Measurements on camera circuitry: using a multimeter

[Andreas Thaler]

A multimeter can be used to measure the three basic electrical quantities: voltage (volts, yellow arrow), resistance (ohms, cyan), current (amperes, magenta) as well as continuity (ohms, white).

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When it comes to electronics and measurements, the term "multimeter" quickly comes up.

What is it, what is it used for, and how is it used for camera repairs?

A multimeter can measure the three basic electrical quantities that always play a role in electronics and that are invisible:

• voltage,
• resistance,
• current.

Current is the flow of electrons in conductors, such as a cable, driven by a force - the voltage - against a resistance, the material of the conductor.

Our cable

conducts electricity (electrons) very well because it has low resistance. The material is often copper or aluminum.

Such cables are always found in electromechanical SLRs; they connect parts of the camera's electronic circuitry.

But the flat, orange circuit boards, which can be bent, also conduct electricity. Here, very thin copper traces conduct the electricity.

A camera's circuitry

ensures that all its functions operate as intended, such as the automatic exposure control program, transport of film or shutter control. It consists of electronic components (for example integrated circuits, transistors, resistors) connected by cables and circuit boards.

This is precisely where the multimeter comes into play

By measuring the three basic quantities of voltage, resistance, and current, it's possible to determine whether the circuit or parts of it are functioning correctly.

To do this, connect the multimeter to two probes, hold them against specific points, and read the values from the multimeter.

These points are specified in service and repair manuals for cameras, along with their target values. Voltage is measured in volts, resistance in ohms, and current in amperes.

Example: Measuring the voltage of a battery

Most often, DC (direct current) voltages are measured in camera circuits. Direct current means that current only flows in one direction.

To do this, you switch the multimeter to the DC voltage measurement function. With this inexpensive multimeter I'm using, you select a voltage limit. More expensive devices do this automatically.

In this case, it's 2 volts. Since I want to measure the voltage of an AA battery with 1.5 volts, I select this range.

Then I hold the probe on the black lead to the negative terminal of the battery and the red probe to the positive terminal and read the battery voltage.

This battery has a voltage of 1.6 volts, slightly more than the specified 1.5 volts. This means it's new.

However, this is only a rough assessment of its condition (open-circuit voltage). The measurement becomes meaningful when the battery powers the camera circuitry and is therefore under load (terminal voltage). The voltage reading is then usually lower and should be around the specified 1.5 volts.

I proceed in the same way

when taking measurements in camera circuits. The black probe tip is usually connected to ground, which in a camera might be the metal tripod screw, and the red probe tip to the point whose voltage I want to measure, e.g., the terminal of a component.

Electrical quantities like resistance and current

are rarely measured in cameras. This is usually only necessary for repairs requiring specialized knowledge, such as measuring the resistance of a coil in a solenoid or the total current flowing through the camera's circuitry.

Instructions for these measurements can typically be found in technical camera documentation, such as the SPT Journal.

Continuity

A commonly used additional function is the continuity test.

It can be used, for example, to determine whether two soldered cables conduct electricity sufficiently well. It is also very useful for troubleshooting circuits.

This method can be used to detect broken connections. To do this, hold the two test probes against the section you want to measure. The multimeter sends a weak current through the circuit, which is transmitted if the contact is intact. This is indicated by a beep.

Summary

1. A multimeter can measure the three basic electrical quantities: voltage, resistance, and current as well as continuity.
2. In camera circuits, voltages and continuity are most commonly measured.
3. Instructions for this can be found in technical documents such as the SPT Journal.

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See also

Troubleshooting the camera electronic: Simple measurements with the oscilloscope

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A word of caution

Before using a multimeter, the accompanying instruction manual must be read, understood and instructions followed. The device can be overloaded and damaged if used incorrectly.

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Please keep in mind that you’ll be using highly flammable solvents for service work, and their fumes are harmful to your health. Soldering also produces fumes that should not be inhaled. Dangerous high voltages can be present when handling electronics, especially in conjunction with electronic flash units and mains. Therefore, familiarize yourself with the safety regulations beforehand and ensure your safety. Depending on the activity, protective gear may be advisable.

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

Acknowledgments

Without the work of Master Larry Lyells, in particular, we wouldn’t have the detailed technical information we need today for service/repair of SLRs.

The SPT Journal and The Camera Craftsman, which he helped shape significantly as an author, and which contain all the necessary information and instructions are available for a fee through Learn Camera Repair.

We owe our access to them to Eugene Pate, the founder of Learn Camera Repair.

 

[Emily Cook]

I would also point out that you don't have to use the probes that came with your meter. A lot of meters, especially cheaper ones, come with probes that have very thick tips that can be hard to accurately place. If you expect to use your meter a lot, it can be worth considering whether there are other probes that better match how you'd like to use them. For example, I use a set made by Pomona (part number 6352) that has very fine, sharp, spring-loaded tips that are easy to hit very small targets with (and the sharp tips even let you poke through the colored "solder mask" layer on top of the board to measure the traces under it, if you're willing to slightly damage the protective layer). You can also spend upwards of $1k on the meter itself, but that's really unnecessary for most people. A good set of probes is a great upgrade for only around $40 (for the ones I like anyway).

 

[Andreas Thaler]

I would also point out that you don't have to use the probes that came with your meter. A lot of meters, especially cheaper ones, come with probes that have very thick tips that can be hard to accurately place. If you expect to use your meter a lot, it can be worth considering whether there are other probes that better match how you'd like to use them. For example, I use a set made by Pomona (part number 6352) that has very fine, sharp, spring-loaded tips that are easy to hit very small targets with (and the sharp tips even let you poke through the colored "solder mask" layer on top of the board to measure the traces under it, if you're willing to slightly damage the protective layer). You can also spend upwards of $1k on the meter itself, but that's really unnecessary for most people. A good set of probes is a great upgrade for only around $40 (for the ones I like anyway).

I can confirm that; I have a collection of different test probes.

The fine ones you mentioned are important, for example, when measuring small pins on ICs, so you don’t accidentally touch two contacts at once. Sometimes test points are hard to reach, so you need longer probes.

 

[LenB]

Andreas,
I had a quick look at the specifications of the Oscilloscope you are using. The bandwidth as stated is in the KHz range.
Have you ever run into any limitations with that? I am guessing not. I still have to read a lot of your threads.
I have a decent MSO with a bandwidth of 70MHz. I can upgrade it with software but have not had the need so far.
Oh and here is another vote for multiple probes for the multimeter.

 

[Andreas Thaler]

My photos show two multimeters.

The small blue one costs less than 20 EUR and is perfectly adequate for most of my work. Only when it comes to measuring currents in the microampere range does the larger one have an advantage because it is more sensitive.

Voltages and currents in analog SLR circuits (which I have seen) are often not precise to the second decimal place. Often, it’s just a matter of plausible values that fall within a tolerance range.

Digital multimeters, in particular, suggest precision with their numerical display—and they do deliver it, given their accuracy. It’s just that the things being measured aren’t that precise. Whether an electrolytic capacitor reads 6.45 or 5.61 volts is often irrelevant to how the circuit functions.

In digital circuits, there are two voltage ranges for LOW/HIGH that also accommodate fluctuations, e.g., LOW from 0.0 volts to 0,8 volts and HIGH from 2.0 volts to 5.0 volts. The measured value must fall within one of these ranges to be defined.

So it’s not quite as precise as, for example, when measuring the flange focal distance with a depth gauge, where the tolerance is +/- 20 micrometers (article on this to follow).

 

[Andreas Thaler]

Andreas,
I had a quick look at the specifications of the Oscilloscope you are using. The bandwidth as stated is in the KHz range.
Have you ever run into any limitations with that? I am guessing not. I still have to read a lot of your threads.
I have a decent MSO with a bandwidth of 70MHz. I can upgrade it with software but have not had the need so far.
Oh and here is another vote for multiple probes for the multimeter.

I’m not even sure what the bandwidth of that little oscilloscope is. For my simple measurements on camera circuits (e.g., whether or not there’s a square wave), it’s always worked just fine.

The newer model has two channels and a bandwidth of 20 MHz. That should be enough for the clock speeds of 1980s SLRs.

My lab power supply isn't top-of-the-line either. But it's more than enough for my work.

 

[Andreas Thaler]

Oh and here is another vote for multiple probes for the multimeter.