Components check

So you've made and electronic device, as far as you know everything appears to be ok, but, device ain't working like expected, or even worse refuses to give any signs of life. What do you do? First, check the circuit for mechanical failures, like disconnected wire, broken vias on the board, connections between two adjacent vias which are not to be connected, bad battery contact inside the case, broken pin of some component, cold solder joint, etc. If this doesn't come with a result, you should compare values of components and their connections with values and connections between components on the schematic, maybe you put wrong component in the wrong place, or you have read values in the wrong way, maybe you forgot that k in front of an Ohm, maybe you connected supply to the wrong pin of some IC. Next step, in case everything was ok with the previous one, would be fault test for each of the components on the circuit board, then if you find one, you should replace it with a working one. You start troubleshooting by measuring DC voltages at certain points of the board, and comparing these values to the ones on the schematic. So, by knowing the operation of the circuit you start the process of elimination to find the “suspect” component.
If there are several “suspects”, and this is not a rare occurrence in complex devices, the testing is divided into groups of components, you start checking in reverse soldering order, this means that you start with components last soldered, because those are the most sensitive components on the circuit like integrated circuits, transistors, diodes, etc. After that, you should check electrolithic capacitors, since they have a somewhat limited lifespan and leave resistors and block capacitors as last in line since they are sturdy little thingies and can take a lot of beating.
The fastest and simplest method to do your troubleshooting would be using an instrument for measuring resistance or the “ohm-meter”. Now, in almost any case you wouldn't have an ohm-meter all by itself, since it is commonly built together with an ampermeter and voltmeter in one instrument, called AVO meter or multimeter. Another option if you're more on the nerd side would be to build your own instrument we presented at the end of this chapter, called “Conductivity tester”, but having this instrument is no replacement for a nice and shiny multimeter.
The safest and most accurate method would be to desolder the component from the board when testing it, because other components could lead to wrong measurement, so you would have to be very careful when testingin-circuit.
Ok, you should know something about mutlimeters now. There are two kinds of those: analog and digital. Analog ones are items of the past, and since they use a needle to tell you values, it can be a bore sometimes decyphering the right value. Digital ones, on the other hand have a nice display and everything. You should go for this one, although both types of components come in different sizes and with different abilities. Their price is from several dollars, to several hundreds of dollars for really good professional ones. Truth is that a two dollar digital multimeter manufactured in some sweatshop of the far east will do a very good job for you for many years, so you should really buy big pricey ones when you get accustomed to working with electronics, and when your ubergeek ego demands a novelty instrument (there is a slight possibility to make a beginner's error and burn your instrument, so there isn't really the need to burn hundreds of dollars when you can burn only two, right?).
Two instruments on 11.1 are some of the cheapest brand-name multimeters. They are manufactured by VOLTKRAFT, first one is an analog instrument with mark A801, and the other is a digital one marked as VC404. As you can see, on the analog instrument, round switch is in position marked with Rx1kOhm, which means that, if for example, needle shows the value 36, actual resistance between the probes is R=36kOhm.
Switch on the digital instrument points to a diode symbol.

1. Diodes and transistors

When using an analog instrument to test a diode, diode is ok if it's resistance value is very small in one, and very high in other direction. According to 11.2, positive end of the diode is connected to one probe and negative end to the other probe (11.2a), and then it is turned around (11.2b). In the first case, value should be very low, and in the other it should be very high. When the multimeter shows low value, diode's anode is connected to the positive probe of the meter (red probe). If the value is equal to or near equal, either low or high in both directions, diode is faulty, and should be replaced.
Digital instrument has another method to test the diodes. It has it's own switch position, as shown on 11.1b. When we connect probes to each other, multimeter should sound a buzz which signals a short circuit, and display tells 0. When we distance the probes, buzzing stops, and a symbol for open circuit is displayed (this can be either 0L or 1). Now we conenct probes to the diode (11.3a). Then we rotate the diode and connect it again (11.3b).

If the measured diode was ok, one of the two measurements would have shown a value which represents a minimum voltage that could be conducted through the diode (between 400mV and 800mV), and the anode is the end of the diode which is connected to probe A (red one). Diode is faulty if you hear a buzz (closed circuit) or some value which represents infinity.
Transistors are tested in a similar fashion, since they act as two connected diodes. Both diodes should be tested in described way, and if both of them are functional – transistor is functional as well. According to 11.4b, positive probe is connected to base, and the negative probe is first at the collector and then emitter afterwards, in both cases resistance should be low. After that, you do the same thing, only with switched probes, negative probe is connected to the base and you test collector and emitter with a positive probe, both cases should produce a high value on the meter.

When testing PNP transistors, all steps are the same, but the measurements should be opposite: on 11.4a they are high, and on 11.4c they are low.
If you test transistors using a digital instrument, process remains similar to the one with diodes. Each diode should produce a value between 400mV and 800mV. Many modern day digital multimeters have a tool for direct transistor check. There is, as displayed on 11.5, a special plug where low and medium power transistors fit nicely. In case when there is a need to test high power transistors, thin wires (0.8mm will do fine) should be soldered to transistor's pins and then plugged into the instrument. As displayed on 11.5, transistor is plugged into specified slot according to it's type (PNP or NPN) and the switch on the instrument is brought into position with a hFE marking. In case transistor works, display shows a value which represents the current amplification coefficient. If, for example, transistor BC140 is tested, and the display shows 74, this means that the collector current is 74 times higher than the base current.








2. Transformers and coils

Grid transofrmers are tested by measuring the resistance of the copper wire on the primary and secondary coil. Since the primary coil has more curls than the secondary one, and is wound using a thinner wire, it's resistance is higher, and it's value lays in range between several tens of ohms (in high power transofrmers) to several hundreds of ohms, even to kiloohms (in low power transformers).
Secondary resistance is lower and is in range between several ohms to several tens of ohms, where the principle of inverse relations is still in place, high power means low resistance.
In case an instrument shows an infinite value, it is a certain sign that the coil is either poorly connected or the curls are disconnected at some point.
Coils can be tested in the same way as transformers – through their resistance. All principles remain the same as with transformers. Infinite resistance still means disconnected coil.

3. Capacitors

DC capacitors should produce an infinite value on the instrument. Eexceptions are electrolithic and very high value block capacitors. When the positive end of an electrolithic capacitor is connected to a positive probe of an analog instrument, and a negative end to a negative probe, needle jumps to the lowest value and then gradually comes back towards infinity. This is a proof that the capacitor is ok, and the needle's movement is the charge stored in the component being discharged. (Even small capacitance components get charged while testing, but their discharge time is very short, so the needle doesn't have the time to move.)
Variable capacitors are tested by connecting an ohm-meter to them, and turning the rotor. Needle should point infinity at all times, because any other value is a certain signal that the plates of the rotor and stator are connected which means that the component doesn't work.
There are digital instruments that have the ability to measure capacitance, which simplifies the process to comparing the value on the capacitor to the one on the display, any other value means that the component is faulty. With this said, it is worth mentionig that the capacitors have considerably wider tolerance than resistors, which goes even to 20%.

4. Potentiometers

To test a potentiometer, or a pot, or a variable resistor, process is rather simple – you connect the component to probes of an ohm-meter and turn the shaft, and values should do what component's name says – vary. Of course, this behavior should be linear – jumps and twitches in values mean that the component is not working as supposed to. If that component was to be used in some audio amplifier, speakers would produce loud noise or no sound at all while pot is being turned.
(“Noisy” pot can be repaired using a special spray, or oil or even a graphite pencil, but this is not a good solution since it is weak and short-lived, and should be practiced only if there is no replacement for that potentiometer)

5. Speakers, headphones and microphones

When testing speakers, their variable coil has either 4 or 8Ohms in most cases, and the meter should show those values.
When using an analog instrument on the speakers, not much of attention is being paid to the value shown on the actual instrument, because by only connecting the probes there should be a short noise heard. If there is no noise, speaker is broken. Same goes for headphones and dynamic microphones.
Electret microphones don't produce the noise. And another source of trouble could be the built-in FET amplifier.


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