I'm told by a tech in the US that it's missing from the authorised Fender tech's service folder, and Fender's Customer Service guys don't even have one. This amp doesn't use the same footswitch design as the other amps on this page.
There is an internet myth going around that fewer than 500 Bassman 20s were made. My research shows that around six thousand Champ IIs and Bassman 20s (total) were made and it looks like around half were Champ II, half Bassman 20. own it because it's a great amp, not because it's "rare".; the Fender Super Champ, Princeton Reverb II, Deluxe Reverb II, Concert (1982-86 model sometimes called Concert II; eleven knobs on the front), Bassman 20, Champ II, Twin Reverb II, and Fender 30, 75 and 140.The 30, 75 and 140 weren't in the Rivera-era, as they pre-date Paul Rivera's time with Fender.I'm not an expert, but looking at the schematic it seemed there were two optocoupler circuits, going alternatively to ground, but: the one belonging to the clean channel was routed to the plain jack, while the lead one was routed to the red jack! So I just moved the tip wires from the plain jack to the tip of the red jack, and vice versa. Now you only need a SPDT switch plugged into the red jack and it acts just like the front knob switch (which is not working if a footswitch is connected), nothing more, nothing less! If you want to have LEDs you can still use the blue wire in the schematic." You can see Silvio's mod on the schematic here.I am indebted to Vincent D in the US for weighing his DRII (with the original Blue Label speaker) and finding it weighs 42lb, and not the 32lb as listed in the 1983 Fender amp catalogue.