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Guitar Amp Channel Switching Problem and How to Repair
By Gary |
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.
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 | 19 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://aztechmusic.com/resources/
Excellent book,
Gary
August 2nd, 2010 at 1:01 am
Hi, my problem is Fender Champion 300 amp, Channel one does’nt work, channel 2 works, any ideas what may be wrong with it?? Thanks.
August 2nd, 2010 at 1:21 am
Hi,
I would read the above article if you havent, check the easy stuff, if it’s not the easy stuff, probably a bad FET.
Gary
August 2nd, 2010 at 8:22 pm
Hi Gary
Where can you get the J111 FET – Looking at all the info on it (and there is not a lot) it seems to be a bit of an odd fish – Peavey only list it as 35v Special Application – other specs are
J111 (TEM) N-FET-J 35V IDss> 20mA Rds30R TO-226AA (TO-92)
Is there a modern easy to get FET that one can pop in or is this something that needs to be ordered from Peavey
Thanks
Paul
August 2nd, 2010 at 9:00 pm
Hi,
J111 is popular, just punch it into google. It looks like you are in UK, I just saw a place called Amp Doctor that has them. You can always do a cross reference check in google to…there’s something that all the electronics companies put out called a semi conductor cross reference book so you can get it at your local electronics parts house.
If all else fails just walk in and tell them you need a FET J111, TO92 case style…just make sure the legs of the FET are the same, by that I mean the Gate, Drain, Collector have the same orientation as the old one.
With that said the FET may or may not be your problem, I would check voltages first with one in the circuit that you know is working. And there is nothing to pop in, it has to be soldered. Some people cut the legs and tack the new one in and some pull the board and desolder correctly and replace…
Gary
December 21st, 2010 at 4:08 am
Thanks Gary! Your advice was spot on.
I have a Fender Champion 110 which is my first amp. I bought it about seventeen years ago and then two years later my friend plugged a Radio Shack amplified microphone into the clean channel and it went dead. Well, not exactly dead because you could control the volume through the dirty channel but the switch had no effect and both channels sounded like the dirty channel.
I always liked the clean channel and have missed it all this time. I tried a Frontman 25R and it seemed flimsier and not quite the same. So I followed your advice.
There are two J111 transistors on the board. The board has to be flipped to get at the soldering side but I took my time and finding the screws was not too bad. The only things I had to unplug were the speaker and the reverb. The knobs on the front pull off and the 1/4 plugs unscrew with a socket (I just used my fingers and no wrench so as not to apply two much pressure).
I didn’t test the transistors because I don’t have the instrument and it was easier just to replace both. I also figured if one went bad then they were probably low quality to begin with.
I used my RadioShack solder sucker gun with the little pump on it to pull off the old solder and the transistors came out easily. Soldering the new ones in was a little harder because I’m terrible at it and have shaky hands, but there’s enough space it’s not impossible. My one mess up I was able to fix with the solder sucker.
Anyway I reassembled everything and fired it up and it works.
A plug for my transistor source: Newark Electionics.
http://www.newark.com/fairchild-semiconductor/j111/transistor/dp/58K8917
This company is awesome. They charged 17 cents a transistor and shipping was like $6.50. They put them in a nice box with foam protection in it and a static free pouch for the transistors. I’m sure they lost money on this transaction (I ordered 10 transistors just to have some spares), but now they have a customer for life.
Safety-wise I don’t recommend anyone else copy me but all I did was leave the amp unplugged for a week beforehand and I wore shoes and tried not to touch too much stuff. The back of the amp says that there are no user serviceable parts inside.
Thanks again. I love this amp and am so glad to have it back fully. And you saved me at least $90.
December 21st, 2010 at 2:30 pm
Wow, you’re one of the very few that actually uses this info, congrats! Sounds like you did it perfectly. If you want to know more about discharging a supply look around the net or drop me an email and I can give you a few pointers.
Gary
admin(at)aztechmusic.com
January 19th, 2011 at 4:00 pm
Hi Gary,
Great site. I’m trying to help a buddy who has a switching problem with his Champion 110 – just like Jack(above). Your advice looks promising!
just curious, do older Boss pedals use this same type of system?
Thanks for your site!
January 19th, 2011 at 4:15 pm
Hi,
From what I remember yes. Mainly for the reason for reduced parts in the supporting circuitry on a circuit that is switching the signal path. With that said, in a pedal it might also just be a mechanical switch with circuitry to clamp the annoying pop but keep it quick so you will probably need to look up the exact model schematic.
Gary
January 26th, 2011 at 12:52 am
Here’s a problem my fender blues deluxe(1997) just developed.i was playing thru the clean channel when the drive channel light went on and i lost output.then the light would go out and the output would come back.this happened on and off consistently
Any thoughts?
Joe
January 26th, 2011 at 12:54 am
Hi
That has a pedal right? If so switch the pedal between the ch. a lot, also if you can unplug it and plug it back in a couple of times. Hopefully it’s something simple at this point.
Gary
January 28th, 2011 at 1:44 pm
hi Gary
this is Joe with the drive channel problem on my vintage blues deluxe.these are the things I did…..
pushed the drive switch numerous times; o.k.
used a foot switch numerous time;o.k.
unplugged the foot switch numerous times;o.k.
left the amp on for 4 hours(about the length of rehearsal)
noticed that when I turn off the power switch,the drive channel goes on( i don’t think that matters,heck,the amp is off).
plugged in my guitar and……..,I could not duplicate the problem
oh ,how I hate intermittent failures.quite time consuming
joe in PA
May 10th, 2011 at 8:59 pm
Hi Gary,
I have a Fender Dual Showman Redknob which has developed a loud buzz on both channels. When I select the clean channel, the gain pot on the other channel increases the volume, (and the noise). I cleaned the pots with switch cleaner, blew off the dust with compressed air and sprayed the pcb’s with laquer. The amp sounded sweet with no noise for about ten minutes then the buzz came back.
Could this be something simple or should I take it to a shop? (expensive here in the UK)
Kev
May 11th, 2011 at 9:13 pm
Hi,
I don’t know, could be the power supply if it’s common to both channels.
Probably shop time.
G
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August 27th, 2011 at 5:01 am
Hey man I have a Fender Hot Rod Deluxe. Today it randomly started to have an issue after a drive, it was facedown in the back. The Master Volume knob has no control over the clean channel anymore, the clean channel has a low volume, probably a standared of 3 on the master volume if it would control it but it doesn’t. However the drive channel does work with the Master Control, so I’m really confused. I don’t know what to do and its so frusterating!!!
August 27th, 2011 at 2:40 pm
Hi,
With the problems you’re describing, not sure how laying it in the back of the car would cause anything, something was probably ready to go anyway. Make sure all your tubes are in good. Really not much you can do except take it to a shop.
G
November 30th, 2011 at 2:39 pm
wooow this is very interesting and very helpful thank you….