This poor lady hobbled in the other day. She had certainly been round the block a few times, her petticoats been up and down more times than she could remember. Knocked about and falling to bits, dented, bruised and dirty. Just like the way I like my women.
The owner said she suddenly went mad, loud hissing noises, bad audio and the channel button making no difference. So, power up with the trusty power monitor / limiter. After warm up and the standby on, indeed there was uncontrollable racket from the speaker. Altering any of the tone controls caused oscillations and instability, drawing kilowatts from the national grid. Then noticed that the 6L6g outputs valves were glowing cherry red, probably due to a lack of negative pixies on the grids.
Now I am a lover of Fender gear, but this red knob range of kit, which seems to have been designed by a group of techies determined to upset the well being of service engineers. The boards are upside down, no component marking and to remove the main board, or lift it to a position where servicing is just about possible, the control board and all the pots has to be retracted. Doing this will see the stiff connecting wires snap off. With that done, the boards can be tilted back a little, given the mains transformer cables placed especially to impede this. Everything runs quite hot, the board always had blackened areas where hot resistors live. Capacitors dry out. The designers have ensure the copper tracks on the PCB are extra thin, creating dry joints appearing as you stare at it with hatred in your eyes.
Looking at the diagram, the neg bias and the voltages for the silly channel switching optocouplers all come from the same source. The 250mA fuse had failed due to short cct diode D104. Its associated smoothing capdensors were tired. A handful of new parts and 2 kilos of solder taking care of the dry joints, got the old girl was back up and running. The only other remaining fault was a duff OP amp used to drive the reverb tank. Asking a bit much of a poor TLO72 in my opinion.
Anyway, back together, the amp sounded really good. HiFi almost. Guess it was worth the foul language.