If you're not familiar with resistor color codes, go to the
resistors page and read the information provided there. If you don't understand the basics (which aren't covered on this page), you won't understand this section. The section on color codes is the most important for this discussion but you should understand everything on that page.
- Component 'Series'
- Resistors You Need to Stock
- Capacitors You Need to Stock
- Dual Emitter Resistors
- Current Sense Resistors
- Flameproof and Fusible Resistors
- Replacement Resistor Selection
- Replacement Film Capacitor Selection
- Special Markings on Small SMD Capacitors
- Special Markings on Unusually Small SMD Resistors
- Selecting Replacement SMD Resistors
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Virtually all components have a limited number of values. The values are determined largely by their tolerance range. A resistor with a 5% tolerance will have an actual resistance of it's rated resistance plus or minus the 5% rated tolerance.
As an example, a 100 ohm resistor with a 5% tolerance will have a value between 95 and 105 ohms. A 100 ohm resistor with a 1% tolerance will have a value between 99 and 101 ohms. Typically 5% components are good enough but if you're repairing an amp with 1% components and one fails, you need to replace it with a 1% part. For testing, you can use the closest 5%† tolerance part but you should replace it with the correct part before returning it to its owner.
†If you don't have a critical 1% value resistor, you can use a series/parallel group to match the desired value. This can save time if you're going to have to delay further troubleshooting, waiting on parts. Having all parts in one order will save on shipping costs.
When the component classification is listed as a 'series', the following applies. To have all of the possible values for a decade (10 ohms to 100 ohms, 100 ohms to 1000 ohms), you need 24 values for the 5% components. This series of values is called the E24 series. The E96 series has all of the available values for 1% components. You can see the various series' in THIS PDF file.
E12
10% tolerance — silver tolerance band
E24
5% tolerance — gold tolerance band
E48
2% tolerance — red tolerance band
E96
1% tolerance — brown tolerance band
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As a bare minimum, I'd recommend that you stock all of the values in the E12 series (10 ohms to 1000 ohms) but buy the 5% or 1% tolerance components (E12 series components are ±10%). To stock the values between 10 and 1000 ohms, you would buy 10-82 ohms and then continue with 100-820 ohms. If you want to buy kits, that's OK but you will RARELY need values over 1000 ohms. The most common failures are low value resistors. For example, the gate resistors for the power supply FETs will generally be between 10 and 100 ohms (10, 22, 27, 47 and 100 ohms are most common). You'll need more of these than others so you'll probably want to buy at least 100 of each (especially 47 and 100 ohms). You should stock 1/8 watt (now sometimes referred to as 1/6w) and 1/4 watt axial resistors (resistors with wire leads on each end). You should also stock the E12 series values in 0805 and 1206 size SMD resistors.
THIS
text file has suggestions for resistors for several common amplifiers. As you'll see in this text file, I recommend stocking 0.1 ohm resistors. These commonly fail when the output transistors fail. You'll see more on this as you progress through the tutorial.
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Capacitors are a bit more difficult to order before you need them. The ones that are most likely to fail (large electrolytic capacitors) vary greatly. You have to get the correct value, voltage, pin spacing, height, diameter and operating temperature range (to mention a few of the possible variables). I've always ordered approximately twice the number I need for any repair. If you need them once, it's likely that you'll need them again.
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In some amplifiers, you'll find dual emitter resistors (like the one below). To check these, you measure the resistance from the center leg to either of the outer legs.
Below, you can see that the resistive element has a center tap (or two resistors both welded to the center leg). The resistance between the outer legs will be twice the rated resistance. This is a 0.33 ohm resistor. It would read 0.33 ohms from the center leg to either of the outer legs and 0.66 ohms across the outer legs.
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The photos in this section show various styles of low value resistors that are used as current sense resistors (shunt resistors). Resistors with values below about 0.068 ohms are rarely found in anything other than class D amps. Rockford used some 0.03 ohm resistors as source resistors for a small number of amps but that was an oddity. Common values for current sense resistors are between 0.01 and 0.04 ohms. These are used in conjunction with a comparator (or op-amp). If sufficient current passes through the current sense resistor, the voltage across the resistor becomes high enough to equal/surpass the comparator's threshold and the comparator's output will toggle (transition from high to low or low to high) and trip the protection circuit.
The following resistor has an extra terminal. On high precision resistors of this type, you'll sometimes find 3 or 4 terminals. The terminals tap off of the resistor at precise points and the tapped terminals have essentially no current flowing through them that could cause errors in the voltage at their point of tap. In car audio amps, you'll generally find only 2 legs on resistors of this type.
The shunt resistor below has 4 terminals. If I'm not mistaken, these were 0.002 ohms. If you look very carefully, you can see that the two terminals on the right are directly connected. This is generally done for the terminals connected to the heaviest trace (on the bottom of the board here) which is generally connected to the rail voltage. On the left, you can see one terminal connected to a heavy trace (and also to the output transistors) but not connected to the other terminal on that end of the resistor. Again, looking carefully, you can see that the last terminal is connected to a small trace that goes back to the over-current protection circuit.
A bit of a clarification... The statement that the shunt resistors are generally connected to the rail voltage is accurate (generally) but, MTX... being MTX, they followed a different path. Not wrong but different. The shunt resistors in this amp are between the high and low-side output FETs and the output going to the output filter inductors is taken from the connection point for the two resistors.
From an Xtant amp with a very similar circuit, you can see the connections mentioned in the text above the previous image, a bit better in THIS image and THIS image.
This last image shows the over-current shunt resistors in a Rockford BD amplifier. They are clearly marked for value and tolerance (not common for shunt resistors).
Copper has resistance and when current flows through that resistance, there is a voltage loss. This loss can induce errors in sensitive circuits where small voltages are critical. For each of the shunt resistors in the two previous images (above the Rockford image), you can see the small traces (green arrows) leading away from the shunt resistors. These are going to the current-sense circuit. You can see the circuit four images down. These tapped terminals are connected to dedicated traces (Kelvin lines) that are not connected to any circuit other than the over-current protection circuit. If that circuit input was taken from a trace that was passing significant current from multiple circuits, noise and loss due to resistance would cause the over-current detection circuit to be less reliable, less accurate in sensing the true current passing through the shunt resistor. The shunt resistors have a specific resistance and measuring accurately across that resistor is critical.
Don't let the above statement that current through a trace is definitively bad for over-current detection. As a matter of fact, Massive and a few other amps use a dedicated trace as a shunt resistor. They actually use one trace for speaker line over-current and another for the current flow through the output transistors (the common use of shunt resistors). The manufacturer below went a step farther and put a heatsink on the trace. They rarely fail from overheating. The heatsink reduces that possibility, even more.
The following type of resistor (marked 12FR04E) often fails and blows soot onto the board. When this happens, you MUST clean the board thoroughly. Any remaining carbon can be conductive and could cause the amp to fail prematurely.
Below, you can see (barely) the current sense circuit for an Infinity Reference 611a amplifier. You will have to open it in a new tab (link below image) and zoom in to see the details. The entire circuit can be found on page 33 of the service manual.
Click
HERE to open this graphic in a new tab. Right-click to zoom. Left-click to drag.
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Flameproof and fusible resistors vary in appearance from standard resistors. Standard resistors are coated with a paint or synthetic resin. These are not typically flameproof and will burn (and generally make a mess) when the resistive element burns. Flameproof and fusible resistors typically have a ceramic coating that cannot flame up. They typically show little or no visible defects in the coating when their fusible element burns.
In some amps (mainly Phoenix Gold), you will see some strange resistor color codes. The last band is generally tolerance. On the resistors below, it has another purpose. The black band means that it's a fusible resistor. These are not exactly the same as flameproof resistors. Both fusible resistors and flameproof resistors will open without flaming up but fusible resistors have a relatively predictable point (referring to power dissipation) where they will open. They are a safety device.
You read these resistors just as you would standard 5% tolerance resistors. The first 2 bands are the significant digits of the resistor value. The third band is a multiplier (it tells you the number of zeros to the right of the 2 significant digits). The fourth band is the tolerance band. The last (black) band tells you that it's a special purpose, fusible resistor.
Below you see a 33 ohm resistor. The first three bands give you the digits 33 (orange = 3, black multiplier tells you to add '0' zeros to the base value). The fourth band is the tolerance band (gold = 5%). This means the value is 33 ohms ±5%.
Above, it was stated that the black (last) band on the resistor indicated that it was a fusible resistor. This information was from someone with factory contacts. The partial diagram below shows components from a similar amp (not sure if it's the exact same model). The resistors above are marked with an F to show that they're fusible. Flameproof resistors don't typically have a special marking.
At least one manufacturer of fusible resistors uses the following color code for the fusibility of their resistors. The 'times' rating is the power dissipation that will cause the resistor to open to 100x it's rated resistance within 60 seconds.
The following example is a pair of resistors that appear to use the coding above. These are from an older Orion 2350GX amplifier's preamp board. They are 10k resistors with a white fusing band.
The following image shows a pair of resistors used in the over-current circuit of a Kicker amplifier. The resistors are 0.03 ohms each. The significant digits for the value are 0 and 3. The multiplier is silver which means multiply by 0.01 (or divide by 100). The tolerance band is gold (±5%). The blue band is likely a temperature coefficient band. The temperature coefficient tells you how much the resistor's value will change when its temperature changes.
When you encounter defective resistors with unusual markings, you may have find other resistors in the circuit with the same markings and check their value. To get the best accuracy, you'd need to pull one leg from the circuit. If there are no other resistors with the same markings, contact the manufacturer. Many times, the manufacturer will not provide a schematic but will give you the value of various components. If you contact the manufacturer, you'll need the circuit board designation (R101, R305...). If there are multiple circuit boards in the amplifier, you may need the number of the circuit board itself. You'll also have know the model of the amplifier.
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When replacing resistors and capacitors, there will be MANY options. Semiconductors have a part number (LM324, TL494...). Resistors and capacitors rarely have a part number. Their markings will tell you their value and voltage rating but little more. There are also many different types of resistors and capacitors. To get precisely the right part, you may have to do a bit of research. I'll try to cover the basics here.
When selecting resistors, other than the resistive value of the component, power dissipation is the most important specification. In general, for resistors that are not designed to be mounted to a heatsink, the physical size of the resistor determines the power handling. Larger resistors have more surface area and can therefore dissipate more heat. The wattage rating of the replacement must be equal to or greater than the original.
Note:
There are resistors that can operate at higher temperatures than standard resistors. Phoenix Passives makes several series of resistors that are smaller than common resistors with equal power ratings. The miniaturized versions are ~2/3 the size of the standard components. If you use the miniaturized components, go to the next higher wattage (if it will fit). If you use the miniaturized version and the owner sees that they are much smaller, they will likely be unhappy (and many will go into the amp if you don't use tamper-evident stickers). When ordering resistors, it's best if you order the replacements by size.
The chart below is from THIS datasheet. It shows the common sizes for 1, 2, 3 and 5 watt resistors. This covers most of the power resistors used in regulators and emitter resistors. You'll notice that the measurements are in mm. THIS is a dial caliper (without the mm markings). THIS is another option. It's a digital caliper and they will generally have the option to switch between mm and inches.
The tolerance for resistors is typically ±1, 5, and 10%. Most manufacturers use 5%. Some use 1% for all of the components even though it's only critical for a few components. If the defective resistor is a 1%, replace it with the same. If it's a 5%, a 1% or a 5% will serve as a suitable replacement. If it's a 10% and it's not a large resistor (2 watts or larger), replace it with a 1% or a 5% component. If all you have is 5% components and need 1% tolerance, you can hand select the resistors that are well within 1%.
For resistors that use color bands, the tolerance is indicated by the last band (for most resistors). For resistors with printed markings, the tolerance is sometimes given with a letter. In the following image, the J on the resistor indicates that the tolerance is ±5%. A K would be ±10%.
We recently covered flameproof and fusible resistors so there's nothing really to add to that.
Voltage is not generally a problem with car audio equipment. Most resistors are good to about 300 volts. If you happen to be working on an amplifier that has 300+ volts in it, you may want to check the voltage rating of the resistor.
The resistive element for the most common resistors is a film of conductive/resistive material on a ceramic or glass core. These are called film resistors. Carbon film and metal film are the most common film resistors. Carbon film resistors tend to be 5% components and tend to have a tan body color. Metal film resistors tend to be 1% components and typically have a blue body color. Larger resistors are sometimes wirewound resistors. These have a resistive wire wrapped around the core. Most all of the large ceramic (cement) resistors are wirewound resistors. I mentioned the dale LVR resistors earlier. They are a metal strip resistors.
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In short, a 'film' capacitor is a capacitor that has some sort of thin film (most commonly, a plastic/polymer) with a vapor deposited layer of metal. These are commonly called 'Mylar' capacitors but the generic term for Mylar film is polyester. Mylar is the trade name used by a company (DuPont) that developed and produces the polyester film.
The markings on film capacitors can be confusing when it's new to you. When selecting a cap, you can be more confident that you have the correct value if you look at the part number. The following examples are a 0.0068µF (marked 682), a 10µF (marked 106) and a 4.7µF (marked 475). Looking at the part numbers on the DigiKey and Mouser labels, you can see that the numbers on the cap are part of the part number. If you needed a 0.47µF cap (would be marked 474) and you were looking at the cap with a part number that included 475, you'd realize that you hadn't selected the right cap.
The tolerance tells you how much variance you can expect from the rated value. For small capacitors, the common tolerances are ±5, 10 or 20%. A 5% tolerance (marking=J, example above) will vary no more than 5% from the rated value. A capacitor marked with a K is a ±10% component (example below). A capacitor marked with an M is a ±20% component (the stylized M on these two examples are the manufacturer's logo, not the tolerance code). Larger, electrolytics are generally ±20% but they may be rated at +50%/-20%. Most are much closer to the stated value than the tolerance may suggest.
This is probably the most important parameter when selecting a capacitor. Using a capacitor with a working voltage below the circuit voltage can lead to capacitor failure. Electrolytics that are subjected to working voltage significantly higher than that for which they are rated can explode (although they generally vent — which isn't pleasant). When choosing replacements, you'll likely see 'working voltage' (AKA WVDC) and 'surge voltage' ratings. You want the WVDC to be greater than or equal to the maximum possible circuit voltage.
Note that the 'working voltage' is in DC volts. The AC voltage rating will be less since the AC voltage (average or RMS) will be much lower than the full swing of the waveform. For many film caps rated at 250v, their AC voltage rating may only be 160, or even 125v. For those with markings for both they could be marked 250v and on a second line 125 V~. Below is a voltage rating from THIS capacitor.
Some film capacitors have no specified AC rating but that doesn't mean that they cannot be used with AC signals. As an example, THIS capacitor has been used MANY times on the output filter of various class D amps (example HERE). It has no AC rating but not one has ever failed, even though the signal applied to it is pure AC.
The value of the capacitor is important but if you can't find an exact replacement that will fit into the alloted space, you can generally use the next higher or lower value (especially for filter capacitors). For example, let's say that the original capacitors were 1800µF and all of the available replacement 1800µF capacitors were too large, you could probably get by with a 1500µF. This has happened before. I think it may be due to the fact that some capacitors cheat on their specs so that they can promote them as a superior product.
In general, electrolytic capacitors are rated for either 85C or 105C. It's important to replace high temp capacitors with the same type. If you can find high temp capacitors that are the same size and value as the lower temp caps, those are generally OK to use but not necessary.
Many people claim that you must use 105C rated capacitors in all locations in car amps but that's not the case. In general, the primary filter caps are better candidates for 105C rated caps. This is because there is likely much more ripple current at the primary 12v supply input. The ripple current in conjunction with the ambient temperature can make the primary caps run hotter than the secondary filter capacitors. The secondary filter caps rarely heat up due to ripple current because they're charged with a square wave that has very little deadtime between the square wave pulses.
For those who disagree with this (85C vs 105C), have a look at the JL amplifiers, like the 500/1. There are probably no higher quality amplifiers than those made by JL. They use 105C caps in the primary and 85C in the secondary. Other manufacturers follow that same model.
Most filter capacitors on the B+ side of the supply are low ESR (Equivalent Series Resistance) types. The high current draw and relatively high ripple currents require low ESR caps so they don't overheat. Many times, you'll see relatively high voltage capacitors used across the B+/ground connections. The reason is that the higher voltage caps have lower ESR than the lower voltage caps of the same value. When searching for low ESR caps, you will generally see this feature mentioned in the general description of the component. If ESR isn't mentioned, try to find the ESR in the specification. Compare the ESR of various capacitors and select the one with the lowest ESR for that package size and value.
The temperature coefficient of a capacitor tells you how much a capacitor's value will change with a given change in temperature. For most capacitors, it's not critical but for capacitors that determine timing (like those used to set the operating frequency of a PWM IC), the temperature coefficient is critical. For the tightest tolerance, you should choose capacitors with a COG or NPO rating. The COG/NPO rating tells you that the value will essentially remain the same over the capacitor's operating temperature range. THIS datasheet covers temperature coefficients in more detail.
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For metal poly capacitors, here are a few characteristics that you should be aware of. Not all of this will hold for every manufacturer but it generally holds.
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MKP capacitors are metalized polypropylene components. Metalized film (polypropylene film in this case) has a layer of metal deposited onto it. These metalized film sheets are stacked on top of one another. More sheets will result in more 'plate' area. See the capacitors page of the tutorial if you don't understand basic capacitor construction.
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MKS/MKT capacitors are metalized polyester (also known as polyethylene terephthalate or PET). The MKS/MKT nomenclature varies by manufacturer but it's the same material.
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FKx (could be FKM or FKS) uses a metal foil (like that used in electrolytic capacitors) instead of a metallized sheet of poly material. The 'M' in FKM denotes a plastic insulating layer. The 'S' in FKS indicates that polyester is the insulator between layers.
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As far as I know, there is no set color code and each manufacturer uses whatever color they want for the various types of film capacitors.
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In all of the above, you'll see that there is a 'K' in the part number. When the K is the second character in the designation, that 'generally' indicates that it's a poly (plastic) film capacitor. film capacitors (not common in car amps) generally start with MP. More on film capacitors can be found HERE. THIS is a datasheet for a metallized paper film capacitor. As a side note, paper film capacitors are typically used for DC or low-frequency AC applications.
This is obvious but easy to forget when going through the thousands of choices. Bear in mind that there is very little clearance in some amplifiers and a capacitor that's only a tiny bit taller may not fit.
This is also obvious. Most capacitors will have lead spacing in 0.1" (or 2.54mm) increments.
You need to use capacitors with the same lead type as the original. This is most critical for large filter caps. The snap-in types will not fit into a location designed for a wire-lead capacitor. Wire-lead capacitors won't mount securely in a snap mount location.
This is important for tightly spaced boards. Some capacitors are wider than others.
The capacitors below are the same. One is reversed so you can see the back. These are 1000pF Golden Max capacitors. The image below is a clip from the datasheet. The markings give you most of the information you need in case you need to order a replacement.
This next photo shows several different styles of ceramic capacitors. Ceramic capacitors generally have a bad reputation but it's generally undeserved. They can perform perfectly well in many applications. The trick is to know the way they're being used and the properties of the cap you're considering as a replacement. In this photo, all values are 102. For small capacitors, the values are almost always stated in picofarads. 10 and two zeros equals 1000pF which is the same as 0.001µF. The first silver band indicates that the capacitor is a ±10% component. I'm not sure what the last band indicates. It could be voltage. These are old capacitors and few manufacturers use this color code now.
THIS
is the datasheet for the yellow axial capacitor (the one that's upside down). Scroll down to the second page of the PDF file.
Note:
Axial ceramic caps are easily damaged by excessive heat. If you're soldering the leads very close to the body of the capacitor, the capacitor may come apart.
The next image shows three 0.1µF film capacitors. The outer capacitors are metallized polyester film components. The center capacitor is a metallized polypropylene film capacitor. This is the information that you can derive from the markings:
This is the datasheet for the blue capacitor. It's a metallized polyester film capacitor.
The WIMA capacitors use at least two features to help identify the type of capacitor. The first is variety of ink/marking colors. These are used to print the markings. The second is the combination of the case color and epoxy fill color. The red cap below has a red fill. Others use a yellow epoxy fill. If so inclined, you can find more information on the WIMA site. This is the datasheet for this WIMA
capacitor.
This
is the datasheet for this capacitor. If you want to know more about capacitor construction, read THIS file.
Some capacitors (like the Mylar capacitor below) use a code for the voltage rating. The first 2 characters of the code are the voltage rating. The next 3 characters are the value (in pico-farads). The letter after the value is the tolerance.
Below is the chart for the various voltage codes. This chart was clipped from THIS datasheet.
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The following chart shows the marking code found on some small SMD capacitors (unfortunately, most have no markings). The capacitor Immediately below the chart is a 10pF. The capacitor was gray but for some reason, came out as pink in the photo. The first letter is the logo (Kemet). The next letter and the number are used with the chart to determine the value.
Click
HERE to open this graphic in a new tab. Right-click to zoom. Left-click to drag.
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It's common to see '0805' sized SMD resistors in amplifiers. These are 0.080"x0.050". A resistor this large provides ample space for marking with up to 4 digits. This allows direct marking for 1% components (3 significant digits plus a multiplier). Smaller resistors require a different marking scheme when tight tolerances (1% — 4 digits) are required. To read the value, you must look up the value on a chart (unless you have a remarkable memory). The chart below is the EIA-96 (E96) marking rule table. E96 indicates that there are 96 values per decade (96 values beginning with 100 and ending with 976 for the first decade — 1000 > 9760 for the next decade).
Below, you can see several resistors. The fact that the third character is a letter instead of a number tells you that the markings will NOT be read as you would for direct markings. It's difficult to remember not to read the first two characters as significant digits. Luckily, you won't have to deal with resistors this small on a regular basis. You should also notice that both upper case and lower case letters are used. As far as I know, the 'case' (upper/lower case) is unimportant/insignificant. If you look at the lower-right resistor, you can see that it's marked 01C. To find the value, you would look up the first two number of the code on the chart (upper green arrows). The value is directly to the right of the code. This means that you use 100 as the beginning of the value (3 digits — just as you'd have for standard, 1% through-hole resistors). The multiplier is C which means you'd add 2 zeros after the first part of the value. This would be a 100-00 or 10,000 ohms. The other resistors are 1780 ohms (25b - upside-down) and 4990 ohms (68B).
Note:
The resistors above look large but they're actually tiny. Their width is about the same as a 20g solid copper wire. That's approximately 2/3 the thickness of a US dime (0.033" vs 0.050").
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As with any resistor, the wattage rating of an SMD resistor is important. To try to prevent being repetitive, if you haven't read item #7 above, do so. When selecting a replacement, the wattage has to be equal to or greater than the original. In most cases with SMD resistors, there is no option to go to a higher wattage (as you can often do with through-hole components). The wattage is generally determined by the size. If you have two options for wattage for the same size part, buy the one rated for higher wattage... Or, buy both. Resistors are typically cheap and there may be a reason to use the one you didn't expect to use.
Use dial calipers (photo example) to measure the resistor. The most common sizes are 0805 (0.080" x 0.050") and 1206 (0.120" x 0.060"). If you don't have a dial caliper, you can use the method shown under (TT7, item #8).
The most common types are thick film and thin film. In most cases, either will work.
Note:
If you're not familiar with dial calipers, follow THIS
link. They are regularly needed to accurately measure electronic components, screws and other items that you'll need to replace. For more information on other types of calipers on the Equipment page, click HERE
.
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Copyright: Perry Babin 2000 - Present -- All rights reserved