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Graphic Equaliser question

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  • Graphic Equaliser question

    Dear all - a quick question about graphic equaliser troubleshooting. Some of the bands cut but don't boost. It might be dirty contacts on the centre indent faders but to me that sounds like there's something else going on as logic would suggest that cutting involves increasing resistance and boosting involves amplification (and I've cleaned them all!). Should I be looking at the op-amps (row of 4558s, one for each band)? Schematic attached. Any advice about what and how to test would be excellent. Thank you!

    This is part of getting an old Yamaha EM-150ii powered mixing desk back to work: the thread of the story so far is here:
    https://music-electronics-forum.com/...wer-amp-repair
    Click image for larger version

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  • #2
    Pots are far more failure prone than other parts.
    How about checking the resistance between the boost track end and the wiper, at various settings?
    My band:- http://www.youtube.com/user/RedwingBand

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    • #3
      Hi pdf64, and thank you for your response. The pots’ resistance checked out ok, but your confidence in the other components led me to recheck connections, and I found a broken trace which connected the boost end of two of the bands. Jump wire in, and function restored. Still got some slightly dodgy pots - this thing is old, and has had a hard life - but we’re nearly there in resurrecting it!
      Thanks for your help.

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