From the Rain City Audio Repair Blog:

This Bose® 901 Series IV Active Equalizer, serial #219533, came through the shop for an overhaul. The owner was reporting the left channel went out, so it’s clearly time for an overhaul!

A little uneven, the first step was to give this Active Equalizer a new set of feet. Inside, it looked all original.

One interesting thing to note is this Active Equalizer came with IC sockets fitted from the factory, which were still populated with the original National LF 353 op-amp chips. These chips are a big, known problem in Series IV Active Equalizers. So much so that in January 1983, Bose® issued a product service bulletin describing their unusually high tendency to exhibit high noise or fail completely due to lead contamination in the manufacturing process. The factory recommended using the TL072, or RC4558 or RC4559 op-amp instead, and that all op-amp chips be changed if a unit came in for service.

New parts, and new chips, were an easy install:

As was repairing cracked joints on the rear panel connectors:

Good as new!