I have worked on a number of these amps and found the most common cause of intermittent channel switching is dry joints on the foot switch jack or the contacts becoming contaminated. In either case you will require a tech to look at your problem as the main board will require being lifted out of the metal chassis for inspection. Because of the ribbon type cables between main board and the board that supports all the valves I recommend professional help . Not a DIY job
SOURCE: My fender hot rod delux is switching channels automatically.
In refering to the schematic i find that the MOST probable cause would be a bad switch contact on the jack the pedal plugs into. There is a normally closed contact that passes the select signal within the unit to the control circuit. If the contact vibrates open, then the unit will switch. These jacks are a pile of.... prone to failure if you use them very much. With the pedal plugged in the switch is no longer in the circuit and control is solidly by the pedal.
If you remove the amp from the cabinet you can probably bend the spring contact to temporarily fix the problem, however, use that jack a bit and the problem will come back. You could cobble a good quality Switchcraft jack with switch as a replacement and add pigtail wires to the circuitry for a little better reliability.
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: Loud static noise in drive
The thing you can try is to swap tubes V1 and V2 which are 12AX7's... if major change occurs including loss of any signal, replace the one that WAS in V2 position.
More than this gets into the electronics and if you don't have test equipment it is not a DIY thing so you would need to take it to a shop. There are several components including FET's Q1 and Q2 and what looks like BAD engineering design around these particular parts that could cause the problem.
The bad engineering looks like tinkerers designed the circuit rather than an engineer. Gates of these FET's are left floating on the end of a 1N4148 diode according to my schematic... I have seen MUCH poor engineering or lack of in a lot of the audio equipment.
SOURCE: my fender hot rod deluxe
Realize that the number on the volume control is only relative... it means nothing regarding what the amp can produce. With a high level output guitar you can easily drive the amp into distortion at very low volume control level settings... what counts is the actual output sound volume and specifically the voltage output going to the speaker. Once the amp reaches saturation, any further input just pushes it into distortion or "flat topping".
An oscilloscope on the output will quickly show when amp starts to flat top.
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