Bad Capacitors Strike Again

I’m working on another Bose 901 Series 1 equalizer. It looks to be the same manufacturing run as the first one I wrote up last July, uses the same components, and those components look like they’ve had the same failure mode.

The rather dubious EM-branded capacitors have the same discoloration on the top middle I saw on the other Bose, as well as on the Farnsworth K-262P which had been repaired once before. I’m curious what the physical failure is, as the discoloration would suggest to me overheating, but that seems very unlikely in this circuit and application.  Newly manufactured replacements aren’t likely to have this problem as the film technology has improved significantly in the last 40 years.

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9 Responses to Bad Capacitors Strike Again

  1. jwk says:

    5% will work fine, yes.

  2. Julie McKee says:

    Could the resistors be 5%… Hard time finding 2 and 1%

  3. Julie McKee says:

    Will do!

  4. jwk says:

    Good luck. Send me some photos of the repair after you’re done and let me know how it sounds, I’ll post your pics up here!

  5. Julie McKee says:

    Thanks again so much for this, I’ll keep you posted when its finished! I still can’t believe I had them the wrong way out all this time….Oh knowledge…. 🙂
    Bye!
    Julie

  6. jwk says:

    Julie, that’s correct. You can use any voltage 100 volts or higher for the film signal capacitors; I had 400/600V lying around for that one so that’s what I used.

  7. Julie McKee says:

    Wow, that was fast :), I thought I left the comment on the original blog 🙂 Thanks so much for this! So if I read this right,
    You replaced the filter capacitors. 2x500uF and 7x100uF @ 25V by the exact same caps, values and all, but you replaced the metal film signal capacitors by identical values but higher voltage between 400 and 600 volts? Is that correct?
    Thanks again for your time!!
    Julie

  8. jwk says:

    Hi Julie, the article linked from the above post contains a schematic with the parts called out. It’s kind of small, but the capacitors are 6 x 0.015uF 100V+, 2 x 0.022uF 100V+, 4 x 0.047uF 100V+. I replaced the resistors with appropriate value Vishay 2% 1/2W metal film replacements. The transistors don’t need to be replaced. Unfortunately you can’t use another lamp there but any NE2/NE2A neon bulb in series with a 150K 1/4W resistor should do the trick there.

  9. Julie McKee says:

    Wow, this is exactly what I was looking for. Your article is complete and very helpfull!
    I have a Bose 901 serie 1 equalizer, with series 2 Speakers. I learned only recently that the front of the speakers was the flat side, and that the V shape had to face the wall…after all these years they were backwards.. 😀 I’ve noticed some intermittent interference Humm buzz sound, and one of the four speakers have all of the drivers foam in pieces 🙂 So I started by replacing the foams around the drivers…
    Now its the Equalizer’s turn..
    Do you still have the list of the replacement parts you chose? Resistors and Caps.
    Also did you change the low-noise 2N5088 transistors?
    And finally 😉 can I use another light bulb instead of the neon one (its dead) and what should be the spec of the light bulb?
    Well, that was a lot of questions hahahaha! Thanks again for this blog!
    Julie

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