The Guitar Amplifier Players Troubleshooting Guide
This rarely happens, but additional damage and to more components is common. For all projects on The Audio Pages, there are quite specific absolute maximum supply voltages, and minimum specified load impedances which may vary with applied voltage. It is extremely important that this information is adhered to, or individual device ratings may be exceeded, resulting in premature failure. None of the project amps is designed to work with a 2 ohm load, and simply adding parallel output transistors for example just makes the driver transistor subject to failure, and having failed, it will nearly always result in output device failure as well.
When one or more output devices fail, it is usually a good idea to replace all output and driver devices, even though they may seem to be alright. It is almost certain that they have been stressed, and may be more prone to failure at some later date. This rarely affects the input stage, which normally survives even the most destructive failure. Note that in some cases the fault current can be so high as to open-circuit the emitter resistors usually not all, but one or two can fail. Always check these if an amp fails, preferably after you have removed the power transistors and drivers.
Unless the failure can be positively attributed to counterfeit transistors which will fail at much lower power levels than the genuine device , try to determine exactly what went wrong before re-commissioning the amplifier. Check speaker leads, supply voltages and speaker impedance - something caused the amp to fail, and it is better fixed than allow it to happen again.
Voltage measurements must be done with the greatest of care. A simple and cheap fault can easily turn into a complex expensive one with just the slip of a probe! In keeping with the general nature of this article, I will not refer to any specific voltages until a little later, but will rather give an overview of what to look for. At this point, a good understanding of the basics of transistor operation is expected and necessary, otherwise you will not be able to understand what you are seeing on you meter or oscilloscope.
Countless man-hours person-hours? This is the very first voltage measurement you should make - always! For example, on a PNP transistor, with the red meter lead to the emitter, there will be around mV between emitter and base, and anything from negative a few volts to several tens of volts between emitter and collector.
An oscilloscope will show perhaps almost no AC voltage at all on the base, but a large AC signal on the collector - this is usually quite normal. The DC voltage readings will tell you if the transistor is correctly biased, and therefore able to do its job. A voltage of mV between emitter and base, but full supply voltage on the collector is not necessarily wrong - you must read the voltage with reference to the circuit diagram.
Figure 1 - Amplifier Input Stage. The emitters are tied together, with perhaps small resistance values in series with each emitter in some designs. The voltage at the bases will probably be a few millivolts negative, and the emitter to base voltage should be around mV. The collectors will be at almost the full supply voltage in most circuits there are exceptions though. If you were to see that the output was stuck to one of the supply rails, then that will upset the long-tailed pair, and all voltages will be wrong.
This could mean that one of the long-tailed pair transistors is faulty, but maybe not! This is where you need to play detective, to ascertain why the output is stuck to the supply rail having eliminated all the previous fault types - incorrect components, bad solder joints, etc. The next device to test is the class-a driver Q5. Check the emitter-base voltage, and make sure that it is around mV. If that is correct, then the collector should be at close to zero volts, but it won't be. Instead, you may find that it is sitting at or near one supply rail voltage. Look at the circuit - the class-a driver is PNP using the previous example and the collector is at full positive supply, that means that the transistor is fully turned on Or is it?
Troubleshooting & Repair Guide
The next step is to look at the current sources Q3 and Q4. Between the emitter and base of each there should be mV or thereabouts, and the current through each is easily determined. Measure the voltage across each emitter resistor - it should be about The answer is a little further down this page - section 5. The collector of Q3 should be at around mV, and that of Q4 at around zero volts. If this is the case then the amp should be working.
Assume that the collector of Q5 is almost the full supply voltage, and likewise that of Q4 - there are either of two possibilities - Q5 is shorted or turned on fully , or it has no collector current. The job of Q5 is to pull the output high as it turns on, and let it swing low when it turns off, but if Q4 were supplying no current, then the output will swing high.
The input stage will try to turn Q5 off, but will become unbalanced by the voltage at the feedback input. This will make the circuit inoperable until the fault is located - this is your mission, should you choose to accept it, of course ;-. So, Q5 has full positive supply at its collector, give or take a volt or so not important at this stage. The collector voltage at Q4 should be about the same, and the current should be about 6.
If everything were working as it should, the amp would be functional, so there is something amiss - but we knew that already. What is the voltage at Q4's collector? Is the voltage across Q4's emitter resistor 0. If the collector voltage is near the negative supply rail, or the emitter voltage is a lot lower than 0. If the collector voltage were at close to positive supply, then the emitter resistor could be open - probably a bad solder joint, as resistors rarely go open without a lot of smoke and fuss. Check the value carefully - was a k resistor inserted by mistake? Figure 1A - Amplifier Example P Figure 1A shows an example, in this case based on P You need a multimeter and Ohm's law, and very little else to monitor and verify the voltages and currents that should exist in virtually any amplifier design, regardless of topology.
Let's look at the schematic above. Voltages are shown for each major point on the circuit, and from those voltages we can work out the current through resistors and many of the transistors.
As an example, R5 is 47k and R6 is ohms. There is 0. Why didn't I subtract the 1. There is an obvious error, but it's important to realise that the exact value is unimportant. What matters is that the voltages, currents and resistances make sense. This applies to every part of the circuit, and there is one thing of which you can be certain If the output voltage is not close to zero, all other voltages are likely to be wrong! If the output voltage is close to zero, then the amp should be working, but only if it has power.
For this reason, I generally never bother to show voltages at various parts of any circuit, because the voltages will only be correct when the circuit is working properly. It would be silly for me to try to give voltage readings for every possible fault scenario, and the information would be completely useless to you anyway. Most of the time, you can analyse the circuit and calculate the likely voltages that should appear at various points. They do not need to be accurate, but they must make sense.
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It doesn't make sense if the base-emitter voltage of a transistor measures 15V - that immediately indicates that the transistor is either the wrong type, is inserted incorrectly, or is faulty. Double check the datasheet, then replace it with a new one of the correct type! If you suspect that a transistor has been inserted the wrong way around, once power has been applied to the circuit you've probably damaged the device.
Do not reuse damaged devices - there's a place for them - the rubbish bin. Circuit analysis for servicing is not a simple task, but if you apply logic and basic principles you have a good chance that you'll find the problem. Sending me an email saying "It doesn't work. Most of the time, voltage readings are of no help either, because they are often taken the wrong way. Look at how the voltages are shown above. The voltage across R6 is 0. The latter reading is pointless, because the supply voltage will vary as you take a reading, and the reading will probably be so far in error that it's unusable.
Many other readings are taken the same way. Needless to say, you must take great care when readings are referred to the supply rail s , because a slip of a probe can easily cause much greater problems than you started with. It is not possible to cover every possibility here, even with the simple circuits shown, but by carefully measuring the voltages you will be able to track down the most likely cause, without having to rebuild the whole circuit! The answer to the little riddle for Figure 1 above There must be about mV across the emitter resistor of the current sink because there are two diodes in series.
D1 balances out or 'cancels' the emitter-base voltages of both Q3 and Q4 - also mV. Whatever voltage exists across D2 and we know it must be mV , must also appear across the emitter resistors. It really is that simple, but it may take a bit more experience before you see it clearly. A useful thing to remember about transistors - if it gets hot, it is working or trying to. Looking at Figure 1 again, if Q4 gets hot and Q5 is dead cold, then Q5 is probably the faulty device - not Q4 as you may think at first. These guidelines are as far as I can take you in a basic article.
The ability to think logically and methodically and to work your way through the circuit is essential. Blindly measuring voltages without understanding what they mean in context will not reveal an answer, but if you can go about the task as outlined here you'll learn a great deal more than you might have expected. Transistors can be tested for basic functionality with a multimeter. If you use an analogue meter, be aware that when on the ohms range, the red probe is negative. Digital meters retain the 'correct' polarity.
As with any diode, they should conduct in one direction, but not in the other. All BJTs may be tested this way, revealing open circuit, leaky or shorted junctions. The test tells you nothing about gain, voltage breakdown, or anything else, only that the device is likely to be functional. Figure 2 - Basic Transistor Test Model. Reverse red and black and measure again - in some cases, one connection may still show mV because of a connected power or driver transistor - this is normal. By using this method, the proper conduction of each diode can be checked - as with any diode, the forward voltage drop is around mV which as explained above shows on most digital multimeters as 0.
In-circuit tests can also be done like this, but the results may be misleading because of other devices in the circuit. In case you were wondering and you are by no means the first to do so , you cannot use two ordinary diodes wired as shown as a transistor. Transistor operation relies on the junction between the 'diodes' hence bipolar junction transistor.
All diodes should show proper conduction and blocking as the probes are switched from one end to another. This is not a useful test for LEDs or zener diodes, but at least you will know if it is open or short circuit. Capacitors really need a capacitance meter as well as an ESR [Equivalent Series Resistance] meter to test properly, but you can still get a fair idea with a multimeter. Shorts are uncommon in film caps, but can occur, although in most projects this is highly unlikely.
Electrolytics should show a low resistance at first, which will rise as the cap charges. Reverse the leads and make sure that the cap discharges expect to see silly resistance values at first , and charges up again. Low voltage reverse polarity will not harm electros. Most other components transformers, connectors, wiring need only to be checked for continuity, and that all wiring is connected to the proper place. Verify that voltage actually goes somewhere - an open circuit or dry solder joint will show up as voltage present at one point, but not at another that is meant to be directly connected.
This can be especially true of a printed circuit board that has been damaged. A broken track may be invisible, but it will still be an open circuit for the voltages that are normally present. There is not much that can go wrong with an opamp circuit. Most linear circuits as used in preamps have one thing in common - the two inputs should be at almost exactly the same voltage, and so should the output. The most common problem is oscillation - especially with very fast opamps.
The ESP boards are designed so that bypass capacitors are as close as possible to the opamps, and there is also additional filtering using small electrolytics. It is still possible to make an opamp circuit oscillate though, so sensible precautions should be taken - keep inputs and outputs shielded and apart, and always use a ohm resistor in series with the output of any opamp that connects to a cable - regardless of length. Other problems can occur, but normally they will be the result of bad solder joints as always , damaged PCB or incorrectly installed components.
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All ESP boards will function first time, every time if assembled according to the instructions, but if yours doesn't, then there is a mistake in the component placement, or the opamp is faulty. Yes, opamps can be faulty from new - it doesn't happen very often, but it does happen.
As with power amps, leaving off accidentally or otherwise the power supply earth zero volt line is quite a common 'fault'. This is the area where it all goes to pieces. Hum or buzz is the usual symptom, but unfortunately, there are no fixed rules that can be applied in all cases to cure the problem. The distinction between a 'hum' and a 'buzz' is extremely important! If you describe a noise as a hum, then the expectation of anyone knowledgeable in the field will think "low frequency, no or few harmonics". This describes the noise made by an earth loop - a situation where two or more pieces of circuitry are joined by the mains safety earth lead and the shield of an interconnect for example , forming a loop.
This can inject a very low voltage but sometimes surprisingly high current into the loop, and the signal is picked up by the inputs. You hear hum - a single low frequency tone. Buzz is caused by any number of things - input leads close to mains wiring, power transformer or bridge rectifier and associated wiring , bad or no earth connection, loops they can cause buzz as well as hum , the list is almost endless. Sporadic oscillation in an amp can also create a buzz or hum in some cases - follow the guidelines above to ensure that the amp is stable under all conditions - low level oscillation can usually only be detected with an oscilloscope, but you may be able to detect it using an RF 'detector' probe - see the projects page for a suitable example.
With any of these problems, it is almost impossible to give a standard 'fix'. The solution is different in nearly every case, and sometimes the best result is obtained with an arrangement that should not work at all. My normal approach is to keep lots of separation of input cables from anything else, and for locating the optimum earth location, I use the following methods This method usually works well, and if you really do find the optimum location, you will need another amplifier to be able to hear any noise.
Storing your guitar in a case also safeguards your instrument from the likes of cooking smoke, spilled drinks and any direct sunlight that could ruin the finish. Which brings me to my next point…. In addition, if you have any damp spots in your house, keep the guitar well away as dampness could potentially set into the wood and warp your guitar. Get a set of straplocks A good set of straplocks are an essential accessory for your guitar. Condition the fretboard The build up of dirt and grime on a fretboard may look cool, but after a few years, it will turn the playing surface into a sticky fly paper-like mess.
Add 2 or 3 drops to a lint free cloth and rub it down the fretboard once every 2 or 3 months. This will increase speed and articulation in your playing whilst protecting your fretboard from damage caused by dirt and grime. Clean the nut Your strings will pick up dead skin over time and so will the nut. Every time you change your strings, give the nut a little attention with some dental floss or the edge of a thin nail file. This will reduce tuning problems and sustain issues. Tighten up all the loose screws, nuts and bolts Screws, nuts and bolts will inevitably come loose over years of playing.
Check under your volume pots to make sure the nuts are tight too. Adjusting truss rods or replacing pickups can be a tricky thing to get right and you risk damaging your guitar if you decide to try and modify it yourself. Now on to guitar amplifiers. Read on…. Keep your amplifier away from a heat source Your amplifier is full of electronic components that if placed in direct heat can become damaged. Just like your guitar, keep it away from a heater and out of direct sunlight at all times.
This will make sure all the innards are kept safe. Mold spreads to unclean surfaces quickly and can get worse in damp rooms. Amps are evolving at an unbelievable rate and the technological advancements bring with them seemingly infinite sound options. Allowing players to effortless switch up their sounds and even find a unique individual tone. When it comes to choosing the best modeling amp for your needs there are many factors to take into consideration Fortunately we have compiled a practical list of the top ten best modeling amps in no particular order.
Line 6 Spider Jam Classic With newly developed high fidelity algorithms Fender has outdone itself with its Mustang series line and makes it into our top The GT40 is a fine specimen of versatility in a modeling amp. It precisely models realistic amp response offers modern-day compatibility capabilities.
Packed with 21, classic voices and more than 45 different effects the possibility this amp allows for is endless. In addition to everything the previous model had to offer the pathway signal processes have been refined, you are now free to move effects anywhere in the signal chain. In keeping with the current direction of modern Fender have created and included a tone app which gives users access to downloads and a database of stored presets created and saved by other artists. You can also directly connect with an online community of creative fender musicians.
Why We Liked It - Fender have taken their experience in the amplification field to provide tube-like emulations in a versatile modeling amp and effects loop to the masses. This is a sweet modeling amp that puts classic line six effects neatly inside a 15 watt digital version for liberating tone experimentation. It is a durable but lightweight and economical alternative to top of the range models from Line 6.
Musicians will find it easy to adjust their sound by simply tweaking the knob which allows users to control their tone and combine two effects simultaneously. In addition to your usual reverb, delay and modulation settings Some of the other featured effects include tape, sweep, echo, and tremolo , with fully adjustable parameters. It has a closed back cabinet and a custom built 8-inch speaker and comes with an FBV foot controller for remote switching. It also sells with a copy of spider jam editing software which takes the tone shaping capabilities to another level and gives players access to exclusive online content.
Why We Liked It - It is an exceptional low budget amp model ideal for classic as well as modern rock. This is a range of guitar amplifiers that honestly allow musicians to dial in and find their own unique sound.
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It is a stunning modeling amp replicating the companies legendary high-end amp tones. Consisting of 14 preamp models, 4 power amp models, 8 speaker cab models. Players can adjust gain, volume, modulation, delay, reverb accessing around presets and 24 effects. Enabling customers to produce a blend of Marshall's authentic tones with pro-quality effects including that critical glistening clean tone.
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It really is quite limitless. This Yamaha THR10 is a great example of an excellent modeling amplifier. With 5 separate channels and a whole host of effects, this modeling amp has plenty to offer. The product was developed and aimed at the heavier rock guitarists in mind who love high gain and distortion. Emulating the response of a cranked up stack it delivers subtly diverse distortion, fuzz and much more.
Some of the more notable effects include a Flange, Phaser, and tremolo, it also has spring-reverb as well as hall style reverb settings. It also benefits from an additional compressor and noise gate exclusive to the THR 10 X model. Sporting vintage aesthetics with its army green metallic look case and strong carry handle for portability. The Yamaha THR10 comes with an assortment of useful guitar accessories and a copy of Cubase for branching out into a recording.
Why We Liked It - It can also be powered by batteries making ideal for outdoor jamming on a sunny day. You can also silently practice leads along to the track whilst wearing the studio headphones that are part of the package. Another fantastic product from the amazing Katana series by BOSS, the line beautifully represents the company's intentions of providing exceptional 'rock sound'.
Featuring finely-tuned processing over the generations relying on the same processes that perfected the popular Waza amplifier, this portable watt amp head is a powerful, compact modeling amp. It delivers genuine tube-like' amp tones in a convenient modeling amp. Expertly manufactured It presents five amp well-known characters: Clean, Crunch, Lead, Brown derived from the Waza amp , and Acoustic which you can simultaneously use a combination of up to 3 at a time.
The additional BOSS Tone studio editing software is a digital perk which offers up 55 delectable sounds, all can be fully customized, enabling users to build sounds from scratch. You can store 15 of the effects on board for quick retrieval in live situations. Why We Liked It - This is a great value for money modeling amp, it offers a fair few watts for your dollars and incredible integrated software meaning custom sounds can be recalled for performing. This analog valve modeling guitar amp features revolutionary circuitry and a newly developed modeling engine. Utilizing Virtual Element Technology which carefully analyses the compo0nants of the circuit itself VOX have taken manufacturing to the next level.
Fabricated with a tightly sealed cabinet which enhances responses for better reproduction of classic tube modeling tones. The VOX Valvetronix valve modeling guitar amp features a preamp design which contains analog circuitry Including a vacuum tube capturing unmistakable subtleties like vacuum amplifiers. Choose from 11 realistic amp models which can be run through 33 preset programs. Modify with 13 high-quality built-in effects. Why We Liked It - The VOX valve modeling guitar amp has been Skillfully calibrated improvements to circuitry, it offers a good range of amp models providing electric guitar players with lots of options.
Boasts unique distortion tones and a propriety bass-reflex. Unbelievably this micro modeling guitar combo amp delivers 8 guitar amp models including the renowned JC The battery-powered electric guitar amp models benefit from Roland's breakthrough composite object sound modeling COSM the latest DSP modeling technology which produces near perfect computer reproductions of sound wave effects.