Our realtor friend knows my interest in audio, so she brought me a whole Bose 901 Series III speaker set that was abandoned in a property. The 901 speakers were re-foamed, however a couple needed redoing due to voice coil rubs. The set is from 1978, so the equalizer is by now in need of refreshing. I record here the process for the future 901 owner who may find this useful:
Replace all electrolytic capacitors:
There are 10 capacitors to replace, all 35V rated (or use higher):
2 - 470 uF One was upgraded to 3300 uF, other remains at 470 uF, make sure bigger cap clears cover, mine did not!
4 - 10 uF These were replaced with Nichicon FW
4 - 1 uF - these were replaced with film caps, it improved sound a bit
Replace 14 small signal transistors:
12 - BC239, replaced with BC547, available at Mouser.com: NPN
512-BC547CTFR
BC547CTFR
2 - Motorola 2N3906, replace with complimentary pair to BC547, the PNP BC557 - Note pin orientation different than Motorola, turn 180 degrees. These are PNP
512-BC557BTF
BC557BTF
Replace 2 rectifier diodes:
1N4002
After replacements, I noticed residual power noise, that is why I upgraded the 470 uF cap to 3300 uF, and installed a 2-3 watt 0.47 ohm resistor between the rectifiers and the first cap, had to cut the trace. See picture.
Replace 8 RCA phono connectors:
These were upgraded to gold as the aluminum connectors were oxidized and sometimes failing to make a solid connection. Available from Mouser:
568-NYS367-0
RCA PANEL JACK GOLD
1568-NYS367-2
RCA PANEL JACK GOLD
Sound is now crisp and clean with no lose connections. I hope this will help the next Bose 901 Series III owner.
Replace all electrolytic capacitors:
There are 10 capacitors to replace, all 35V rated (or use higher):
2 - 470 uF One was upgraded to 3300 uF, other remains at 470 uF, make sure bigger cap clears cover, mine did not!
4 - 10 uF These were replaced with Nichicon FW
4 - 1 uF - these were replaced with film caps, it improved sound a bit
Replace 14 small signal transistors:
12 - BC239, replaced with BC547, available at Mouser.com: NPN
512-BC547CTFR
BC547CTFR
2 - Motorola 2N3906, replace with complimentary pair to BC547, the PNP BC557 - Note pin orientation different than Motorola, turn 180 degrees. These are PNP
512-BC557BTF
BC557BTF
Replace 2 rectifier diodes:
1N4002
After replacements, I noticed residual power noise, that is why I upgraded the 470 uF cap to 3300 uF, and installed a 2-3 watt 0.47 ohm resistor between the rectifiers and the first cap, had to cut the trace. See picture.
Replace 8 RCA phono connectors:
These were upgraded to gold as the aluminum connectors were oxidized and sometimes failing to make a solid connection. Available from Mouser:
568-NYS367-0
RCA PANEL JACK GOLD
1568-NYS367-2
RCA PANEL JACK GOLD
Sound is now crisp and clean with no lose connections. I hope this will help the next Bose 901 Series III owner.
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Ditto. I fully respect the art and pleasure of restoring an old piece of gear.
That said, you could get better sound with a MiniDSP 2x4HD, plus it has digital inputs, and it would only take about 15 minutes to get the EQ within the ballpark.
I wrote a sidebar about the Bose 901 EQ (the engineering of the 901 EQ system relative to the low port tuning Fb of the small drivers is really quite ingenious) in this article in AudioXpress: "The DSP Assisted Reflex" https://audioxpress.com/files/attachment/2721 page 43 of the magazine / PDF p. 7 - the 901 is Exhibit A of this technique and Bose executed it quite well in the analog domain back in the day.
That said, you could get better sound with a MiniDSP 2x4HD, plus it has digital inputs, and it would only take about 15 minutes to get the EQ within the ballpark.
I wrote a sidebar about the Bose 901 EQ (the engineering of the 901 EQ system relative to the low port tuning Fb of the small drivers is really quite ingenious) in this article in AudioXpress: "The DSP Assisted Reflex" https://audioxpress.com/files/attachment/2721 page 43 of the magazine / PDF p. 7 - the 901 is Exhibit A of this technique and Bose executed it quite well in the analog domain back in the day.
I saw that DSP device, would probably not be as noisy as the original Bose equalizers. Thanks for the replies, all great information, especially for folks that get a 901 set without the equalizer. I don't have the schematic, searched all over and could not find one.Exactly what I was thinking when I saw the thread title. What are the odds?
jeff
Series I schem. in post #12 here: https://www.diyaudio.com/community/threads/bose-901-series-i-active-equalizer-diy.244191/
jeff