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phil mailloux 18th June 2012 09:27 AM

Onboard bass preamp troubleshooting
Hey guys, I'm a bass guitar builder and have run into a wiring problem on one of my customer's bass so tought I'd ask the electronics Experts over here :)

I've installled an Aguilar OBP-3 preamp inside of a Jazz Bass clone I built, I must have installed about a dozen of these preamps in the last couple of years without a hitch, unfortunately i'm now stuck, here's the schematic used

As can be seen I added a passive tone pot to it, I've also wired a few of these with this schematic in the past without a problem, the difference this time is that i used a Fender 250k/500k dual concentric pot for both the volume and the tone controls

I essentially wired all the ground wires to one star ground and sent that to the jack. Frm the tone pot drawing, i soldered the cap to one lug then the other bit of the cap to the casing (same side casing as the lugs making sure not to touch lower casing for the other part of the pot) and then sent a ground wire from there to the star ground.

Here's the problem, absolutely everything in the circuit works fine as it should work besides the tone control. From turned up full on to about 1/3 of a turn nothing happens to the sound (tone doesn't work) then once turned the pot back to that 1/3 the sound goes dead on the bass, volume shuts completely off.

The top part of the pot (volume knob) works fine, just the lower part of the pot, the tone doesn't work

Any ideas anyone? I'm at wits end here and need to send this bass back to its owner...:(

geraldfryjr 18th June 2012 01:13 PM

Sounds like a bad pot.
Have you check its function with an ohm meter?
It kind sounds like the resistance element maybe open at some part of its travel.
I have had similar situations due to bad pots on a few of my guitars.


jer :)

geraldfryjr 18th June 2012 01:27 PM

Tie a voltage source across the element (ie. 9v battery) and measure the voltage on the wiper and V- with a VOM or a scope.
By actuating the control, the voltage should go up and down smoothly.
If it is erratic or suddenly jumps high or low then it has a bad element.

Good Luck !!

jer :)

Markus2006 20th June 2012 06:25 PM


There are three options:
- either bad pot,
- or bad capacitor,
- or ground wire not connected to the ground.
At least two of these three options can be asily checked with a cheap digital multimeter (set to Ohms measurement). In many cases you can also measure capacitors with such a meter.
There is one more possibility to check the pot: since this is a double pot, you can switch wires between sections (tone <-> volume) and see whether volume still works after the wires were switched (and whether tone control started working suddenly) :).


Markus2006 21st June 2012 05:50 AM

Even simpler test is to short the pot with a wire and listen whether this changes the tone.


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