This Bose® 901 Series IV Active Equalizer, #509052, was just purchased and sent to Rain City Audio for an overhaul.
This unit was originally a 220V model but had already been converted to 110V on a previous repair. The op-amps had been replaced as had a couple of electrolytic capacitors.
Unfortunately, the pads and traces on the bottom of the unit were badly damaged around the op-amps. Ultimately, it ended up being too risky to replace them, but since they had already been replaced with reliable models this wasn’t a major loss.
New electrolytic capacitors, and silver solder later, and this one works and sounds good as new.
RETROVOLTAGE 
Do you know what voltage the transformator has on this model?
My transformator is broken and i can’t find the specifications anywhere. And i can’t test the voltage because it’s broken.
I think it’s a 15-0-15V transformer, and very low amps. Output ~19V DC after the rectifier, before the dropper resistors to 15V for the op-amps.