Clean channel starts to distort after a couple of hours of use and eventually the sound cuts in and out, at the same time the red mains light flickers and goes dim when the problem happens. I've checked for bad connections on the mains side of things but I can't imagine how that could be linked to the sound issue.
Check the power supply voltage and look for a hot component. If it takes a while to bring on the symptom it's probably something getting hot.
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: My hot rod deluxe has internal distortion problems.
There are many components in the system. Electrolytic capacitors are most suspect. C49, C50, C45, C51
There are many others but these are top suspects. You MAY be able to detect a change by cooling these components with circuit cooler spray.
ALSO check the +/- 16 volt power sources in the unit.
Testimonial: "I haven't fixed the problem yet, there is more to it now that I have pinpointed the general problem, but he was right on the mark."
SOURCE: hotrod amp is distorting on
to rule out the speaker, you could try one of 2 things.
1. remove the speaker from the cabinent and gently push the diaphram in 1/2 an inch to 3/4 an inch, as far as it will gently go- BE GENTLE. the speaker should recess and return to normal position smoothly, without any scratching or rubbing, or internal resistance., the only resistance should be from the paper cone itself, not internal to the speaker. if there is any rubbing or scratching from the inside (inside the area surrounded by the magnet), or any changes in resistance (not smoothly moving) as the speaker goes in or out, then the voice coil is damaged.
2. find another speaker of suitable wattage and ohmage, and disconnect the hot rod's internal speaker, and hook the replacement speaker up in the internal speakers place. a speaker of 50-100 watts should be suitable, and it will probably need to be 8 ohms- a standard bookshelf speaker may be suitable if it is a suitable wattage and ohmage rating (stamped on the back of the hot rod's speaker magnet and/or on the back of the external speaker/cabinent as "8ω",) do not, and again, do not, under any circumstance disconnect or connect a speaker while the amp is on, or even plugged in. touching the positive and negative speaker leads together, even briefly, while the amp is powered will fry the amplifier circuit, and you will smell it!!!
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My local repair workshop isolated the fault to a driver valve - replaced the valve, checked the contacts on the pilot light and all's well!
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