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Hum problems: HELP

Discussion in 'Pickups & Electronics [BG]' started by OmegaZ27, Feb 7, 2006.


  1. OmegaZ27

    OmegaZ27

    Sep 18, 2005
    California
    Alright. I replaced a broken tone pot that I had on my Heartfield Prophecy II. I realized that the extra pin on the existing pot was just ground.

    Anyways, after I changed the pot (which is a 250k), I noticed that I get a crazy hum when I set the controls a certain way. Just FYI, here are the controls I have:

    Master Volume
    Blend (Precision to Jazz)
    A Bass cutoff (just leaves mids and highs)
    Tone

    I get the hum like crazy when I switch all the way to the Jazz setting. It was driving me nuts, so I decided to look at the whole circuitry. I noticed that all the original pots were 20k. And that one tone pot I changed was 250k. Is it possible that the one pot I changed affected the whole circuitry? If so, should I change the other pots too?


    Phew. Sorry, this is a lot, but I would glady appreciate the help!
     
  2. 20k pots...it must be an active bass...in which case, you should do like-for-like replacements unless you know what you're doing.

    Instead of changing the other pots, change the tone pot to a 20k like the original.

    and...yes...changing one pot can change everything, especially if your talking about changing the gain in an active circuit, which in this case could take your tone to some places where the preamp was not designed to go.
     
  3. seanm

    seanm I'd kill for a Nobel Peace Prize! Supporting Member

    Feb 19, 2004
    Ottawa, Canada
    Yup, I believe those did have active preamps. The fact that you mention mids seems to reinforce that.

    You must replace the tone control with the same size as the original (20k). 250k is for passive basses, not active. You are probably messing up the biasing in the preamps.
     
  4. OmegaZ27

    OmegaZ27

    Sep 18, 2005
    California
    So you guys don't think it would be a good idead to change all the pots to a higher value?
     
  5. seanm

    seanm I'd kill for a Nobel Peace Prize! Supporting Member

    Feb 19, 2004
    Ottawa, Canada
    No, that will make it worse.
     
  6. OmegaZ27

    OmegaZ27

    Sep 18, 2005
    California
    Alright. There's no way I can find an exact 20k mini pot. The closest I can find is a 25k mini pot. I also noticed that the blend pot is either a 50k or a 500k. Are you sure I can't change the other pots?

    And also, What are the best value pots for an active bass?
     
  7. seanm

    seanm I'd kill for a Nobel Peace Prize! Supporting Member

    Feb 19, 2004
    Ottawa, Canada
    25k should be fine.

    Unlike passive basses, the "best value pots" for an active bass are the ones it was designed for. Some pots can be changed but you have to understand the preamp circuit completely before you make a change. Each preamp is different.