A few caveats with audio and RF amplifiers:
1- Never operate an amplifier without a load. To do so means the amplifiying device must disipate the energy, or the output transformer has to disipate that power, and many will fail in milliseconds. It is never good.
2- Line voltages and B voltages can kill. When a circuit is plugged in- whether live or not- use one hand in the circuit only and make sure your body is not in contact with anything conductive.
3- If using junk speakers, keep controls at mid level and volume down.
Now, many people have different opinons on what “servicing” an audio amp entails. For some, this means essentially totally rebuilding the amp, for others, it can mean replacing just what needs to be replaced to make the amp operate within expectations, much as it means for your car or in the past with TV sets. If you are doing this for money, you need to discuss with the amp’s, or device’s owner just what it is they want. And if some snobs want to be ciritcal of the lesser definition- that is their problem for being unrealistic in a real society. If the snobs want to rebuild the item with new film resistors, new electrolytic capacitors, and replacing ceramic disk caps with polyester or other film type, new low noise transistors, low noise op amps, etc, this is beyond “normal” servicing and constitutes totally rebuilding the unit- but they cannot call it that because they are usually reusing the original controls. If someone is expecting that level of work to qualify it only as being “serviced”- they tend to be the first ones whining when they get their repair bill for their vehicle- especially if the service station were to use their definition of “servicing” applied to their automobile.
If someone wants to do that much work to a vintage amp they typically should already know what is involved, why they are doing it and many would certainly not being paying someone else to do it for them as they could do the work competently themself. It is simple economics to not do that level of rebuilding to try and sell it, but some try.
When I do a repair, I clarify what it is that person wants. Because one can invest the equivalent of a thousand dollars or more in parts and time on many amps with the end result of the amp not being salable because it’s street value is just not there even with the 4 amps considered the “creme de la creme” of solid state amps. Especially if one takes the extreme definition of “servicing” an amp. When I have seen those types of whiny ads posted on Craig’s List and elsewhere, I usually know who it is (of a group of about 3 people) and they are trying belittle everyone else to try generate business for themself because they cannot sell the amps they have because they sank too much time and materials into them and no matter how much 1950’s era “salesmanship” they apply- people are not buying the amps. I spoke to one earlier this year thinking he had some potential need to fill in some lower price points so he could appeal to more people and generate business for himself. Instead he was trying to sell me on amps he had worked on already and put too much into them as if I just floated down on a big white balloon and knew nothing of electronics.
Now it does make sense to replace the filter capacitors in older equipment, but if that is all you do, or reconfigure a tube amp from an autobias type to one where you set individual biases, neither one constitutes “recapping” or “servicing”, but there are some people out there who do sell that small amount of work as “Service”.
Sometimes capacitors do not need replacement unless you are wanting to change the “type” of coupling capacitors. Mylar capacitors have been around since the 1950’s, paper caps on the other hand have been around from the early days of radio- some have one preference of one over the other. Some people want “Paper in Oil” types, (which is another type from the 1950’s adapted from other oil bath and other paper types that date to the 20’s). “polyester” and “Mylar” is the same material- Mylar is just a trademarked name of Dupont and nothing more than that. However, Polyester is a different material than Styrene, or polystyrene, which is a little more modern with it’s own audio characteristics. Many people can hear no difference between the dielectric types, yet some can- many times that difference is very subtle, just like the differences between Coke, Pepsi and RC colas. Not everyone can taste the difference, but some of us can. With audio caps it is no different. But if you take the time to compare them, and listen for clarity, listen for bass reproduction and detail, listen for treble tones- are they clear, or are they harsh, or are they “mushy”? Think of it like tasting wines. The first two wines I sampled many years ago, my uncle was disappointed because they were in theory two diffferent wines, but largely the same grapes were used for both- yet I could taste no discernable difference because I had no preconcieved notions, and the two wines were not significantly different as both were “varietals”, but this is not to say I could not distinguish them from other wines even then, the fact is I can. It is just that some wines you need to surpass a certain cost per bottle to really start tasting differences. The same is true with bourbon, the same with vodka, the same with scotch whiskey. If you go in with preconcieved notions, you have an expectation and the result is usually a self fulfilling prophecy, and therefore not accurate. Just because you cannot hear any difference however, does not mean it is not there. But there is the matter too of expectation, which can lead many people to invest in a lot of materials that really may not be doing what they owner thinks they are doing.
So, once I clarify what someone wants done on the radio or amp, or other device, it is straight forward from there. Sometimes an amp or reciever only needs controls cleaned- some do not see that as “servicing”, and as long as you do not present that as “servicing”, you will not have any misunderstandings. But you still do have time invested in that amp doing that work to make sure it was functional to a minimum level. Most of the people actively advertising for “Pioneer” and “Kenwood” equipment are only looking for the top end units and are only wanting to pay about 10 bucks for the amp if they find it, and they will dismiss anyone else’s time that may have been spent on the amp cleaning it up, locating parts, etc. I will not deal with those people when I do come across those amps because I know how they are, and I have other people to sell to.
If the amp is quiet, (turning up the volume with no input signal reveals the “Noise Floor” of the amp. If it is quiet- in that hiss is not noticed, such as a vintage Sherwood I have, the amp may not need much attention, but if there is a great deal of noise, then you want to look into things much further, such as checking transistors for poor solder connections, poor general condition of certain transistors and electrolytic capacitors (ie leaky) or bad solder joints on the circuit board or chassis, or dirty tube sockets, defective tubes, noisy composition resistors or dirty controls and switches, and checking idle current draw with zero signal on tubes and transistor units, etc. The procedures vary between tube and solid state for setting or testing idle currents and for right now I do not plan on going into much detail on that aspect anytime soon, later it may become a post though.
If at normal listening levels, if the audio signal is distorted, then you need to decide a course of action- replace all transistors? Replace just the leaky/noisy ones? Or if tubes, find the weak ones or misbalanced or otherwise defective tubes. If microphonics are a problem, which is a situation where physical bumps to the floor in the room or amp result in a corresponding noise through the amp. It is not uncommon for headphones to reveal microphonics before you hear it through speakers, and many vintage solid state systems with PNP type transistors suffer from noisy finals. Microphonics can come from a loose grid wire in tubes, to failing connections in transistors, electrolytic capacitors or resistors not being as tight as they were when new, etc. Even disc capacitors can become microphonic due to internal delamination or leads pulling loose.
This is where actual troubleshooting begins.
Integrated circuits can pose their own problems- avaiability is one of them moreso in tuners and tuning sections of recievers than elsewhere in the system. (An integrated amplifier, or amplifier has a separate radio tuner outside of the amplifier. A “Reciever” has that tuning section integral in or on the amplifier chassis) Audio preamp ICs are constantly improving and somewhat interchangable, with the most common one: “4558” which is a design dating from the 1970’s and is still in use today in many amps and still in production. While it is dual op amp chip based on the single channel “741”, it is much quieter the 741 and the 1458 is nothing more than a dual 741. However, over the years there have been several improvements by other manufacturers that are even quieter and bear different part numbers. For example, in the late 1970’s National intorduced the LM387 dual preamp. Great amp for phono preamps ( and I made a few back in the 1980’s with great results) which was made to capture the tape deck preamp market based on how simple the tape preamp with “NAB” equalization was in it’s design. It is designed primarily as a “single ended” amplifier (one power supply), but has not been in production for almost a decade now. The vast majority of modern preamps are a “dual supply” or “Double ended” supply, which is how most solid state audio amps are made. There is nothing wrong with a “dual ended supply”, but it does become an issue when dealing with low voltage systems such as in automotive applications.
There are some other common “FET” or “BiFET” op amps, such as the LF353, TL082, TL072, and some others. Most are still made today, and the LF353 and TL072 make some reasonable quality utility preamps and can be used in any circuit that has a 1458 or 4558, and the TL081/TL071 can be used with the “741” circuits that were in many guitar circuit construction books from the 1970’s onward. The TL7x series of op amps are lower noise than the TL08x series. Plus most of the FET and BiFet op amps are quieter in general than the 741, 1458 and 4558.
The 4558, 741 and 1458 had many prefix letters, but they are the same ICs for numerical type regardless of who’e prefix is on them. Today there are some low voltage preamp chips/op amps that can be used on 12 volt audio, but they need to be handled differently as they are dual supply types which complicate things as the audio ground cannot be the same as the chassis ground. Some newer op amps too are low enough in noise to be used for radiation detectors, which makes them quiet enough for the most discerning phono preamp circuits. That is a topic for later discussions
Servicing a tube amp that has not been use for years usually entails replacing electrolytic capacitors, checking resistors in the power supply with some potential replacements for those damaged or out of value, testing the tubes and replacing weak or defective tubes, replacing defective tube sockets, input jacks and terminals, checking idle current, cleaning controls, switch contacts and tube sockets, and confirming the amp is quiet. Replacing all audio coupling caps, and comp resistors is a case by case situation depending on the preferences of the owner. And if the amp is to be sold- replacement of all of those parts is a waste of time and money because doing so puts prices in the stratosphere well above “Street Value”, and if the amp already has polyester capacitors- these are the most reliable types in use, and have good sonics.
Now for amps and electronics in general, the basics for troubleshooting are:
First step, especially if the item has come to you and you know little of it’s condition, the obvious things are the first order of business- checking the power cord and visible wiring for damage. Essentially checking the first obvious things if there is some evidence of damage from mishandling. Sometimes an issue arises at the wall plug where the insulation is pulling away from the plug leaving conductors exposed. Close inspection usually reveals some tearing of the insulation and expose wire conductors. If the cord is in good condition otherwise, replacing the plug end with new one will suffice. If the cord is in fragile condition- just replace the whole cord.
Second order of business is cleaning controls and switches. This is not a hard thing to do, it usually entails simply the application of a contact cleaner, or LPS-1 into the gap of a potentiometer where the terminals come out, or the “rotation stop” stamped into the cover. Some controls use a resistance wire, and contact cleaner will work, but is sometimes not needed. LPS-1 is a lightweight lubricant with some solvent action and is not conductive. Contact cleaner can be conductive if it is water based, sometimes older cans of cleaner used a CFC- which are effective and have a distinctive odor, and some are a petroleum solvent base. The label will tell you directly or indirectly what is in it. Depending on the materials used, you need to use plastic compatable chemicals.
Often you will see people suggesting a product called “Deoxit” produced by Caig. This has it’s uses, but not in potentiomenters and absolutely not in slide pots as this softens the substrate that the carbon track is applied to and will destroy those. Now if you dismantle the control and only apply it only to the metal contacts, that is a different story, but that is usually more work than it is worth. If Deoxit is applied sparingly to the exposed contacts of a switch, you will be fine. Some rotary switches have a different issue in that a vinyl plastic was/is used to hold the center contact that is rotated through the positions- these often fail just from age and fall apart, so are best handled with as much care as you would a “Ukranian Easter” egg that is several years old. Even then though, the Ukranian egg will be more durable. If you have one of these switches and it is already broken, but the pieces are all there, then you need to draw out the schematic of the switch contacts and in what position the center portion makes it’s contacts and then find a modern or vintage bakelite type that can be wired to work in that location. Bakelite is also known more generically as “Phenolic”.
Once those steps are done, then a physical inspection of the chassis is in order. If the power cord was replaced because it was a natural rubber insulated type that hardened, you may find wiring inside that is in the same condtion or worse. If mice got into the unit, that is nothing short of a minor disaster if you do not have a schematic or cannot identify the chassis. Mice will chew on everything, plus leave a corrosive mess behind. When dealing with newer electronics that utilize an IC, locating an “Application Note” or “Data Sheet” for that IC will give you a good idea of what the rest of the circuit around it is. At a minimum you can identify the voltage pins and inputs and outputs.
If the unit is solid state, if older than about 10 years, replacing the filter caps in the main power supply is usually a real good idea before applying any power, and these rarely “reform” (explained later). The same if you are working on an older 5 tube radio- most of those filter caps dry out. If plugged in and left that way, it is fire hazard. Older collectible audio gear and other older radios that have a metal can capacitor should be checked at where the leads come out of the capacitor- any evidence of corrosion, soot or leakeage from inside means replacement is your only course of action.
If the leads or terminals of a “can type” of capacitor “look” clean, you might consider bringing up power on the unit with a “Variac”, which is a transformer that allows you to dial up the applied voltage. Begin at 50 to 60 volts for an hour, and then increase 10 – 20 volt increments (after the first hour at the low voltage) every 40 minutes to an hour. Doing this reforms the oxide layer on the capacitor plates to increase the insulation factor between capacitor plates. This works about 40 to 60 percent of the time, and can be worthwhile to at least try it. You can gut that capacitor out later and stuff new ones in if the capacitor does not reform and you want to retain the original look, or solder a terminal strip on the underside when possible and install the single unit capacitors under the chassis. Some chassis’ are close toleranced, such as under a Sherwood S5000 series audio amplifier, and the only real option is to gut that capacitor that is held with a clip located at an angle to the chassis- gut it and stuff the new component capacitors inside it after wrapping them with a high quality electrical tape or shrink tube, and these is barely enough room under the chassis for careful placement of the other electrolytic capacitor values.
What happens if you have a capacitor that does not reform the oxide layer is the capacitor shorts out and overloads the rectifier tube and potentially burns out the “B voltage” winding of the power transformer if left on long enough. The transformer in that situation gets hot enough to melt out any wax that was in it, and the older radios from the 1920’s into the 1930’s were often potted in asphalt/tar. If in your initial look at the underside of the chassis, if you have a transformer that has leaked out the asphalt potting, or a great deal of wax, don’t even try to reform the filter caps as they are likely shorted out already. The chances of a transformer surviving getting that hot are exceedingly slim (you have better luck betting on 99:1 horses at the horse racing tracks.)
If you isolate the transformer leads from the radio when you suspect a shorted transformer, you can check the transformer by plugging it into the wall for short periods as you check to see what each winding pair shows for an AC voltage. If any are significantly lower than they should be, such as seeing only 3 volts on a 12 or 6 volt heater winding, the transformer is likely junk. The transformer will usually also get hot quickly. 60 volts on a winding that should be 250 volts is definitely an indication the transformer has a problem and likely is junk.
Even if it seems the filter capacitor(s) have reformed, check the transformer for heat. In normal operation that transformer can get warm, but only warm enough that you can keep your hand on the transformer covering bell. Also check the can capacitors to make sure they are not heating up. Sometimes they will feel like something is boiling inside of them, which on a case by case basis, should be monitored, but at any indication of excsseive current flow in the form of heat- replace that capacitor.
If you replace the filter caps- make note of the can polarity and any isolating hardware, washers, phenolic mounting plates, etc. There were some can caps that had a common positive can. If in doubt, look at the schematic to make sure. If you are looking at some European gear, sometimes it is not entirely clear what the polarity is of the orignal capacitor, and you need to rely on the schematic.
If the transformer looked okay and nothing looked melted or burned (some paper capacitors will look a bit melted, but that is normal for many of that type. If however the wax is visibly very much melted out of the paper or ceramic jacketed coupling capacitor, it should be replaced. More on these later.
When reforming is attempted, monitor the transformer for excessive heat. A “warm” transformer, one that you can rest your fingers on for an extended period of time, is usually okay. But watch for any tubes showing redness on the outside tube structure, or purple glow within the tube. These are indications of a problem- if the outer structure of the tube turns a dull red or brighter- that means the filter capacitors are junk, or something else has shorted on the output circuit of that tube. If the tube develops a glow inside- unless it is a mercury rectfier or gas rectifier which are supposed to glow, that is an indication of gas ionizing inside of the tube. Some power amplifier tubes will show a tiny amount of blue in the vicinity of the heaters or plates, this is something to be watchful of, but not an automatic condemnation of the tube or capacitors unless it becomes a glow that fully engulfs the tube envelope.
If the tubes light up okay, then you can proceed to some other basic tests if it is not functioning.
With Solid State gear/radios, look for electrolytic capacitors that may have vented, or may have leaked. If they are bulging or there is a smoke residue on the PC boards, or just two leads poking out of the PC board (indicating the body of the capacitor is somewhere else), or they show a black or dark brown substance on the top of the capacitor- just replace the capacitor(s) and those that are around it. Much of the solid state electronics made from the 1950’s onward have cemented the capacitors to the circuit board and they were made that way so they would survive the vibrations of shipping from Asia or Europe and remain bonded to the PC board. If the stuff looks thick and rubbery like contact cement, it is just adhesive. If it is crumbly, it is likely from the inside of the capacitor nearby. That adhesive can be corrosive, so if one capacitor shows corroded leads from being in contact with that adhesive, just replace all of those that were cemented. If you are just wanting to do some functionality tests, plug it into the variac and bring up power over the span of a minute to reach full power and watch the meter on the variac the entire time- if it shows excessive current, which is anything more than about 20% of it’s power rating on the device’s label(once you work on a few things, you develop a “feel” for where to draw that line, and not everyone is going to agree on where to draw that line, but anytime the unit draws more power than it’s label indicates, then you have a problem), the filter caps are bad typically, but there may be other problems as well.
If you do not have a variac, then simply take a lamp, wire an outlet so the lamp is in series with the bulb, and then use an incandescant lightbulb (or do this in steps) 75 to 100 watts in size to see how relatively bright it gets. A Solid State stereo for example will marginally operate with a 75 Watt bulb or 100 Watt bulb, but if the bulb lights to normal intensity, just shut it down as the amp has something shorting. The lightbulb in this case limits the amount of current able to flow through the amp or radio thus minimizing further damage. It is essentially the same thing as old appliance testers.
Dim Bulb/Appliance tester
Many of these following tests can be done with a solid state amp on a dim bulb tester with a smaller bulb (if you get some audio activity you may not have that many issues in the amp), but until you know nothing is seriously wrong keep it on reduced power then go forward from there. A tube amp may or may not begin responding audibly with a dim bulb tester, but the tube amp is a bit simpler in general construction and therefore easier to troubleshoot without being under power.
If the unit passes the dim bulb test. and if things appear normal visibly, but the unit does not operate, you can do “noise injection” on units where one channel may be defective or if you know the finals are fully functional. Noise injection is just using an insulated (plastic handled) screwdriver and touch each grid terminal of tube amps and tube radios, or the base and collector terminals of each transistor in solid state radios and amps starting at the output, and work your way back. Where the “buzz” fails to appear is usually the area where a problem exists- such as a “coupling capacitor” that has pulled apart internally leading to an “Open” capacitor in tube equipment, or a transitor has failed in solid state gear.
Paper capacitors and some other material coupling capacitors indicate the outside foil layer with a “-” sign or band or special dimple if US made. These can be replaced with “Stacked film”, “Polyester”/”Mylar”, “Paper in Oil” (PIO), “metalized film”, “Ceramic disk”, etc. Many of these do not indicate an outer foil, but if you can identify what lead attaches to which side of the “Stack”, or “Disk”, you can place the capacitor in a manner where that one side that would be the “Outer foil” of the original capacitor is now the side facing away from the chassis, and you can use the chassis to act as the other part of that shield. I have in some rare instances actually replaced paper capacitors with high quality ceramic disks, and in doing this I still had acceptable results, but I much prefer the stacked polyester types or wound polyester or styrene types.
With solid state gear, the problems can be more than just a failed capacitor. Transistors themselves can become noisy as can any type of capacitor. Integrated circuits and composition resistors can become noisy as well.
Another problem area with tube era gear and some older solid state gear are the composition resistors used. The old “Dog bone” resistors are the least stable and often drift off value are primarily found in pre World War 2 radios, though some early film types used a similar construction (largely Japanese origined radios and European radios, but should not be confused with the older composition dogbone type). Newer composition resistors can also drift in value, and in some cases both of these can be fire hazards. When they just drift off value, this can affect sensitivity of a radio, or lead to inconsistency between two channels of a stereo. In tube gear, you can usually measure the resistors without much interaction from other components when the unit is off and removed from power due to tubes not conducting when not energized to see if the resistors deviate much from their color coding. Transistors on the other hand are different and direct measurement of resistorswithout removing at least one lead of the resistor is at best a gamble. However if you are measuring the relative resistance between the 2 channels of a stereo that has one working channel and one faulty channel, that is a different story if you are compairing resistance between identical points of each channel. But noise or signal injection are probably going to be the fastest way to narrow the area down by compairing the corresponding devices found between the two channels. In the event you cannot generate noise in the output of a solid state unit, you can signal trace with an audio source such as the audio output of a signal generator, or even just a music signal from another radio or tuner and using an oscilliscope or another amplifier such as is found in a “Heath” visual and aural signal tracer, or even a set of high impedance headphones with test prods on the ends. Then you can follow the signal path through the unit until the sound becomes distorted or disappears. Then you search around that area where the signal disappeared to resolve that issue.
When using a pair of old high impedance headphones, one lead can be clipped to the chassis. This allows you to apply a signal to the device being repaired and while powered up- starting from the input side of the circuit working your way through the audio chain along “Base” terminals of transistors until the signal drops out, or to the input terminals of signal IC’s (this is where you want to find the “Datasheet” for the device via it’s part number, such as “LSC 4558” so you can identify lead functions.) and their output. Where the signal disappears, is where to look for the issue. Electrolytic capacitors can dry out, short out, or just fail “open”. Transistors, IC’s or diodes can also just fail, or can partially fail causing a great deal of noise.
If a solid state unit appears to be just “dead”, check fuses (I have had fuses that looked okay, but had corroded through under the metal contact sleeve.) with a continuity tester regardless of their appearance, the rectifiers and power switch and cord to make sure they are okay. Then check the continuity of the transformer windings with the unit unplugged, then plug it in and see if you are getting the primary voltage (120 VAC nominally). If yes, then check secondary voltages. If no voltage is present in one or more areas of the power supply.
One of the often overlooked issues are the solder connections. In the late 50s into the 1960’s some antimony got into some solders. Antimony is not entirely stable and will break down leaving the remaining materials it was alloyed with porous (and sometimes deformed when too much of it is used in diecast parts.). When it fails in solders, the surface may appear okay, but underneath, the connection is weak and resistive. So if you have a device you are working on and nothing is apparent after everything has been checked over and nothing was out of sorts, reflow solder connections. If you are looking at some of the radiation detectors from that era made for the Civil Defense Department, many of those had components that had their leads were nickel plated, which in itself is a problem as it does not always alloy on the surface with the solder used.
Starting with units made about 1994, silver based solders were adopted as the solder of choice due to some innitiatives like RoHS (Reduction of Hazardous Substances) which phased out lead based solders among other things. The solders were also adopted readily because they were a little more durable for the Surface Mounte Devices (SMD’s) being introduced. The problem is the higher melting temperature solders also had a shorter period of transition from solid to liquid, so “cold solder joints” (weak and poor conductivity) were a more common problem than they had been previously. If nothing is apparent, and if you research to find if the company had poor quality controls in place, suspect bad solder joints. Polaroid TV’s had this affliction with sets made prior to their bankruptcy and reorganization.
Another thing to watch for is the older fiberglass PC boards- these are usually not heat tollerent, and you will have traces that lift easily no matter how good you are with sldering iron and sometimes the conductive rivits (eyelets) will also lift. Most PC/component boards were not meant to be serviced on the component level. But ChipQuik has a product intended for SMD device removal that aids in working with these fiberglass boards too. It is low melting point “desoldering alloy”.
Isolating what may be functional from what is not is largely a matter of logic- “does it work” or “does it not work”, and many service manuals have that type of breakdown for troubleshooting. However, there are times when some other factors may come into play that can derail that type of “decision tree”, but it is a starting point. Sometimes you can find a service manual or schematic online, sometimes not. A big part of troubleshooting is looking at the circuit to see what it does and evaluate why it may not be doing what it needs to do. Many times there are part number references inked onto the circuit board that have their location indicated in the schematic and are easy to locate in that manner. Again, if a schematic is not available, you can usually make some headway with an Application Note or Data Sheet from the maker of the IC. This is especially true with the newer digital TV’s and monitors.
With a simple amplifier, it is going to be basically a power supply, a preamp/driver stage, a power output stage and audio switching system to switch between inputs. If you are looking at a “Reciever” instead of a simple integrated amplifier, the only real difference is the tuning sections are part of the circuitry of the amplifier instead of being a separate component, and is switched essentially the same as it would be with the exception the power is also part of the input switching schema. With Things like modern TV sets, block diagrams can get complicated, but the issues usually settle in around the power supply, the inverter board for the backlight, the backlights themselves, and the display. Sometimes the board that controls the inputs develops an issue, and this is usually why people just swap boards rather than repair them at the component level- because sometimes additional failures developed as a result of capacitors failing.
##As complicated and involved as all of this sounds so far, as this is a broad subject. Once you have an idea of what each area of the amp, reciever, tape deck, etc is supposed to do, Power supply, Preamp, Voltage amp (Driver stage), Phase inverter, Power Amplifier, RF stage of recievers and tuners, etc- then it is a matter of determining what stages are working or what stages are not working. Typically in an amp, if the power supply is the fault, both channels are affected and in some cases the unit will just not turn on. In a Flat screen TV, the power supply is often the issue- usually capacitors, but sometimes another fault caused the problem and capacitors were just marginally functional.
In an amplifier that is stuck in “Protect” mode, it is usually an issue with the power amplifier stage, which can be failed transistors or failing capacitors and resistors that have failed as a result of the other component failures. Some use rectifiers as snubbers to protect the transistors, and these can fail. If Zener diodes were used and they are not marked, that can be a problem, but sometimes their voltage can be determined by test voltages when they are included in the service manual or schematic in locations close to the Zener diode. Sometimes if the problem develops after the repair, polarities of transistors, rectifiers and electrolytic capacitors and even tantalum capacitors should be checked. When working with power transistors, hybrid IC’s and some othe components that have special mounting hardware, that hardware has to go back into the original position. Some examples are mica insulators under power transitors, plastic screws and nuts or plastic washers to isolate the conductive parts of the device from the chassis or heatsink they were attached to.
This sums up the very basics of troubleshooting, and for some this may be enough to give a strong idea of what they need to do with other pieces of equipment. In the cases of power inverters, it is the same approach- find what is working and where it is not working and focus in that area in between. Some specifics on testing devices and components will be in one of the next posts.
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