Bose 901 Series III Equalizer #181986 Repaired

Cross-posted from the Rain City Audio Repair Blog:

Bose 901 Series III Active Equalizers are uncommon compared with the other series, but I did have one through the shop just recently. The Series III design – while a great performer when in good condition – was susceptible to reliability problems with the somewhat more complex circuit design. The Series III was the last generation Active Equalizer to use discrete transistors; while functionally interchangeable, the later Series IV used op-amp chips.

This one originally came from Europe and was fitted with a 220V plug. Voltage conversions aren’t generally possible with later series, but the Series I, II and III were easily convertible by just removing a couple of parts and changing the plug end.

Starting with the Series III, Bose used high quality resistors which don’t drift, and good film capacitors – but like most other electronics, it’s the electrolytics which cause the problems. In this case, the owner also requested an upgrade to audiophile-quality film output capacitors.

This equalizer got a full set of new Nichicon capacitors, including Nichicon Fine Gold capacitors in the signal path, and top of the line ERSE PulseX film capacitors with excellent transient response and a vanishingly low dissipation factor for the best clarity possible in the output signal path.

That’s a few parts pulled out! The new replacements are all up-rated and will provide many years of reliable service to come.

EDIT 8/29/2022: Rain City Audio isn’t doing repairs anymore, but here’s a diagram showing electrolytic caps and their values.

Series 3 Template

This entry was posted in Audio, Bose, Projects, Vintage and tagged , , . Bookmark the permalink.

5 Responses to Bose 901 Series III Equalizer #181986 Repaired

  1. Jesper Nordström says:

    Hey! Im trying to get my eq series iii to work properly 🙂
    Ive scanned www at least four laps now, but cant find help with a list of components to order. The old owner fucked up and put random caps in it, so i cant know what was there to begin with🤯
    Could you help me here🙏 pretty please 🙏
    Ive refoamed the speakers an they wanna play!

    • jwk says:

      C2, C3: 470 uF 50V+ Radial Lead
      C4, C5, C112, C312: 10 uF 50V+ Radial Lead
      C101, C301: 1.0 uF 50V+ Axial Lead
      C116, C316: 1.0 uF 50V+ Radial Lead
      Polarity should be marked on the board, or you can use non-polarized electrolytic caps.
      You could also use film caps (all nonpolar) for the 1.0 uF if you preferred.

    • jwk says:

      I also put a placement diagram up at the bottom of the article.

  2. Frank says:

    Good day, Is the Bose equalizer serie III for sale?

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