Ok campers, I get quite a few questions in emails asking me about how to repair channel switching problems on newer Fender and Peaveys and such.  Most of the time the actual repair is simple to an experienced bench tech but to someone sitting at home wondering if they should fix it themselves to save money or take it to the shop, the answer most of the time from me is take it to a shop.

But with that said, maybe some of my visitors would enjoy seeing what the problem most likely is.  And some may decide to repair it themselves.  BE WARNED, THIS INFORMATION IS FOR ENTERTAINMENT ONLY, you go into your amp and screw it up, don’t come back here complaining.

Here is a definition of a FET: (FET) A voltage controlled transistor in which the source to drain conduction is controlled by gate to source voltage.  If you want to compare it to a tube, think of gate=grid, drain=anode, source=cathode.  Of course they are different animals.  And if you’re really want to dig, find out how a field effect transistor (FET) is made and works.

Now let’s move on. On a lot of Fender’s and Peaveys and other amps that have channel switching, there are little FET’s like the J111 that go out.  This isn’t the problem all the time but a lot of the time.

The first thing to check is something simple like the foot switch or channel switch on the control panel.  Remember to always check the simple things first.

Look for clues, like does it switch with the panel but not the foot switch?  Does the LED lights change but not the sound? This can help you narrow it down.

I’ve included a partial schematic of a hot rod deluxe for an example.  In the first pic notice the red flares, these will be places of interest.  Study the whole circuit but for this post I will focus on the main areas of interest.

The circuit in the first pic is responsible for generating signal voltages to do switching is a good way to look at it.  Notice to the left you have the foot switch input and the channel select switch.

These are basically shorting and lifting a ground on the input of the IC’s to vary the input voltage.

One chip delivers current to drive the LED lights and relay and the other is driving the input to our focus point of the post the J111 FETS.  Look at the MORE_DRV

Notice on the second pic the MORE_DRV, this means a continuation of the same circuit, which is an actual copper trace on the circuit board, sometimes wires.

Ok, this is where it gets interesting. Notice the MORE_DRV is tied to both J111 FETS.

Which in turn, the J111’s are tied to the cathode’s of V2A and V2B, thus shutting down or turning on the tube according to the voltage that the J111’s receive from the circuit of the first pic.

Now that we have the ability to turn tubes on and off, how do we control them individually?  By the relays K18 and K28 that again are controlled by the circuit in the first Pic.

If you want to study them closely you can figure out which one is doing what at which time, personally from a repair tech point of view, my aim is to get in, repair the circuit gracefully and get out and move on.

Of course from an engineering point of view, every fine point would be studied and understood.

Now back to point of this article. I know from years of experience that most of the time in a switching problem is going to be the FETs. Why?  Because usually it’s the weakest point in the chain, it’s the nature of the component to fail for various reasons: heat, voltage spikes, excessive current for a short amount of time, alignment of the planets, pick one.

The beauty of a FET and the reason it is used, FETS have the ability to interface seperate circuits with reduced supporting circuitry.  The accountants with the manufacturing companies love them.  And they do their job well, the downside is they are vulnerable to the items I mentioned above.

So it is a tradeoff and if you happen to be one of the unlucky ones, you have a reapir job on your hand.  Believe me FETs have come a long way in their dependability.  In the early days of solid state electronics used in music gear, after a major lighting storm that causes large volume of static electricity, the gear would march in the door the next day. Especially in the early effects units.

How do you test if it’s bad?  The easiest way is to take voltage readings while your using the switching components, like the foot switch.  You should have a swing of so many volts between the different feet of the FET.   It can vary between amps

The simplest thing to do is A/B with good ones in other part of the circuit. Watch how they behave as you switch between circuits.  A bad one just sits there and maybe switches .5 volts, there is little variance.  Now make sure you have continuity from pic one to pic two for example.  If there is a physical break between the two circuits, then the FETS are not the problem, the problem is they are not getting a signal.

What I  usually do is watch it real quick on my scope and see how it behaves. Make sure it’s getting it’s switching voltage from the PIC one, and because I’ve seen so many fail, I replace it, I don’t do a lot of troubleshooting.

But anyway back to the article. Here’s the dilemma on the newer style amps like the reissues.  This little inexpensive part takes awhile to replace correctly.

To get to the part, everything has come out, like the main board, jumpers, pots have to be freed and you have to carefully contort various items to get the board out and be at an angle you can get to it.

Then once there you have to remove the transistor, which I have a desoldering gun that makes it a snap but can be done with a spring loaded solder sucker or solder wick.

Your repair soldering chops are going to have to be up to par or you’re going to gimp up the board and have more work on your hands.

Now some techs might just clip the transistor from the front-side and tack on another one. I’m not going to comment on that one way or another, each tech will decide that one. Each situation is different.

I’m including a video that explains FET transistors with examples pretty well. It’s talking about MOSFETS but it is basically the same thing.

This is kind of a “the basics” of transistors but it has a section on FET’s

Video Transcript

In this tutorial I’m going to go over the basics of a popular transistor and show you how you can use it to control different gadgets. What is a transistor? A transistor is a device that allows you to use small changes in voltage to switch things on and off. They are kind of like a valve in your plumbing system but instead of controlling the flow of water, you’re controlling the flow of electric current. To make things as simple as possible I’m only going to talk about the easiest type of transistor to work with: The N-channel MOSFET. Basically they work like this: When the transistor is off, no current can flow. So it’s as if one of the power wires on your gadget has been disconnected so obviously the gadget will stay off. When the transistor is on current can flow and it’s like both of the power wires on your gadget are connected now so the gadget will activate. So where can you get an N-channel MOSFET? Well there are many different types of N-channel MOSFETs but they all work in pretty much the same way. You can get an N-channel FET from Radio Shack or you can scavenge them from from old computer hardware. They usually look like this. Google the part number on the transistor to double check exactly what you’re working with. Here I have an IRFZ44. Anything else you need? Well in addition to the transistor, you’re going to need a couple of other things. You’re going to need the gadget that you want to switch on and off. And I’m going to use a car’s headlight as an example here. You’re going to need an external voltage supply that your gadget would normally require. And in this case it would be the car’s twelve volt battery. And finally you will need some sort of signal that is either 0 volts or 5 volts. Basically a digital logic signal… and I’ll give you a few examples later. Okay so you’ve got all that? Let’s talk about how you connect the transistor. N-channel MOSFETs always have 3 pins called Gate, Drain, and Source. I know the names are kind of funny sounding but you will have to memorize them. Gate Drain, and Source. Drain is the pin that current will drain into. Source is the pin that current will flow out of. And Gate is the pin that will turn the transistor on and off, kind of like how a water gate valve will control the flow of water. Connect up the transistor like this: The source is connected to your circuit ground. Connect the negative side of your load to the drain of your transistor. Connect the positive side of your load to the positive terminal of your external power supply. Now whether the transistor is off or on will depend on whether the gate is at 0 volts or 5 volts. Here is the equivalent circuit when the gate is at 0V. The transistor stays off, so no current can flow, so the headlights stays off. Here is the equivalent circuit when the gate is at 5V. The transistor turns on and starts acting like a very low resistance current path so current can flow. Current will flow from the power supply through to your load, into the drain of the transistor, and then out from the source of your transistor into ground. So when the transistor is on, your gadget will turn on too. Now let’s talk a little more about the signaling voltages that are going to the gate. There are a lot of different ways to do it and that’s why transistors are so much fun. Here is an example with a little mercury vibration switch to turn on the transistor. When you hit the switch, the gates receives five volts so the transistor turns on. Here’s an example with a computer’s parallel port pins. When the parallel port outputs a 1 (which would be five volts) The transistor turns on. And here’s another example with a 6 volt solar cell. When the light shines on solar cell, the gate receives at least five volts, so the transistor turns on. And there are hundreds of other ways you could switch the transistor on so basically you can control anything with anything. Now I would like to clarify something for safety’s sake. Over here on the gate side, you want to keep the signaling voltages less than fifteen volts. 0-5V is fine, 0-12V is fine but if you try to signal things with a 0 to 30V signal you will blow something up. However on the Drain side of things you have a lot more freedom in the voltages you can use. The only limitation is what the transistor can handle. This IRFZ44 is rated for up to 60V so it can switch 12V loads, 50V loads, whatever I want all the way up to 60V DC. So I could switch LEDs on and off. I could switch a string of low voltage christmas lights on or off. If you add a diode over here you can switch a motor on and off, or switch a solenoid on and off, or switch a relay on and off. And once you have a relay being switched you can switch light bulbs on and off, you can switch toaster ovens on and off and you can switch your refrigerator on and off. Basically if you can get a system that puts out a 0 to 5V signal, you can attach a transistor to it and you’ll be able to switch any gadget on and off. Now remember, I just showed you the basics of one type of transistor. There are many kinds of transistors out there with many different operating modes. If you are interested in learning about other kinds of transistors, google “NPN transistor tutorial” “PNP transistor tutorial” “P channel MOSFET tutorial” and “JFET tutorial.” That should be enough to give you a headache. But for now, you know how to use an N-channel MOSFET and that is all you need to turn any DC powered device on or off.

So hopefully this article will give you a better understanding of your guitar amp. Sometimes this very same arrangement is used in rack gear and synths, etc. Either for switching or what is known as clamping circuits.
For example to clamp down the output audio signal chain while all the digital circuits stabilize so you don’t get annoying pops and clicks. Maybe this is another article.