While some other projects have been pending information and parts, I’ve had a chance to work on the Bose 901 Series 1 equalizer that’s been on my bench for a couple of weeks.
The equalizer is a necessary component of the Bose 901 speaker system, Bose’s highest-end hi-fi equipment. The speakers are an array of small drivers designed in a way that requires the signal to be equalized and pre-amplified, and if you don’t use the Active Equalizer they’ll sound pretty bad, lacking much in the way of bass or treble response. It’s surprising how many people have forgotten this fact about the 901-series speakers over the years, using the 901 system without one is likely the origin of the derogatory slogan “Bose: No Highs, No Lows”.
This particular model came to me from a client complaining about distortion and eventually signal loss in one channel, and general sub-par sound. He had the equalizer for about a year after purchasing the set from a collector, and it never sounded quite like it should and rapidly degraded from there. The capacitors in the unit had probably been going bad for a while but only crossed the threshold to completely dead after some time in use.
The Active Equalizer offers 30-some combinations of curves to select and enough pre-amp gain to maximize the speaker’s output.
The equalizer is fairly simple construction, using a single-sided PCB with hand routed traces, ten transistors (five per channel) and an assortment of capacitors and resistors and a few inductors to do the work of shaping the frequency curve.
With the top cover off, you can see inside clearly. This particular equalizer came to me with reports of distortion and low gain. Obvious immediately are the large orange and red capacitors. The red models especially have visible discoloration at the very top. I also noticed that many of the resistors are the original carbon composition type, which is known to absorb moisture and change values. As the resistors in the signal path, any drift can change the equalizer’s effectiveness. I spot-checked a sample of the resistors, and found that very many of them had drifted past their stated tolerance and were also going to need replacement.
To start, I began replacing the electrolytic capacitors and resistors from the top down. I’m using high-precision metal film resistors instead of carbon composition resistors, all rated for 1% tolerance to ensure long-term precision and stability. In addition, metal film resistors have a much lower noise figure than carbon composition resistors which will further improve performance.
After service, this unit was still giving me some trouble with the channels being slightly different volumes, so I pulled the transistors and selected new units for very careful gain matching.
Here’s the results of the matching:
Nice and level! With that replacement, the channels were perfectly balanced. Not all equalizers need their transistors replaced, but it’s easy enough if they do. And with it all cleaned up, the performance curve looks great!
With brand new capacitors, transistors, precision resistors, and a new neon power lamp this Bose 901 Series 1 Active Equalizer should be good to go for many years.