Hack for ISO above 400 on Yashica GT?

[Gabriel M.A.]

Oh Enlightened ones, I have been pondering how to actually make my Yashica Electro 35 GT be usable beyond ISO 500. Is there some circuitry hack that can be done to, say, shift the ISO values by two stops?

Meaning: ISO 100 would now be ISO 400. I don't think I will ever use the ISO 10 or 12 setting, so I'm thinking, perhaps there's a resistor that can be replaced that would allow you to effectively shift these values by two stops, so that the lowest value would be, say ISO 50 (I would like to use Ilford Pan 50 or Kodachrome 64 while it still is on its last breaths).

I'm sure G-man would know... 🙂 Any more in the same club of Enlightened Ones?

 

[pshinkaw]

The schematics for the exposure system are here:

http://www.provins.org.uk/

Good luck!

-Paul

 

[Sonnar Brian]

Time to add a GSN or GTN. The ASA goes up to 1000.

GSN's are inexpensive, GTN's are a more. The GT could always be sold to cover the cost. I would go with a GSN.

Your GT is an early one, like mine. Later GT's, according to Yashica-guy.com, went to 1000. My GT has the cosmetics of the "Profrssional" as well.

http://yashica-guy.com/document/chrono.html

 

[Gabriel M.A.]

Well, I *really* like this camera; I got it from G-man. The rangefinder is calibrated, the shutter is extremely quiet, and the body is black...I still have to see the rolls I've run through it, but if I could *just* make it at least meter at ISO 800; up to 1000 would be very sweet, so I could use Tri-X to be developed in Diafine...

 

[Sonnar Brian]

One easy Hack that was used with the Polaroid SX70 to shoot ASA600 film is to put a white card in front of the electric eye. The meter is calibrated for 18% grey. When it sees the higher relative reflected values, it is going to essentially think there is more light than there is. This trick was listed in Pop Photo way back when.

 

[Gabriel M.A.]

Hmm...interesting, Brian. I don't know how to do that, but that's a thought.

Paul: thanks for the link; I think I found that PDF, but I don't think it addresses the specific part I'm looking for. It has the diagram, but it doesn't say which resistor controls the eye's current, and what resistance it is.

 

[Sonnar Brian]

The Pop Photo author simply taped a piece of white card board in front of the EE. It was at an angle, so that the electric eye looked into the white card. You did say Hack...

Did G'man adjust your camera for 6v batteries? If so, he would certainly know the adjustments to make.

 

[Gabriel M.A.]

Ohh, I see! OK, thanks. Now about the other question, I don't know...G-man?

 

[Gabriel M.A.]

just a mini bump...

 

[Gabriel M.A.]

A quadratic linear? 😉 Do you know what rating the resistor is and with which I can replace it with? Thanks for chiming in, G-man.

 

[Gabriel M.A.]

ok, thanks Dave

 

[ErnestoJL]

The resistor that´s connected in paralell with the CdS cell should limit cell´s value when in low light condition. May be this is used to limit the opening time to a maximum of 30 sec.
The capacitor value will alter drastically the shutter timing due to the design, so changing it will bring either a shorter or longer opening time for a certain light condition.
May be experimenting with capacitor value, (reducing it will also reduce opening time), may bring the equivalent for increasing ISO setting.

What makes difficult to "play" with it´s value is that CDs cells are not equal in terms of resistance vs. light, and capacitor´s tolerance in value is usually also very wide, from -20% to +80%. Besides,´I´ve heard of several values used ranging from 2.2 to 4.7 uF.
It would be interesting to know the changes in component values from model to model, it is from the YE35 to the GSN. I think that it was easier (and cheaper) for Yashica engineers to alter some component values instead of changing the mechanical assy. for the ISO setting.
Unfortunately I have both a GT and a GSN and both go up to ISO 1000...

Ernesto

 

[Gabriel M.A.]

Thanks Ernesto. Yeah, I'm just hoping that the "bump" for this can be achieved by replacing a capacitor or resistor; the ISO selector as a variable resistor would be the same on all models, I would guess; the only change they would have made would have been the capacitor or resistor that sits in front of it; (i.e. this is what "calibrates" it at 0 resistance from the selector).

Of course, this is all speculation.

 

[ErnestoJL]

I´ve been watching the schematics at the Roger Provins website, and what I see is that the capacitor alone will do the trick unless I missed something else. Worst problem would be that the new calibration of the shutter timer would be quite complicated unless having the right stuff: you need a shutter speed meter and a "calibrated" source of light. The latter is simple as you can use any light bulb of any power, but to make things easier, the resultant exposure must be lying on the full seconds area so any stopwatch would be usable to measure how is it working.
The timer is based on two things: to set the shutter speed the capacitor is charged through a CdS cell, then a certain amount of charge is stored in the capacitor (shutter speed set). Upon shutter opening the capacitor discharges through the base emitter junction of the transistor, and some resistors which set the aperture (connected to the transistor´s emitter terminal).
Then, if the cap value is reduced to a half, the shutter speed will be increased (opening time reduced).
What I don´t know is how linear the circuit will react, it is if half the capacitor value produces half the opening time or what.
I think that it deserves some experimentation if someone wants to bring the camera able to handle 4x at ISO 800.
If it´s linear, then the only problem is to measure the capacitor value, and replacing it with another of 1/4 the original value.

I´ve got to admit that this idea is tempting me... a fixed lens RF that goes from ISO 100 to 4000 at a bargain price...hummmm
May be I will do something with my GT that needs POD replacement...

Regards
Ernesto

 

[Jeroen]

Hmm... Now what about a 35 CC that goes up to 1600? ;-)

 

[ErnestoJL]

Well.... as long as you have the schematics for the electronics and hopefully a functional description of it, and if electronic components are not custom made, and being it commercial/industrial types, not custom numbered... you may perform some experimentation.
Of course no warranties!

But in the real world, unfortunately few if any of the above conditions are met.
Sometimes is easy to modify a moving coil based exposure meter either to enable it to run on alkaline batteries or to increase the ISO setting. But if there is some electronic circuit involved, things won´t be that easy.

Regards
Ernesto

 

[john neal]

If I'm reading this right, the intention is to move the "program" so that, say, 1/500 @ f16 was given instead of 1/500 @f4?

I have the service manual for the Yashica series which includes those at the provins website and think that there is more in play than just the capacitor. There are resistors, both in the aperture ring and in the control module, that determine when, and for how long, the capacitor is charged/discharged.

Since the circuitry is bespoke and slightly different between models (there are 3 different types), it would seem to be entirely hit and miss in terms of which components to change to affect the shutter - whatever you do you will never get anything faster than 1/500 as this is the max mechanical speed of the shutter. If you want to achieve this at smaller apertures, you may be better working out how to change the resistor chain behind the aperture ring so that the camera thinks it is set at f4 when it is actually set at f16.

This would be much easier to achieve than playing with the capacitor due to the odd values originally fitted and the (relatively) huge tolerances of that device - and resistors are cheap too 😉

 

[zuikologist]

Is it possible to just change the lightmeter element for one with different sensitivity and leave the rest of the circuit untouched.

 

[john neal]

The CDS cell has a variable resistance from ~0 ohms to ~20,000k ohms, with another resistor in series to give a min value. Replacing that will not affect much other than either the fully light/dark setting, it will probably not change any of the intermediate values.

I suppose you could measure it's response curve to many absolute light values and plot it to see if it's linear or logarithmic, then all you would have to do is find one that offsets the curve by the amount you need. Personally. I think life is too short......

 

[Gabriel M.A.]

Hey guys, apparently I'm not the only one interested in this. John, you have some very good points. But I'm thinking in simpler terms; perhaps it's the computer scientist/engineer in me with some simplified thinking bestowed to me by my electrical and mechanical engineer dad...the ISO speed selector serves as, let's just put it in lay terms, a "dimmer" switch.

Before I go any further, does anybody know what is the sensitivity of the electric eye of the models that go up to ISO 500? I've been looking for that info, and it may be buried somewhere, because I can't seem to find it, not even in the PDF file of the "manual" I found.