Also: setup on overdrive channel, gain & volume to 0.Nevertheless I can hear a little bit (at very low volume) the note distorted on the speaker. I think there should be two problems: 1) Wrong separation between the two channel (maybe due to "electronic switch" and not "mechanic" one) 2) Gain of the overdrive channel preamp too high. I'd like to have your opinion. Do you know where I could find the circuit diagram? Thanks a lot and my congrats for the service.
If you are using headphones, then I'm guessing you are practicing and running the amp as quietly as possible while still able to hear it. Unfortunately, most channel switching guitar amps will have a tiny bit of bleed-through from the other channel when turned all the way down and listened to at close distance from the speaker. And yes, using a mechanical switch or relay to switch the connection from one channel to the other would mostly eliminate this, but due to the high amount of signal gain in guitar amp preamp sections, this type of switching causes very loud pops and other unwanted noise when switched. The amps are made with either photocell (optical) resistors or reverse biased transistors or op amp chips to switch channels. These are 'softer', partially because they don't fully turn off when in the off state(or fully on either). This creates bleed through, but it is better and easier than a CRACK THUMP POP!! every time you stomp the channel switch pedal. Amps are designed to be used at output levels generally much louder than a dorm room at 11:30 pm, and at stage volume in a club, the channel bleed is so far below audible, it is never heard by anyone. That's just the way it is.
SOURCE: Fender Concert II preamp is faint and distorted on 2 channel
It sounds like a job for a technician. There are high voltages in the chasis and the potential for a serious shock to the unwary.
That said, I can suggest several possibilities.
First, make sure the controls aren't causing the problem. Contacts can build up dirt and oxides and block the signal. Try twiddling the controls and see if the problem comes and goes, or if you hear scratchy noises. If symptoms are found, buy a contact cleaner spray, take out the chasis after unplugging and letting it set for an hour, being very careful not to touch any exposed conductors inside. You'll see some holes in the control bodies; squirt cleaner for 2-5 seconds and work the controls.
Also see if the problem happens without the footswitch, and try the channel switches several times - same problem, dirt and oxidation.
Other possibilities - cathode bypass capacitors drying up, and it looks like the amp uses some opto-couplers at various stages.
SOURCE: I have an Allen &
NEVER, NEVER put contact cleaner into the faders!!! It removes the lube and the faders will be damaged in short order. Try using CRC226 available at Home Depot in the electrical dept. Use ONLY the CRC226 as there are other CRC products that are not applicable. Squirt some in the center of the slots... not too much and work the sliders... if they improve try a little more... If they are still bad, replace the sliders... it is a MISERABLE job involing removing mass number of knobs, screws, etc... and then getting/finding the replacement fader control. Usually I get them from Digikey.com or Mouser.com and sometimes the actuator tab has to be trimmed to length with tin snips so knob will be at right height. The fader must have the same resistance and TAPER as the original as well as mounting tabs.
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