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Guitar Amp Channel Switching Problem and How to Repair
By Gary | January 14, 2010
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.
 1 New Hot Air Desoldering Gun for 850 852 Station US $25.00 |
 Generic SS 200 Desoldering System w Gun Untested US $66.69 |
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.
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.
Cheers,
Gary
Topics: How to Repair Music Gear | 2 Comments »
January 27th, 2010 at 10:39 pm
Gary,
Thanks for suggesting the “The Guitar Amp Handbook” (Understanding Tube Amplifiers and Getting Great Sounds)!!! Fantastic resource. I love your site because it is good for the layman like me or for people who need to expert help. Your tutorials are excellent and to the point. Great site!
Keith Guitar Stanton
Orlando, FL
January 28th, 2010 at 2:02 pm
Hi Keith,
Thanks for the kind words. If anyone is interested here is a link to the book, a guy at Groove Tubes turned me on to it:
http://www.aztechmusic.com/resources/
Excellent book,
Gary