Intro
The Zenit Quartz 1x8S-2 is a super-8 camera designed and built in the Soviet Union from 1974 to 1993. It is fitted with a Meteor-8M-1 ƒ/1.8 9-38mm lens. It is of solid aluminium (?) construction, with a clockwork motor. Over 300,000 cameras were produced by KMZ, and were sold both domestically in the Soviet Union and abroad, including as the Kinoflex in the US.
Circuit Diagram
| S1 | Aperture Mode | |
|---|---|---|
| Open | Manual | |
| Closed | Automatic | |
| S2 | Flicker Compensation | |
| Open | Off | |
| Closed | On | |
| S3 | Frame Rate Selector | |
| 1 | Common | |
| 2 | 9fps | |
| 3 | 12fps | |
| 4 | 18fps | |
| 5 | 24fps | |
| 6 | 32fps | |
| S4 | Exposure Compensation | |
| 1 | Common | |
| 2 | +2EV | |
| 3 | +1EV | |
| 4 | 0EV | |
| 5 | -1EV | |
| 6 | -2EV | |
Explanation:
Overview
The light meter is a variation on the Wheatstone Bridge Meter, consisting of the shutter leg and the chassis leg. The Shutter leg consists of R1 & R2 on the upper half and R4-R8 via S3 on the lower half. The Chassis leg consists of R9-R13 via S4 on the upper half and RP on the lower half, and is grounded to the chassis of the camera in the middle. Bridging the two legs is Galvanometer A1 and a smoothing circuit(?) composed of C1 in series with R3 and antiparallel diodes D1 & D2. The bridge is connected to the batteries by S1. The circuit mostly functions as a normal wheatstone bridge light meter, the main difference being how the exposure is adjusted.
The Shutter Leg
R1 & R2 form one of the known, fixed resistances. S2 is in parallel with R1, which is closed when the shutter button is depressed. This adjusts the leg to compensate for the reduction in light from the shutter flicker. One of R4, R5, R6, R7 & R8 forms the second known, fixed resistance, and is selected by S3, which is controlled by a cam and lever connected to the frame rate selector. This adjusts the leg so it can select the proper aperture for the frame rate.
The Chassis Leg
R9, R10, R11, R12 & R13 are selected by S4, which is controlled by the EV compensation selector. Much like the frame rate selector, it adjusts the meter to properly compensate the EV, and is also used to select film speed. RP is the CdS photoresistor. It behaves as a normal photoresistor, and reduces resistance in response to increased light exposure.
Across The Bridge
Galvanometer A1 uses a D'Arsonval-Weston movement to control the aperture. The zero point of the aperture is at about ƒ/4. Current across A1 (IA) is in this post is described as flowing from Shutter to Chassis. ƒ/1.8 is set with -0.6µA and ƒ/22 is set with 8.4µA across A1. C1 and R3 form a smoothing network along with D1 & D2.
Function
Voltages are described with respect to the batteries, not the chassis The circuit is switched on via S1, closed when the aperture selector knob is turned to 'A'. The resistors of the Shutter and Chassis legs form voltage dividers. Current flowing through these dividers produce output voltages at the junction of the resistors and A1. Assuming the settings of S2, S3, & S4 remain unchanged, the output voltage of the Shutter Leg (VS) is fixed and the output voltage of the Chassis Leg (VC) is adjusted solely by RP. The difference between VS and VC in turn form a voltage across A1 (VA), and thus sets IA, and further, adjusts the aperture. An increase in the intensity of light falling on RP reduces its resistance, and, via the Chassis Leg, IA, and thus the aperture. Due to the design of the light path, this change in aperture will inversely affect the illuminance, and thus resistance, of RP. The high gain of the circuit without the light path feedback (as the change in the resistance of RP for a given stop results in a greater change in IA than would adjust the aperture for the same stop) results in a negative feedback loop that allows the circuit to more accurately set the aperture. C1 & R3 act as a low-pass filter to damp oscillations of the aperture drive, both from the feedback loop and from shutter flicker. As the time constant of the circuit is 330ms (1/3 seconds), D1 & D2 bypass R3 when VA>VF of the diodes (~0.6V if Si), so that when the position of the aperture is far off the correct exposure VA changes rapidly, but is damped when closer. This allows the meter to respond quickly to the lighting conditions yet not oscillate once at the correct exposure.
Reverse Engineering
The reverse engineering process was fairly straightforward. The circuit is split into various individual PWBs and components (modules) connected by identical looking wires. The layouts of the PWBs and the relationship between camera settings and switch positions was determined and diagrammed. A multimeter was then used in continuity mode to trace the wires connecting the boards. These connections were then added to the diagram, which was redrawn to a more familiar layout. Aspects of the circuit were then tested in order to understand the behaviour of the circuit and the primary feedback loop.
Using the Camera
As with a lot of photographic equipment from the same period, the 1x8s-2's light meter was designed to run on now unavailable PX625 mercury batteries. Despite these batteries being discontinued, the camera can still be used with various options as to how.
PX625 Substitutes
There are various substitutes for PX625 batteries, including voltage converters Zinc-Air cells, and Silver Oxide cells. Voltage converters allow for an accurate voltage that will properly match the very flat discharge curve of Mercury cells, but take up space and require some power to run, which limits the available charge. Zinc-Air cells are similarly reliable and can provide more charge but are always reacting whether or not the battery is being used. Silver Oxide cells also have a flat discharge curve, although output at 1.55V.
Dissimilar Batteries
During the reverse engineering process the light meter's response to over- and under-voltage was tested, with minimal change in aperture between ~3.5V and ~1.5V. This is in part due to the wheatstone bridge design as well as the negative feedback of the aperture drive. In addition, the meter draws a maximum of about 300µA when on, meaning that a pair of LR9 alkaline cells, which have the same form factor as the PX625, should be able to power the meter effectively.
Manual
The main motor of the camera is clockwork and the aperture has a manual mode, so it is entirely possible to use the camera without batteries.
Still under construction
