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Without a schematic you can still trace the circuit to locate the fault.
Put a music source or sound source into the bad channel and a good channel, and with an oscilloscope or with a pair of high impedance headphones with test leads on the ends- you can set up a low z headphone with an impedance match trasformer, make sure you have test prod ends on one of the leads. The other can go to a chassis ground. Start at the input and work your way forward comparing the signal between the good channel and the faulty channel. When you lose the signal or you encounter severe distortion, you are in the vicinity of the defective part.
Common failures are a Deffective 4558 or TL072/TL071/TL074 IC chip. You may have a faulty electrolytic capacitor, or even a faulty bypass caoacitor in general. Bad solder connections, even a broken fim resistor.
With the IC chips, the usual power pins for an 8 pin plastic or ceamic DIP are pins 4 and 8. usual outputs for the 8 pin are 1 and 7 the remaining pins are the inverting and noninverting inputs. 2 and 3 and 5 and 6.
Also check switches and especially those with interupts on the input and output jacks- they can get dirty.
You can apply an audio signal to the input and with a small amplifier with speaker you can trace through the circuit to locate which IC failed.
The input to the small amp split iattach 2 wires pr test least leads from a meter, ground the ground lead of the little amp to the mixer chassis, and then with the other wire/test lead trace the defective channel 1 IC at a time until you lose signal.
If you download the IC's data sheet, Most likely a 4558 typse, you can speed the testing by simly testing at the amplifier inputs and output of each section of the IC. When you lose the signal, you found the offending IC- use the other channel for compairison.
Digikey has a site search, so you can enter the number on the IC to find if they carry it, or a similar IC, and who makes it- on the item specification page there is a link to the datasheet. While they may not have all of them, they have quite a few. http://www.digikey.com
This can be due to either a RUNAWAY of some power circuit or there is a dry point of soldering which seems to get aggravated when the board is working or heating.
So when the fault occurs check the power to the amplifier, the output stages. If a tap bring about life then the case of the dry soldering is justified.
If not this can be due to power failure due to semiconductor failure. A tracing with voltage/current/signal will be needed to confirm . USe a multimeter to check the voltage line , check the secondary to the circuits, specially the outputs.
Remember that while in tuner mode, the audio output is disabled. Try additional presses of the tuner button to see if the audio comes back on. Also verify the volume and EQ gains are turned up. Verify the battery is new.
colearthur16, Only a slight chance of problem with your speaker. You will need to prove that all stages of amplification of the audio signal from input to output are not being driven into distortion (clipping) because of a bad DC power supply rail to the low level amplifiers or the high power "final" output amp! To prove this, you will need the help of an electronics technician who has access to a variable audio signal generator (amplitude and frequency) and a cheap 5 to 20 megahertz oscilloscope to view the amplified waveforms of the amplifier stages in the Fender Amp. Remember the following statement for the rest of your life!!! "All musicians who use electronic amplification to play musicial notes through speakers should have between 4 and 8 hours of classroom instruction on how typical amplifiers (tube and solid state) operate and how to troubleshoot them when they fail to perform to rated specifications!!!"
Cole, have the Fender amplifier (not the speaker) tested by a qualified technician to prove amp is not at fault!!! 12fixlouie
Common problem... If you drive it hard and have any glitch on your speaker cables or bad speakers or cables not completely plugged in, the amp will arc between layers of the multi-layer circuit board in the amp module. Usually it is the rearmost amp that fries...While the parts cost is nominal (less than $20, the labor to restore is extensive. I have repaired 4 of these and it involves carving out burned board, re-insulating, and replacing blown circuit traces with jumpers as well as replacing 3 to 5 transistors, a Zener diode and sometimes a metering resistor.
One can get a new power amp module online. It is an HCA2400 and I have seen them for around $175.
It is VERY important to NOT have flakey speakers or cables and PLEASE read my tip on here regarding one of the amp output being hot on the outside of the 1/4 inch plugs instead of the tip if you use Speakon to 1/4 inch cables.
When you are changing the DSP effect on the fly, static or noise is NORMAL !!! The DSP is changing the data and having to recalculate on the fly and doesn't mute itself while doing so which results in "garbage sounds out".
The bypass setting should NOT get any tone UNLESS you have feedback going on. If your guitar can "hear" the speakers in the amp, the strings can vibrate and get feedback going.
Unplug the guitar and see if high freq tone is still there on bypass. If the tone is in the musical range, then there is a problem if nothing is plugged in. If it is a VERY weak, very high frequency tone, this may be the digitizing noise from the DSP. On bypass, the audio is likely to still go through the DSP, just not be modified by it.