Minolta X-700 electronics: A closer look

[Andreas Thaler]

I'd like to take a closer look at the X-700's electronics in the coming days.

On the one hand, I would like to understand how the circuit basically works, and on the other hand, I am interested in details that can be seen on the flexible circuit board.

It's always fascinating to me what solutions the Japanese engineers have come up with here


A golden work of art: ASA setting and exposure compensation

Today I will examine how the ASA setting and exposure compensation work.

On the left side of the X-700 there is the rewind crank with ASA setting and exposure compensation dial which can be rotated:

This is what it looks like under the top cover:

A ring with two wipers that are electrically connected to each other rotates on gold-plated contact tracks:

Here you can see the setup parted from the camera.

The arrow points to the two wipers, which run separately on both contact tracks:

The inner contact track is narrower and runs continuously without interruptions (orange arrow).

The outer contact track consists of individual, separate contact segments that are electrically connected to each other via a grey layer of resistance material (blue arrow).

Everything together including the wipers results in a variable resistor:

Variable resistor: changing voltage

A variable resistor allows you to alter the amount of current that flows through the resistor.

This also changes the voltage that drops across the resistor integrated in a circuit, e.g. 2 millivolts or 5 millivolts (2/1000 volt, 5/1000 volt).

And the circuitry of the X-700 can work with voltages.

Here, with the ASA setting and exposure compensation, the X-700 can distinguish between different voltage values.

Because when I turn the adjustment dial on the camera, the position of the wipers on the two contact tracks also changes. And since this is a variable resistor, the voltage values also change.

For example, the 2 millivolts could stand for ASA 100 and the 5 millivolts for ASA 400.

That's my assumption.


Multimeter in action

I wanted to determine whether this could be true by taking a closer look with my multimeter.

With a multimeter you can, among other things, measure the value of resistors. The higher the resistance value, the more ohms are measured.

Here the photo from before again:

I place one measuring tip of the multimeter on the grey resistance layer on the outer contact track and the other measuring tip on the inner contact track (purple circles).

Both contact tracks are connected to each other via the wipers, which act like a electrically conducting bridge (yellow bar).

When I switch on the multimeter, a measuring current flows through the grey resistance layer (+) across the bridge to the inner contact track (-) and back to the multimeter.

The further the bridge is from the measuring tips, the longer the resistance layer that the measuring current has to overcome.

This increases the resistance, less current flows and the voltage increases.


In a nutshell:

When I turn the dial on the X-700, the voltage values change and the camera knows which ASA value or exposure correction is set.


Measuring resistance

Here I measure different resistance values with the multimeter.

As already noted, the further away the two wipers (bridge) are from the measuring tips, the higher the resistance:

10.23 kiloohm (= 10,230 Ohm)

14.71 kiloohm

17.59 kiloohm

Confirmed


Of course, this is only part of the circuit for the ASA and exposure compensation setting.

But I think a very interesting one


By the way, why are the contacts gold plated?

The wafer-thin gold layer ensures perfect electrical contact and is corrosion-resistant.

Here the Minolta engineers played it safe


+++

All information provided without guarantee and use at your own risk.

To be continued

 

[Andreas Thaler]

Open aperture metering: Always the brightest possible viewfinder image for the photographer


The X-700, like all SLRs of their time, works with open aperture metering.

This means that the exposure is measured at the widest aperture and the actual aperture used for exposure is simulated. This allows the exposure metering system to determine the correct exposure.

If the X-700 were to measure exposure with the lens stopped down, the photographer would have a darkened viewfinder image, which is a limitation.


Aperture simulation

The aperture simulator on the X-700 with rotating ring and coupling for the lens should also work on the principle of a variable resistance.

In practical terms, this means that when you turn the aperture ring on the lens, the aperture simulator ring rotates and positions the wipers on two conductor tracks:

Here we see the two conductor tracks (which are located under the bayonet ring removed here), with the upper one being designed as a resistance track:

Two wipers are integrated in the ring and electrically connected to each other to form a conducting bridge:

Depending on the position of the wipers on the tracks, the current has to overcome different distances on the resistance path:

This results in different voltage values, which the X-700 interprets as aperture values and uses for exposure control.

This technique should be the same for all Minolta-X cameras.

+++

All information provided without guarantee and use at your own risk.

To be continued

 

[Andreas Thaler]

Exposure measurement with silicon photocells

The X-700 has two silicone photodiodes built in to measure light.

One sits on the side of the mirror box for TTL flash light measurement and the other above the viewfinder eyepiece for measurement of available light.

A photodiode converts light into voltage, which the camera's circuitry interprets.

The silicon photodiodes switch faster than the previously used CdS measuring cells and were a significant advance in camera technology.

The X-700 controls flash exposure using the flash light reflected from the film which goes through the lens.


TTL flash measuring

The photodiode responsible for flash measuring is covered by a slider in the mirror box:

If the mirror goes up when it is triggered, the slider releases the photodiode:

I took a closer look at the photodiode in the mirror box. It is held by a tab:

After removing the tab, the photodiode can be pulled out with a prism in front of it. The prism directs the reflected light onto the diode:

The prism is mirrored at its lower end:

After the adhesive has been released, the prism can be removed and the photodiode is exposed:

Measuring of available light

And here is the position of the photodiode for available light measurement:

After loosening the associated circuit board, the photodiode becomes visible (red arrow). The yellow arrow points to its housing above the eyepiece:

On the board you can see also two blue tantalum capacitors (C2, C3 on the circuit diagram) and a transistor (Q2).

According to the service manual, C2 is used for circuit stabilization, C3 works as a memory capacitor and Q2 switches the photodiode.

This information makes little sense on its own. As I conclude this series, I will attempt to provide an overview of the X-700's circuitry, greatly simplified.

+++

All information provided without guarantee and use at your own risk.

To be continued

 

[forest bagger]

Nice work!

 

[Andreas Thaler]

Nice work!

Thank you

 

[Andreas Thaler]

How can I understand the circuitry of the X-700?

In my opinion, the circuitry of the X-700 can only be described as a block diagram because it is too complex.

A block diagram shows the essential components of a circuit with functions, without going into the detailed level of individual electronic components.

Here is the block diagram for the X-700 from a Minolta brochure. Unfortunately I only have it in German, but I hope it at least gives an impression:

Minolta X-700, die voll programmgesteuerte Spiegelreflexkamera mit Belichtungsautomatik und dem faszinierenden Zubehör des Minolta-Programmsystems. Minolta, 1984


In addition, most of the circuit is designed in five integrated circuits (IC). These ICs are not documented in detail by Minolta and in turn consist of a large number of components in micro format.

These ICs are largely so-called „black boxes“ that receive and emit signals via pins. Accordingly, information about switching processes relates to these pins without knowing in detail what is going on in the IC.

Therefore, there is little point in dealing with circuit details because there is simply too little information, and if, too complex.


Analog/Digital electronics

The circuit of the X-700 consists of analog and digital elements; a quartz oscillator generates the clock for switching the digital signals.

Analog elements include, for example, the numerous capacitors that store information not in binary (digital) form, but as charge values.

This is a special feature of electronic SLRs of this time, as analog electronics could not yet be largely replaced by digital ones.


Troubleshooting

As a result, troubleshooting instructions remain in the service manual and other technical descriptions to resolve errors.

Defective ICs can only be exchanged for those from abandoned X-700s, as there are no new replacements. However, unsoldering and soldering in is complex because the ICs have many pins, are small and there is little space between them and other components on the board. Two ICs can only be replaced after removing the entire circuit board, which is very time-consuming.


Five ICs form the main part of the circuit

To give you an idea of what the five ICs in the circuit are responsible for, here is a quote from the SPT Journal:

BASIC CIRCUIT OPERATION:

The circuit uses five IC's -

IC1 - top of flex, rewind side. Provides the MOSFET amplifier for the silicon photodiode SPCA. The metering signal (varying with light level, film speed, and f/stop) appears at pin 14 of IC2. IC1 controls the charge across shutter-speed memory capacitor C5 with the voltage at pin 6; the voltage varies with light level and film-speed setting. IC1 also contains the second-curtain control circuit for the shutter magnet, the self-timer drive circuit, and the charging circuit for timing capacitor C6.

IC2 - top of flex, wind side. Contains the switches for the memory circuits, the charging circuits for the memory capacitors, the aperture-control circuit that automatically sets the f/stop in the P mode, and the mode-discriminating circuit that selects the P, A, or M modes.

IC3 - side of flex that folds on top of pentaprism, wind side. Provides the 32KHz crystalcontrolled clock that sequences the timing functions, the 8 Hz signal for the LED indication, the signal that shuts off the LEDs when the shutter releases, and the signals that drive IC5 to turn on the LEDs, disengage the release magnet, and disengage the mirror magnet.

IC4 — driver for the LEDs, side of flex that folds on top of pentaprism, rewind side.

IC5 - inverter IC that operates the aperture, release, and mirror magnets. Also turns on transistor Q1 to provide power to the circuit.

SPT Journal & Service Notes, January/February 1984, p. 12


How processes take place in the circuit is described like this:

The release signal, provided by closing S2, causes pin 35 of IC3 to switch high. IC5 inverts the signal and applies a low signal to C10. C10 discharges through the coil of the hybrid release magnet SL-1. When SL-1 separates, the diaphragm starts closing. Simultaneously, pin 13 of IC3 switches low to shut off the LED display.

ibid., p. 13


Conclusion regarding repairs

• You should try to approach X-700 electronics repairs pragmatically. Troubleshooting tables guide you through the most common errors, you just have to follow the instructions. Added to this is the previous experience in troubleshooting that you already have.
• Some tests require special equipment from Minolta, which is not available.
• What is pleasing is that the ICs rarely break. Most problems arise from defective electrolytic capacitors, damage to the circuit due to moisture or battery acid, and of course force impact (eg. dropping).
• The mechanics also seem to be robust and reliable; only few parts really break.
• You can always try to find errors and try to fix them. But often a solution is not possible. You should come to terms with that.
• Perhaps it is a consolation that even service workshops replaced the entire electronics before embarking on extensive troubleshooting

+++

All information provided without guarantee and use at your own risk.

 

[Andreas Thaler]

I hope this thread isn't too technical or too long.

In any case I would be happy to receive comments or corrections.

And of course a discussion about this topic is very welcome

 

[miha]

I hope this thread isn't too technical or too long.

In any case I would be happy to receive comments or corrections.

And of course a discussion about this topic is very welcome

To my mind your technical threads contain some of the most valuable information and should become "sticky".

 

[Andreas Thaler]

To my mind your technical threads contain some of the most valuable information and should become "sticky".

If I can contribute something meaningful, I'm happy

There are still many interesting details in the X-700, including the mechanics, the combination of light components and massive ones where required. This makes the camera lighter than others without sacrificing stability. Not everything has to be made of metal; plastic has known advantages.

I will now go through the X cameras that I purchased for my repair program. X-700/500/300 are all closely related, if you know one a little you can get along with the others. If there's anything new, I'll let you know

 

[forest bagger]

If I can contribute something meaningful, I'm happy

I'm missing LIKE-buttons here...

 

[Neofito]

Interessant. Have you worked at this level with canon eos cameras?

 

[Minolta93]

This is good stuff. Thank you

 

[Andreas Thaler]

Even if the X-700's electronics might not be understandable in detail, there are still ways to adjust things.

The already mentioned article about the X-700 in the SPT Journal

SPT Journal: January-February 1984

Minolta X-700, Minolta 50mm f2 MD, Minolta Autopak 450E

learncamerarepair.com

shows the places on the board where settings can be made. I've marked a few here as examples:

1. Strobe level (shut off point flash unit)
2. EV
3. ASA inclination
4. LED readouts
5. Manual speeds

1, 2 and 3 are small adjustable resistors that can be used to change voltage conditions in the circuit. The settings are therefore made analog and can be adjusted with the tip of a screwdriver.

4 and 5 are adjusting screws whose connection to the circuit I don't recognize.

I know from experience that fine adjustments with adjustable resistors are not easy to do, as even the slightest turn with the screwdriver changes the values. Given the size of the potentiometers here, I imagine that setting them would be even more difficult. This will also require measuring devices that are not available.

Therefore, here it is best to follow what Thomas Tomosy says:

Most electronic circuitry is provided with some adjustment capability, usually in the form of small adjustable resistors (trim-pots, or pots, for short). It is a grave mistake to disturb the factory settings impulsively. In general, you should not disturb anything unless you’re absolutely convinced that that is where the problem lies.

Thomas Tomosy, Camera Maintenance & Repair, Book 1, Buffalo/NY: Amherst Media, 1999, Kindle edition, position 835.


There is still a lot to discover about the circuitry of the X-700, but for now I will close the topic. There are other camera systems waiting, the electronics of which are hardly easier to understand


+++

All information provided without guarantee and use at your own risk.

 

[Andreas Thaler]

Interessant. Have you worked at this level with canon eos cameras?

No, I wouldn't dare to do that