Cross-posted from the Rain City Audio repair blog:

Another Bose® 901 Series II Active Equalizer came into the shop recently for a full overhaul. These equalizers are getting pretty old and even if they power up and pass some sound, they’re almost certainly not performing as they should anymore and need some reconditioning to bring them back to peak performance.

You can see evidence of some outgassing from leaking capacitors on the shield foil inside the cabinet:

This one’s mostly original and frankly I’m surprised the owner reported it was making any sound at all.

The output capacitors had bulged out from the inside due to leakage and heating:

And the main filter capacitors weren’t doing so hot, either:

It looks like the main filters were replacements from some point in the past, too:

They’re all coming out to be replaced with new models:

Here’s one of the bad main filter caps. Nominally, this should be about 470 uF and have a very low ESR (<0.1 Ohms). Shown here measuring 6 uF and  140 Ohms ESR.

And an output capacitor measuring 19 Ohms ESR.

The silent channel had a totally dead output cap:

Time to replace parts:

Carbon composition resistors like these absorb moisture from the air over time and drift in value; this 22K 10% tolerance resistor has failed and no longer meets it’s spec of 19.9 – 24.2 kOhm:

The rest of the resistors are a simple swap-and-replace job:

The Series II and later generally have reliable film capacitors, but for the SI and SII they’re always replaced.

And finally to replace the electrolytics:

On a standard service, we’d be done here, but the owner requested upgrades to audiophile-grade film output capacitors, with a much lower dissipation factor to offer improved clarity in the output:

After testing, it was time to proceed with the other upgrade requested by the owner: gold-plated panel jacks. Panel jacks are a great upgrade if you’re planning to use new, thick interconnect cables (like Monster® or similar) but the modification is irreversible since the original jacks mounted on phenolic board are destroyed in the removal process.

And making the final connections:

With this, all resistors, electrolytic and film capacitors, and panel jacks have been replaced:

And a quick confirmation test shows everything is still perfect after the jack swap:

With that, it’s ready to go home! With all new top-quality parts and a thorough cleaning, it’s going to sound fantastic for a long time to come.

[Rain City Audio Bose® Active Equalizer Repairs]