more spare time I will remove the resistor from the circuit and see how the metering changes, if it is now suggesting overexposure then I will work out the best resistor to replace it with.

I loaded a short Foma 100 into a cassette to test it, setting the camera to ISO 25 the negatives appear to have come out reasonably well exposed. Once they're dry I'll scan them to get a better idea of how accurate the exposure is.