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Eliminate TiVo fan noise (S3 and TiVo HD)

Discussion in 'TiVo Series3 HDTV DVRs' started by Saturn, Jun 8, 2012.

  1. Saturn

    Saturn Lord of the Rings

    12,278
    14
    Apr 10, 2001
    Ever notice a constant clicking from your TiVo? It is somewhat faint but can be heard easily in a quiet room such as a bedroom at night. Surprisingly, what you hear is probably NOT the hard drive, but rather the fan.

    The TiVo uses Pulse Width Modulation (PWM) to drive the fan. So rather than adjusting the voltage from, say, 7V for low speed up to 12V for high speed, the TiVo sends pulses of 12V at around 32hz to the fan. The longer the pulse (and shorter the pause between pulses), the faster the fan spins. This has many a couple advantages for these types of fans, which I won't go into, but one disadvantage: At low speeds it causes the fan to "click" at that same 32hz.

    This is a well-documented phenomenon:


    http://www.silentpcreview.com/forums/viewtopic.php?t=9340

    This URL actually includes a circuit for fixing it too:
    http://ww1.microchip.com/downloads/en/AppNotes/00771b.pdf

    However, that requires a capacitor, resistor and a transistor. My solution is much simpler.

    Just add an appropriately-sized electrolytic capacitor in parallel with the fan. I've found that "appropriately-sized" is best found through trial and error. My S3 required only 7uF (4.7+2.2 in parallel) but my TiVo HD (with an after-market fan) required 47uF. If you select a value too small it won't do anything. If you use one too large, the capacitor will end up driving the fan while the PWM is off, resulting in a fan that runs at full speed no matter what the PWM is doing. Select just the right value and it "rounds" out the square-ish wave of the PWM signal which seems to prevent the motor of the fan from clicking while still allowing the TiVo to control the fan's speed.

    Now, I'm not an EE but I did poke around with a scope to see the effects on the PWM signal, which I've attached for anyone interested.

    Standard warnings apply: you will void your warranty taking apart your TiVo, and this is clearly a hack and may have detrimental side effects.

    I can tell you that this has been running fine in my S3 for 4 months and my TiVo HD for... about 12 hours. :)
     

    Attached Files:

  2. dlfl

    dlfl Cranky old novice

    7,052
    26
    Jul 6, 2006
    Dayton OH
    Interesting .... but what puzzles me is why they bother to vary fan speed at all. Why not just let it run at full speed? Hard to imagine they're worried about components running too cool. Trying to minimize air rushing noise? Minimizing power consumption? I would never have imagined any of these factors would justify the cost of the circuitry to control fan speed.
     
  3. Saturn

    Saturn Lord of the Rings

    12,278
    14
    Apr 10, 2001
    TiVo fans are pretty loud at full speed. They run full speed when you first plug them in, then sometime during bootup it will slow down (and start ticking).

    I believe the S1s were just a 12v fan run at a constant 7V (with - connected to 5+ and the + connected to 12+...in other words, a huge hack), making for a nice quiet fan but they probably got too hot in enclosed cabinets and such.
     
  4. dlfl

    dlfl Cranky old novice

    7,052
    26
    Jul 6, 2006
    Dayton OH
    I've never heard any kind of noise from my TiVo HD, including when it is first plugged in. I had an S2DT for 3 years too and never heard anything from it.
     
  5. Saturn

    Saturn Lord of the Rings

    12,278
    14
    Apr 10, 2001
    If you have a TiVo in your bedroom and your house is very quiet, the fan is very audible.
     
  6. steve614

    steve614 what ru lookin at?

    10,722
    0
    May 1, 2006
    Dallas, TX
    I will corroborate with what dlfl says about the TivoHD. I have one in my bedroom and I don't notice any fan noise at all.

    I have an unsubscribed Premiere that is a different story.
    When booting up, the fans sounds like a jet engine powering up.
    Once it calms down, I notice clicking noise you describe.

    Does the capacitor(s) have to be electrolytic?
     
  7. Saturn

    Saturn Lord of the Rings

    12,278
    14
    Apr 10, 2001
    I dunno. I didn't try any others. Give it a try and report back. :)

    I think you'll have a hard time finding a large enough non-electrolytic capacitor to make a difference though. Digging through my box of parts the largest mylar capacitor I have is 1uF which I can pretty much guarantee won't do squat.
     
  8. unitron

    unitron Active Member

    16,400
    5
    Apr 28, 2006
    semi-coastal NC
    The reason capacitors with a lot of capacitance (big uF number) are almost always electrolytic is that's the only way to get that many microFarads in that small a space.
     

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