By Steve Weiss

common failure modes of guitar amps

I have been servicing electronic musical equipment for more years than I want to admit and it never ceases to amaze me the misconceptions people have when it comes to troubleshooting their gear. I will present here some of the common problems I see come across my counter and what they actually mean.

Fuses

| can't tell you how many times, I hear this said. "I think its just the fuse." Well it could be only the fuse, but first let's define what a fuse does. It is a protective device that is in the amplifier solely for one purpose - protection. In the event something in the amp fails and causes it to draw more power than it's maximum power rating the fuse will open and keep other components that haven't failed yet from being damaged or in the worst case scenario, it prevents electric shock or fire. It is possible that the fuse has opened due to an overload or power surge and if it is replaced all will be well. Therefore the first thing you should do is try a replacement fuse of the same ratings. If it works fine, you're done. If it blows again there are other problems inside the amp that need attention and it is time to have a qualified tech look at it. I must also add, never under any circumstances replace the fuse with a larger rated one or the infamous Wrigley's gum wrapper 100 amp fuse. This is just asking for trouble. Additionally if your blown fuse is the type that is made from clear glass and the inside of it has black spattered all over the glass, you have a short somewhere in the amp and I guarantee you replacing the fuse is a waste of your time, it's just going to blow again.                                        

On a tube amp you may see more than one fuse on the back panel. One of the fuses will be labeled main and one may say something like H.T. This second fuse is directly in line with the power going to the output tubes and is there to protect the output stage circuitry including your expensive output transformer. Output tubes can develop intermittent shorts and blow that fuse after which the tubes will still work but I recommend when this happens that the output tubes be replaced. It is probably going to keep happening and most likely even though you are hearing sound, the tubes are no longer in spec and do not sound as well as they should.

Transformers

Ahh, the mystique of the transformer. It amazes me how often blame is assessed to a transformer. I even see techs who are not really knowledgeable (or Unscrupulous) blame the transformer (a nice high profit repair). I have been in servicing a long time and I can clearly state that transformers are the least likely components to fail in an amp. A power transformer has three normal failure modes. A short in its primary will cause the mains fuse to blow  and an open primary coil will cause the amp to be completely dead leaving the main fuse intact. If your amp is powering on and not blowing the fuse it is highly unlikely your power transformer is at fault. A short in the power transformer secondary may blow the fuse or possibly start to smoke and smell.  Of course if what the transformer is connected to is shorted it will cause the same symptoms. The transformer must be disconnected from the circuit it is supplying and run unloaded to actually determine if it is bad.


The output transformer also has several failure modes. In the event of an output tube short it can receive damage to its primary. Collateral damage here  usually produce smoke and fire so you will probably be aware something is wrong. Damage to the secondary side of the transformer can come from overload in designs where the transformer is not up to the task or from playing the amp loud with either no speaker connected or an intermittent cable or jack that is quickly working and then not working. The unloaded transformer causes spikes back into the output stage potentially damaging many other components. A blown output transformer may have no output or often times very little output, but it is hard for the non technician to discern this from other failures that will yield similar results. As I said before, I find transformers to be last on my list of potentially failing components.

Jacks

Some amplifiers contain as many as ten or fifteen jacks and they are all subject to problems that will cause you headaches. Modern amps containing printed circuit boards that are created with automated assembly use a style of jack that solders to the PC board. The most common failure here is that the cable gets pulled on and breaks the jacks solder connection to the PC board causing a no sound or intermittent condition. The remedy for this is to re solder the jack to the board or replace it, If the jack has sustained physical damage. Unfortunately most of these circuit boards are held in place by all the front panel controls and many screws, so the few seconds of soldering needed to be done incurs quite a bit of labor for disassembly and assembly.

A jack consists of several sections terminating in- well- terminals. One terminal handles the grounding of the cable plugged into it and one handles the signal coming in but there are several other terminals that may apply also. In the case of the input jack there is a terminal  connected to ground that will short itself to the signal terminal of the jack when no cable is plugged into it. Its function is to ground the input circuit and prevent noise occurring when nothing is plugged in. If you would like to see what would happen if that was not there unplug the cable from the guitar and just hold it in the air. The input circuit becomes an antenna picking up all the noise around it and amplifying it. When your guitar is plugged in and the volume turned to zero it grounds the signal terminal through the guitar volume pot creating the same effect the terminal shunt does with no cable plugged in. If the input jack signal terminal is bent or corroded and doesn't make good contact with the terminal that connects it to ground when nothing is plugged in, the noise condition we  discussed above will occur.

Another common problem with jacks is when they are used as inserts or patch points in a circuit. On a guitar amp you will commonly see an effect loop and maybe also a power amp in jack. These jacks have a terminal that receives a signal from a preceding stage and passes it on to the next stage when nothing is plugged into the jack by touching the terminal that is normally the signal terminal. The signal then goes from the signal terminal to the next stage. An example of this is the output of the pre amp going to the input of the power amp. If a cable is inserted into the return jack the signal path from the pre amp is interrupted and whatever signal is coming down the cable that was plugged in now flows to the power amp. The problem we are looking at occurs when these jacks are NOT used. Over time the contacts of the terminal corrode right where they touch it each other and it interrupts or distorts the transfer of signal. This problem can be notoriously intermittent and subject to humidity, temperature and vibration. Occasionally a high level signal can blast its way through the corrosion causing it to work temporarily until the corrosion builds up again. Several ways you can test to see if these jacks are at fault would be to play a signal through the amp at a very low volume and while doing so take a plug from a spare cable and insert it into the suspect jack several times quickly noting whether this either aggravates or eliminates the problem. You can also try hooking a cable between the send and return jacks to see if the problem goes away. This bypasses the two contacts coming together and fools the amp into thinking it is receiving an external signal. Once the bad jack is identified my feeling is to replace it. Spraying contact cleaner into it generally does not get it where it needs to go as the contacts that need cleaning are pressed together when no plug is inserted as when you are spraying into it. You could file clean the contacts on the jack ( and potentially damage the plating on the terminal), but to do so you need to pull the jack out anyway so why not be safe at that point and install a new one. A classic example of this failure are many of the solid state Crate amps. They have a bunch of these insert style jacks and whenever I see one if you replace all  the patch style jacks the amp is good to go. These jacks are used in a number of positions and they are all possible causes of signal interruption. They can be inserts, effect loop jacks. headphone jacks, power amp in jacks, external speaker jacks and even foot switch jacks that use these contacts to enable the front panel channel switching when the foot switch is not being used. A classic poor design using these jacks is in the Marshall TSL series of amps. The output transformer signal feeds the 16 ohm jack which feeds the 4/8 ohm jacks by closing its switch contacts when the 16 ohm jack is not in use. When these contacts bend or corrode they interrupt the signal to the speaker leaving the output transformer with no load which as we stated above can do lots of damage.

Pots

Potentiometers commonly known as Pots are the controls on your amps front and real panel that  let you vary functions such as EQ , Gain, Volume etc. They have an affinity to attract dirt which came make them noisy, scratchy or intermittent in operation. Usually they can be sprayed out with a professional grade contact cleaner such as Deoxit and restored to normal operation. Unfortunately it is not possible to spray the cleaner in from the front of the pot. It is necessary to get to the opening in the rear of the pot to get the cleaner into it. Due to modern construction methods it is often needed to remove the circuit board to gain access to the pots. Another common failure mode of pots is broken solder at the pot terminals where it is soldered to the printed circuit board. Many manufacturers support the PC board that the pots are soldered to from the terminals of the pots and nothing else so as you bounce your amp in your truck on the way to your gig it can make those solder joints crack and cause intermittent operation of the control. A re solder will take care of this. The other failure mode of pots is when they receive an impact from the front.This serves to either push the pot apart inside rendering it inoperable or it fractures the part of the pot that contains its carbon resistance trace. You can usually crimp the pot back together but if the trace is fractured the pot needs to be replaced.


Intermittent Operation

How often have you had an amp that works correctly some of the time and then all of a sudden it makes a strange noise, some functions cease working or no sound at all comes out? According to how often and randomly the problem occurs can make this a hard thing to track down. The first thing that should be done is to try and isolate the problem,  see if any specific function, volume level etc will cause or stop the problem. Does it only happen when the amp is first turned on or does it take some time before it shows up.  Finding a way to be able to continually cause the problem to happen goes a long way towards helping locate it. Take your hand and rap on the top of the amp in different places and see if that induces the problem. Intermittent problems are caused by one of two things. Either a component is failing and is working only some of the time, maybe when it reaches a certain temperature it goes bad, or there is a loose connection somewhere in the amp. A guitar amp has many components inside and each of these is going to have at least two solder connections. It only takes one bad connection in the right place to cause a lot of trouble. The worst offender for intermittent problems is the combo amp. The electronics are located directly in the vibration field of the speaker and this will bring out any possible loose connection more so than a head/speaker cabinet combination that affords some degree of mechanical isolation between the speaker and electronics.

Next month I will continue this discussion, moving on to problems with tubes and speakers.


Steve Weiss is the owner and main technician of Steve Weiss Electronics Inc. He is experienced in the repair of analog and digital musical equipment. This includes everything from Vintage Tube Amps and Pro Audio equipment to Digital Keyboards There is also a guitar repair shop staffed by some of the areas top guitar repair techs. He is authorized for warranty work on most major brands. Steve Weiss Electronics is located inside of Sam Ash Music at 5460 West Sample Road Margate, FL 33073 954-975-3390 Ext 272. Steve has also spent 25 years on the road as a performing guitarist and is the designer of Primal Guitar amps that can be seen at Primal Audio.com Steve can also be reached at stevew@metromusicmayhem.com
Sam Ash Quikship Corp.

 

 
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