Bass preamp (similar to Fender ToneStack) - no output

Discussion in 'Pickups & Electronics [BG]' started by Simon_Blanch, Aug 19, 2021.

  1. Simon_Blanch


    Jan 18, 2021
    Hi all,

    i've tried to build this bass preamp explained by Rob Mods () in the pedal version.

    Here's the schematic (also attached): https://16523fc6-ac2c-4fa9-ac46-96b...d/1ac20e_da20806d22ae4a9ba1c4221a9bfa8b99.pdf

    However, after soldering everything, i don't get an output when I engage the pedal. However, the LED is lighting up (so stomp switch seems to work properly), but don't hear anything (no noise hum, nothing).

    When I bypass the pedal, my signal moves through the pedal, so no issues with that.

    Does anyone have tips & tricks on how I could start debugging? I have a multimeter and can perform signal testing, but i'm not technical enough to know where to start.

    Thanks in advance for someone's help!

    Kind regards,

  2. @RobbieK is on here. I’d guess he can help.
  3. dwizum


    Dec 21, 2018
    When I get stuck like this on this type of project, the first step I take is to follow the signal and ground paths visually on the circuit. Print out the schematic and the pcb layout and compare them to your project, one piece at a time. Check that the schematic matches the layout, then check that the layout matches what you built. It seems like at least 3/4ths of the time there's no signal, it's because either the signal path is broken (things that are supposed to be connected aren't), or the signal is shorted to ground unintentionally (stray wire, something soldered in the wrong spot, etc). It's often the case that there's an easy "oh duh" moment. Follow the circuit visually. If nothing visual jumps out, follow it with a multimeter. Do this unplugged, on DC resistance or continuity mode. Look for good connections where there are supposed to be connections, and no connection where there isn't supposed to be one (i.e. touch the probes across the input and output and make sure there isn't zero resistance.)

    The other 1/4th is sometimes harder to troubleshoot. If there are tiny surface mount ICs, I reflow the solder on those. Sometimes a pin might not be soldered well even though it looks like it is.
  4. Crater


    Oct 12, 2011
    Dallas, TX area
    ^^ Those are all good suggestions.
    I will also add to do a quick "idiot check" on some items. Make sure the op-amp is oriented correctly, I hope you used a socket. Diodes and electrolytic capacitors (C8, C9, C10) are polarity sensitive and won't work if they're connected backwards, so triple-check those.

    Voltages here are measured from the test point (red lead) to the pedal's ground. I like to ground the black lead by using an alligator clip jumper wire so I only have to fumble with one test probe and not two. (set your meter to DC volts, 0 - 20) The pedal of course has to be powered up and a cord plugged into the input jack to test the voltages.

    Pin 8 of the op-amp is the +v terminal and should measure around 9 volts DC. There will be some voltage drop from the polarity protection diode, usually around 0.4 - 0.7 volts. The + terminal of C9 should also be at this voltage.
    Pin 4 is the -v terminal and should be at zero volts to ground. A resistance check from pin 4 to ground should be close to zero ohms.
    Pins 3 & 5 are connected to Vref, so about half battery voltage, or somewhere between 4 and 5 volts. The + terminal of C10 should also be at this voltage.

    With the pedal unpowered, you can measure the DC resistance of the input and output. The input resistance should be about 2.2 meg Ohms, (value of R1), and output resistance should be a maximum of 10 k Ohms, (value of the volume control) and should go down smoothly and steadily until it's at zero ohms or close to it with the output volume turned all the way down.

    Covering the whole mess with a big piece of shrink tubing is something I haven't seen done to a pedal's circuit board. It's weird, you don't need to do that. o_O Especially not until you've got the pedal working.

    One last thing, those light blue 3PDT stomp switches are really flaky, in my experience. Try trouble shooting the switch contacts with an ohm meter or test it by jumpering the terminals with test jumpers.

    This post brought to you courtesy of coffee.
    dwizum likes this.
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