Where can I get a schematic for the output section of the mixer on a behringer 1280s?

SOURCE: LEVEL POT ON A BEHRINGER EURORACK MX 602A-BAD

It appears that both these use a 50K pot in the identical circuit and it is UNLIKELY they re-engineered very much so it is likely they use the same pot.

Behringer parts service should be able to tell you.

SOURCE: My Behringer Europower Pmp5000 Powered Mixer blew

Probably the channel A went. If you are fortunate, the power supply is still OK. There are protection circuits for the speakers, however you MUST use speakers that are rated at over 400 Watts. If you use smaller ones, they can get blown. There is a weakness primarily on insulation for channel A and an arc between layers of a circuit board can wipe out several transistors and a zener diode and sometimes a metering resistor. Once this happens, have unit serviced before using it again as further damage can result.
ALSO PLEASE read my tip I posted on FIXYA regarding a grounding hazard when using Speakon to 1/4 inch speaker cables... essentially the barrel of the 1/4 inch jack of the A channel is the hot lead, NOT the tip for that channel.

SOURCE: I am an elec tech of 30 years. I have 2 Behringer

Try this site... you may have to log in...

http://elektrotanya.com/behringer_k1800fx_sch.pdf/download.html

If it doesn't work, email me at my email address with K1800FX in the subject line and I will send you the PDF.

SOURCE: Does anybody have a schematic for the Behringer

Look at this one:

http://elektrotanya.com/behringer_k1800fx_sch.pdf/download.html

This is for the 1800 however...

Often they will just double up with two amps, one inverted, and use in bridged mode.at the higher power in what we call an "H" arrangement.

Since the drains are tied together, the heatsinks MIGHT be hot output level.