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Thread: active bass preamp troubleshooting

  1. #1
    Supporting Member StarryNight's Avatar
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    active bass preamp troubleshooting

    Lately I'm getting a lot of basses through the shop for some reason. anyway, I've got one with active electronics and it's making some violent/explosive crackling noises. turning the 3 eq pots does it, moving the wires gently in the control cavity does it (there are about 30 different wires on these things). I've try to resolder a few joints but no change. Batteries are fresh. I was thinking it might be the opamps going south but looking at the micro boards on these preamp there is no way I can replace anything. I'm open to any and all suggestions. The client is willing to put in another preamp system but I'm trying to save him the expense if it's not necessary.

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    The first thing I always check for is mechanical/connection problems, as these are by far the most common issues with active basses. Is the noise from turning the EQ pots identical in every way to moving the wiring? A problem opamp with be problematic whether you move the wires or not and I think your fault lies elsewhere. My approach would be to;

    1. Make sure the pots are clean - used Deoxit.
    2. Ensure the battery connection is good. Snap connectors come loose or wires break internally. Battery box terminals can relax and wires break. Check the other end of the leads and make sure the preamp gets an uninterrupted supply. Make sure the switched negative lead at the instrument's socket is good and that the socket makes proper contact. Battery lead/connection issues are the #1 problem I encounter. If necessary, monitor the voltage at the circuit board while you move the connections.
    3. With the cleaned pots, observe the pot terminals to make sure there's no movement when the pot is rotated. I've had instruments where the whole assembly has been loose and could shift around.
    4. If moving wires causes the noise, try to isolate individual wires and use forceps/tweezers to try to determine which wire is causing a problem. Where you have a bundle, use your fingers to stabilize the rest of the wiring.
    5. If possible, inspect the solder-side of the PCB under magnification. Cracked solder joints often have a circular crack around them and look gritty, but you need good light and good eyesight to see them. I use a pocket microscope or bench magnifier.
    6. If the connections are via a push-on connector, unplug this and inspect the mating connections to ensure they mate properly. Use contact cleaner if necessary. Sometimes IDC ribbon connectors oxidize inside the header and need remaking or replacing.

    I repair dozens of active basses each year and have not yet had to replace a board. I guess over 90% of faults are external to the preamp board.
    J M Fahey and big_teee like this.

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    Supporting Member StarryNight's Avatar
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    brilliant suggestions! I will go through the list and report back. Many thanks!

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    The only defective pre's I see are the occasional OBP-3, 90% of the time it's a bad 9V clip or even an internal defect in the battery. I do see a fair number of mis-wired buffered preamps with floating grounds being incorrectly wired to earth. Audere pre's are the main candidate but Demeters and some Bartolini pre's are also potentially susceptible. That said replacing a preamp with a better one often makes perfect sense. There's some real garbage in fancy packaging being sold these days and much of the up front cost of a preamp is in the pots so that's where you'll often see lesser components being used.
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  5. #5
    Supporting Member StarryNight's Avatar
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    yeah I noticed the battery clips (2 9v) were really tight and I had trouble taking the batteries off. Perhaps over time the wires have been pulled with use. I'll change these out and report back.

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    Finding a quality 9V clip is a PIA. Most of them seem to come with vinyl insulated 28AWG 7 strand shit that breaks when you look at it. The Switchcraft fiberboard clips that EMG uses are pretty rugged but they cost a bundle.
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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by David King View Post
    Finding a quality 9V clip is a PIA.
    Mouser - last time I stocked up, got about 50 hard plastic 9V clips with wire that was thin but a good deal more substantial than the ultra cheapo batt clips practically every manufacturer uses. Both kinds - wire out the side and out the end. Now the PIA part = finding the part number at Mouser. They were only about 16 cents each.
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  8. #8
    Supporting Member StarryNight's Avatar
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    update: replaced the two 9v clips - no difference.
    did a closer inspection of the wires and determined that when very gently moving any of the wires attached to one of the 50k ohm pots (or touching the lugs on the pot) the crackling goes crazy. resoldered all the connections - no difference. I'm going to replace the pot and see what happens...

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    Please tell us something about this preamp, who made it, what are the controls, anything else we'd need to know to help you out here.
    When pots go bad it's often the rivets that hold the lugs to the traces that get loose. Try gentle tapping on the rivet face while supporting the back against something with substantial mass to tighten it, that will often do the trick. I use an automatic centerpunch or the eyelet staking tool to do this provided they fit.
    J M Fahey likes this.

  10. #10
    Supporting Member StarryNight's Avatar
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    that probably would have been useful info from the start! The bass is an Ibanez BTB675. 3 eq, 1 vol, 1 blender. no switches, 18v system. It is one of the EQ pots that seems to have gone tits up. It never occurred to me to try and repair the pot, but trying to source a center detent, 50k solid shaft linear pot has proven to be difficult.
    I do have an auto centerpunch and the stewmac rivet staking tool (might be a bit small for this). I will definitely give that a try! Thanks David.

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    Those preamps are garbage anyway, there must be dozens on ebay at any one time. An Audere 4 band Classic will blow that thing out of the water. Bestbassgear.com sells individual pots but you might find one locally by just calling around to your luthier buddies. i have a drawer full of old ones. Also nothing wrong with replacing it with either a 100k or a 25k (EMG). Many circuits use the B50K-CD pots including Aguilar, Bartolini, Glockenklang, Nordstrand, older EMG preamps etc.

  12. #12
    Supporting Member StarryNight's Avatar
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    yeah, what I know about bass preamps you could fit on the head of a pin. Great to have a recommendation to upgrade! I can at least offer a viable alternative if the customer is willing to fork out the cash. I was looking at the Aguilar OBP-3 but have no personal experience with these things. Strangely bestbassgear.com doesn't have anything resembling a B50K-CD. Usually my luthier friends call me when then need anything electronic! I will check out my local electronic store as well. They may have something.

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    If you're a bone fide business, all of these companies will give you a substantial break, usually "A mark".
    Meanwhile $2 at allparts.com https://www.allparts.com/EP-4482-000...ot_p_1440.html Also available on eBay.

  14. #14
    Supporting Member StarryNight's Avatar
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    found a 50k smooth shaft pot locally (no detent tho). replaced the one that seemed dodgy and the problem was solved. they don't make 'em like they used to i guess.

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