This Bose 901 Series I Active Equalizer came to me from North Carolina, where it had been stored in original packaging for many years. Unfortunately even with careful storage the equalizer wasn’t in running condition and it looks like the years hadn’t been great to it.
This one has an engraving on the back from a previous owner. These equalizers have been around a long time and many have had several owners.
The plating had experienced pretty serious corrosion. The board itself wasn’t in much better shape – corrosion was growing up the component legs through the board and was degrading the copper traces and the board material itself.
This one was tough because the traces were damaged around many pads and required very careful soldering to make solid connections.
The original transistors all had very bad corrosion on their legs so they all were replaced. The 22 mH inductors in the low frequency circuit also were replaced with new precision models as the ultrafine wire had failed from internal corrosion.
The owner of this equalizer requested I install some pigtail jacks to help them connect thicker interconnect cables to their equalizer. The narrow pitch of the RCA jacks from the early 1970s isn’t wide enough to accommodate today’s highest quality interconnects and these are a great solution to adapt to the last few inches of cable run. It’s also a fully reversible modification.
This equalizer got an upgrade to the output capacitors to audiophile-grade film capacitors. These really bring out the fine detail in the music in a way that’s just not possible with the original electrolytic capacitors, and this is a great way to bring some of the advances in materials science over the last 45 years to these classic speakers.
This equalizer sounded great in testing and really delivered on the wide, powerful sound the Bose 901 speakers are known for!
These Active Equalizers are key to the system’s performance and it’s important to have them working in top condition. If you need yours repaired, I can help.