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.
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