Basic Troubleshooting - MultiMeter basics
The multimeter is one tool that is an absolute must have when troubleshooting, simply because it's so versatile. Also, these days, unless you opt to buy something expensive from the likes of Fluke or Tektronix, multimeters are very inexpensive, and a quick search on Amazon shows devices starting at about £10, which for a beginner with a limited budget is quite suitable.
What can a multimeter do for us then?
1. Measure AC (Alternating Current) & DC (Direct Current) voltages. 2. Measure resistance in ohms. 3. Measure current flowing in a circuit. 4. Test circuit continuity.
The above are all functions you should expect from a basic multimeter, but you may find additional functions available. My own device also includes testing ports for transistors, and also allows a connection via serial port to a PC.
This is my multimeter:
[attachment=6]MM.jpg[/attachment]
There are a number of controls and inputs that I have indicated with boxes:
1. Light blue - on/off button. This may or may not be present, as some use the large dial for this. 2. Yellow - the display where you can easily read the results in large friendly letters! 3. Blue - the settings dial. Reading from l-r we have voltmeter, ohmmeter/continuity, diode test, transistor test and 4 different setting for reading current, or amps. 4. Red - Inputs. The black probe, or common is used for all inputs. The 2 empty sockets to the left are for the ammeter setting and the rightmost socket is for voltage and resistance. 5. Purple - These are secondary controls used in conjunction with the dial, and allow switching between AC & DC, the ohmmeter and continuity function. In addition there are controls to move the decimal point and to turn on the display backlight. 6. Orange - Not present on all devices, but this allows for testing of transistors. You plug them in here and set the dial to transistor test.
Clearly this is very specific to my device, and yours may well be different, so make sure you read the manual so that you fully understand how you've set yours.
You will note that the two test probes are black (Negative) and red (Positive). Whilst devices may vary in the functions they offer, and how these are selected, probes will always follow this colour code.
Let's look at the 4 core functions one at a time.
[b][u]Measuring Voltages[/u][/b]
Measuring voltages is one of the key functions you will likely need when working on an old computer like an Atari ST. Typically the voltage you will see when testing is 5v, but 12v is also present, but not required for the machine to boot.
The ST uses a simple code for the PSU outputs of Black = GND (Ground), Red = 5V and Blue 12v. Whilst this colour code is commonly used in the ST, you should always check if working on something else in case it uses another scheme.
To measure voltage it is simply a matter of touching GND with the black multimeter probe, and then touching the red multimeter probe to the positive connector of what you want to measure of. If you reverse this and put black to positive, most modern multimeters are smart enough to not care, but the voltage will be read as a negative value because the polarity is reversed. Some older devices have a display with a pointer and can't measure negative values.
When measuring voltages there are 2 things to consider:
1. Is the voltage AC or DC? You should set your device to measure the voltage type based on the instructions for your device. 2. How big is the value I'm going to measure? Most modern devices can automatically detect the value being received and display it. This is called autoranging. If you happen to have an older device the does not do this, you MUST set the multimeter to the correct voltage range. I.E. You expect to measure a maximum value of 50v DC, so you must set that as the minimum value, or higher, say 100V. Don't set to 10V or 25V maximum as you risk damage to the meter. If in doubt,set it too high and work backwards.
Here the voltmeter positive probe (Red) is touching the VCC (5v) pin of the Shifter and the negative probe is connected to the GND (Black wire) of the STs floppy power lead. The values here should match those at the output of the PSU itself.
[attachment=4]Voltmeter_1.jpg[/attachment]
The multimeter is set to DC volts and as it is autoranging, it is not necessary to set a suitable range, Other than the PSU, there are no AC voltages present so you will most likely always be using a setting of DC volts.
Here we have the reading:
[attachment=3]Voltmeter_2.jpg[/attachment]
Its as simple as that :) Just remember if you get a reading you think is odd or you don't understand, check the multimeter is set correctly before jumping to conclusions.
[b][u]Measuring resistance in Ohms[/u][/b]
Measuring resistance is much the same as measuring voltage, with the exception that resistors really don't care which direction electricity flows through them, so when we use this feature on our multimeter, we don't have to worry about polarity as the multimeter will send it's own power through the resistor to determine its resistance. For this reason, if we wish to measure the value of a resistor that is still in a circuit, we MUST turn off power to that circuit.
You should also be aware that if a device is in circuit, other components may affect the result, though usually the reading is good enough for most purposes.
Here is an in circuit resistor being measured to determine its value:
[attachment=2]Ohmmeter_1.jpg[/attachment]
Note the multimeter probes can be connected either way when measuring resistance. Here the multimeter is set to Ohms, and here is the reading:
[attachment=5]Ohmmeter_2.jpg[/attachment]
Again, the multimeter is autoranging and will present a result based on the value it reads, in this case 33 ohms or 33R. As before, you must check the instructions for your own meter to ensure you're using it correctly.
[b][u]Measuring current[/u][/b]
Measuring current presents a particular problem, because unlike measuring voltages or ohms, the multimeter needs to be in series with the load being tested.
I have done this here to demonstrate by removing a red 5v line from the PSU output plug, and connecting it to the red probe of the multimeter. The black probe then connects to the 5v input of the mainboard where the original connector once plugged in.
[attachment=1]Current_1.jpg[/attachment]
Essentially its 5v into the multimeter, and straight out the other side.
For this reading, I knew 10Amps was excessive, since the PSU cannot deliver that much current. I therefore set the meter to the next highest value of 400Ma, and here is the result:
[attachment=0]Current_2.jpg[/attachment]
Obviously the problem measuring current is that you have to have a series connection. I personally have never need to measure current whilst repairing any ST, so this is useful to know information, but something you probably won't need much, if at all.
[b][u]Testing for continuity[/u][/b]
Continuity testing is one of the most useful things that you can do with a multimeter, and also one that is difficult to show, as it is usually an audio function! Once again, each multimeter implements this differently, so check your instructions to see how.
For my multimeter, I set the dial to Ohms, then press one of the extra control buttons to switch it to continuity mode. In this mode, if I touch the probes together, I receive a continuous tone from the multimeter to say that the circuit is complete. Like the Ohmmeter section, continuity doesn't care about which way it's connected. Unless of course you have a diode in the circuit, which you can determine from your schematic.
But what is continuity?
If I have a piece of wire, and I connect my multimeter in continuity mode to each end of the wire, the circuit is complete and I hear a tone. This means that the wire is good and capable of making a circuit. It is like using the wire to complete a circuit comprising of a battery and lightbulb. The light comes on and we know the wire is good. The light and the tone are the same thing.
If I now replace the wire with a piece that is broken internally, but I can't see the break and don't know it's there, I can now connect the multimeter to either end of the wire, but I don't get a tone. This means the circuit is broken and can't conduct electricity.
So continuity means "My wire is not broken". We can use this to test between points on the PCB and determine if the tracks and vias are working. If we get a tone, they are good, if not, then we have a problem.
I had an example of this very problem on my Mega ST 4 recently.
The machine would not boot and on checking the data lines from the CPU to the MMU with a logic probe, I found 2 data lines that were not pulsing as expected. I checked at the CPU, the pulses were present, but not at the MMU. I powered the machine down and checked the PCB traces incrementally with my multimeter in continuity mode. Working my way from the MMU, I had a tone telling me the connection was good up to a pair of vias that take the track from the top of the PCB to the bottom of the PCB. These vias were only 10mm away from the CPU, but once past them, the circuit was broken and the tone stopped.
Continuity testing is a great way to check that connections on a PCB actually work, and if not, through a process of elimination, where they are failing. Some simple bodge wires were enough to restore my connections and return the machine to life.
You can also check for shorts with continuity mode. If you connect two points that according to the schematic should not be connected, and get a tone (Or however your meter indicates continuity) then there is a chance there is a short which might be causing your machine to fail.
I would strongly suggest that you look for an audio tone as part of the specification for continuity mode on any multimeter you plan on buying, as it it an incredibly useful tool.