Yashica 124-G Minor Repairs

[ic-racer]

A few minor repairs of my 124-G. I got this one new in 1975. Last time I traveled Europe, this was the camera that I used.

For many years during the 1980s I was a 'medium format' snob and did not shoot 35mm and I used this camera exclusively.

The fold out hood is removed.

 

[ic-racer]

Deteriorated foam rubber removed from between the meter module and the flip up hood.

 

[ic-racer]

New foam rubber cut to size.

 

[ic-racer]

New foam rubber in place.

 

[ic-racer]

Viewfinder glass cleaned and mirror cleaned.

 

[ic-racer]

I use Zinc cells for the meter in this camera. Meter works, the shutter speeds are all good. Glass is clean. Camera is ready to go.

I have placed a serrated washer around the battery. The washer is re-usable.

 

[ic-racer]

I use these cells because they are good for ANALOG as indicated on the package....

 

[ic-racer]

I have been using the Zinc-Air cells since Mercury cells became rare. Recently I noticed the Zinc-Air cells going bad in the package. That is, I had a pack of 6 like above, I used one, then about 8 months later 4 of the 5 remaining ones were dead. It is as if they are purposely using porous tabs on the back or something to shorten the shelf life.

 

[Donald Qualls]

My experience has been that the zinc-air cells take 30-60 minutes to ramp up after peeling the tab -- if you test them immediately, they'll read near zero voltage, but an hour later they'll be at proper 1.35V. Unless, of couse, the electrolyte has dried out.

Hearing aid users get a few days from one cell, so they'd go through a pack like that in less than a month. There's no incentive for the manufacturer to make them longer lasting than the expiration on the package (which is much shorter than it would be for alkaline cells). I know a source for regulators that let you replace a PX640 with a CR1632 lithium cell plus the regulator (which ought to last a long, long time in a meter that's shut off or kept dark when not in use). Same fellow has made one for 2xPX625, but the physical size of the regulator modules makes them impractical for smaller batteries. We need a source of 1.3V and 2.6V regulators that are small enough for our needs...

 

[ic-racer]

The Zinc-air cells I have now expire in 2023. See if any go bad before that time. I'll send them back for a refund

Have you done anything with Shottky BAT 43 diodes?

I have played around with voltage regulators (like TCR2EE135LM), but as long as the Zinc-air cells are available, I'll keep using those.

 

[GRHazelton]

I have a 124G in excellent condition, but I've been unable to unscrew the cover on the meter's "battery" compartment. Any ideas, other than a pair of vise-lock pliers?

 

[Donald Qualls]

Have you done anything with Shottky BAT 43 diodes?

I'm not an electronics guy at any useful level. I crammed just enough knowledge to get my ham license in early 2019, and I've forgotten almost all of that since. I understand the physics behind tubes, capacitors, inductors, and some solid state components, but I don't have the knowledge to do anything useful with them.

 

[reddesert]

A Schottky diode can be useful here because Schottkys often have a lower voltage drop than a conventional silicon diode. A regular silicon diode, when conducting a moderate amount of current, typically has a voltage drop of about 0.7 V, while Schottkys can drop around 0.2-0.5 V depending on type. Because most light meter circuits draw a very small amount of current, the actual voltage drop may be less. For example, look at figure 2 in the datasheet for a Schottky BAT43: https://www.vishay.com/docs/85660/bat42.pdf The "knee" of the curve of forward voltage drop vs current is at around 0.4 V at room temperature. But if the circuit draws only 0.5 milliamps, then the forward voltage is closer to 0.2 V, which is about what we want to make a silver oxide battery look like a mercury battery.

Since this does depend on current draw, it may be that the ideal diode for one camera is different from another, but probably close enough.

Actually wiring the circuit would be simple, just put the diode in series with the battery with the correct polarity. The hard part will be getting the assembly into the small space of the battery compartment and making good contact.

Schottky types with low voltage drops to look at include BAT41-BAT46 and 1N5817, 1N5818. The BAT diodes tend to come in very small casings which is helpful, while the 1N5817 are larger. They're all common, inexpensive parts. I have some but haven't experimented with this much.

 

[ic-racer]

Yes that datasheet is similar to one I consult. For those not familiar with the graphs, the more straight up and down, the better for our purposes (of lowering silver-cell voltage in spite of unpredictable current).

reddesert, any success stories to share on using diodes?

 

[Patrick Robert James]

I put a diode (or two, can't remember) in a Gossen Luna Pro and use it with regular alkaline 625 batteries. IIRC I had to adjust the two zones but it still works fine years later and still on the same batteries now that I am thinking about it. The idea of zinc air batteries always struck me as unreliable, but if it works...

 

[reddesert]

I have not experimented with it extensively. I have a lot of these diodes lying around from building electronic circuits, but fitting the diode into the battery compartment and insuring good contacts is the hard part.

Since this came up, I got out my Yashicamat 124G and wedged a BAT46 diode in series with an AG13 alkaline battery that read about 1.45 volts. I tested it only briefly by comparing two readings ("inside" and "outside") with a modern Luna Pro Digital. It seemed that the Yashicamat metered about correct without the diode, and with the diode wanted to overexpose by a half stop. That makes a little sense in that lower voltage means less meter deflection, which indicates a faster aperture. (For a simple meter circuit, more light -> lower resistance of CdS photocell -> more meter deflection.) But not all meter circuits work the same way.

It would be useful to wire up a battery and diode outside the camera to actually take current and voltage measurements (current draw, voltage supplied). On the Yashicamat it's accessible to clip probes onto the battery contacts, but on some other cameras it's harder to get access to the contacts.

 

[reddesert]

I have a 124G in excellent condition, but I've been unable to unscrew the cover on the meter's "battery" compartment. Any ideas, other than a pair of vise-lock pliers?

On my 124G the cover is plastic. It may be jammed shut due to a swollen battery or corrosion. You could probably cut/drill a little slot or pair of holes to get a better twist on it. You could try dripping a little vinegar or alcohol in as a solvent (not too much please). If you can get the battery residue softened up, it will probably come off. Anything rough like vise grips may destroy the cover.

 

[Donald Qualls]

A simple Schottky might be a very reasonable solution to silver oxide or alkaline cells in place of the old mercury cells (long lasting) or zinc-air (correct voltage, but short-lived due to electrolyte drying out). I would probably avoid alkaline, because their voltage drops rather linearly with cell life, vs. mercury and silver oxide cells that have near-constant voltage over 85-90% of their capacity life. The adapter I purchased, to replace a PX640 was $30, including two replacement cells, and the seller threw in the other adapter (replaces 2xPX625) and one or two tiny cells for it). These run on lithium primary cells, nominally 3V, so a simple Schottky probably isn't the right thing, at least for the PX640 replacement (and I doubt he's using half a dozen Schottky diodes in there).

I don't know for sure, but he might be using regulator chips made to go in lithium rechargeable cells (similar to 18650) that will replace a single AA alkaline. You can buy these cells with the regulator factory-installed (inside the standard size case), with combined voltage regulation to 1.4V or so, short circuit protection (current limiting, or a simple fuse -- I don't know which), and a either bypass for charging with a lithium-aware charger, or a buck converter to make 1.6V charge power into 4V to charge the lithium cell (and manage the charge as well -- plenty of room on even a pretty tiny chip, as long as nothing dissipates much power).

 

[GRHazelton]

On my 124G the cover is plastic. It may be jammed shut due to a swollen battery or corrosion. You could probably cut/drill a little slot or pair of holes to get a better twist on it. You could try dripping a little vinegar or alcohol in as a solvent (not too much please). If you can get the battery residue softened up, it will probably come off. Anything rough like vise grips may destroy the cover.

I was kidding about the vise grip pliers.

 

[ic-racer]

(..)

 

[ic-racer]

Looks like the 124G meter schematic is pretty simple, here is it:
https://www.photrio.com/forum/threads/yashica-mat-124g-meter-ciruit-design-and-adjustment.153186/

 

[Jon Goodman]

Occam's Razor or The Law of Parsimony. You're far better off using the zinc-air cells as diode regulation is guaranteed to be fundamentally flawed. Please let me explain as briefly as I can. Electronics students learn this early on in Ohm's Law (and with this I'm going to try hard to stay in the shallow water). If you fail to understand Ohm's Law, you'll never need to buy an Electronics 102 textbook. Basically you have voltage (for decades I've said to think of this as the size of your arm) and you have current (which I compare to the strength of your arm) and you have resistance (in this case you have variable resistance) which is sort of like an elastic band wrapped around your wrists keeping your arms from moving easily. Georg Ohm's basic forumla was I=V/R. Where I is current, V is voltage and R is resistance. But there are some wild cards. Certain electronic components (and we can include wires) observe Ohm's Law and we cleverly call those "ohmic." Ohmic situations produce beautiful linearity. However, certain electronic components do not observe Ohm's Law and what does that mean? It means their relationship automatically and invariably leads us straight to nonlinear town. One of the first non-ohmic components the student learns about is the diode. Introduce the diode and linearity is impossible. Why? Because the amount of resistance is not constant as a function of voltage applied. Let me say that again (because it is basic to understanding why diode regulation can't work): If you use a diode in an attempt to regulate voltage, then the amount of resistance will not be constant as voltage is applied. It will fluctuate, and fluctuation in any math formula causes variation in other factors, doesn't it? Sure it does. This circuit (the meter circuit) depends entirely on linearity. And with a diode, linearity has flown out the window. Now, some will say "Oh, that's bs. I did this and it worked for me" and I understand you might enjoy a correct reading at one point somewhere on the graph, but then again a broken clock will be momentarily accurate at 2 times each day, won't it?

 

[ic-racer]

The non-linearity in diode resistance is exactly why it can be used. A device that does follow Ohm's law (conventional resistor) is unsuitable.

 

[Donald Qualls]

The non-linearity in diode resistance is exactly why it can be used. A device that does follow Ohm's law (conventional resistor) is unsuitable.

Exactly. A constant output voltage is desirable, but a constant drop, regardless of current, is acceptable (providing you're using a cell like silver oxide or lithium with near-constant voltage for most of its life). Neither one is available with ohmic components; only a nonlinear/active component can provide either. Zener, Schottky, or more complex regulator circuits. For our use, with a fairly narrow range of expected supply voltage (either 1.5V or 3.0V, depending on the cell you're designing for) and very low current, Schottky works about as well as a full active regulator, and is cheaper and usually more compact.