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.
I would like to attempt this job for fun. Do you by any chance have the parts list you used? Its my first attempt so I would love to learn from your research. My eq seems to work good for a while then after about a hour of on time the speakers start to put out a buzzing noise. Might as well gut the whole thing like you did!
Hello J.W., You’re my hero! You see? This is why I didn’t bother to get that old Enco scope going. Even the carbon resistors go bad….which is why I stopped trying to resuscitate it! Thank you for showing me that my experience was as valuable as my decision not to keep on replacing stuff on it. And thank you for showing us all how easy it seems to repair this kind of audio unit. You just need a little patience, alot of training and the other 99%, pure sweat! Take care, Bud.
Definitely made the right choice! Those old scopes aren’t that useful, anyway, most of the time.