No sound from bass guitar unless strings are hit very hard: dead preamplifier?

Question

My electric bass guitar (Yamaha RBX374, active pickup) suddenly stopped producing any sound. Only if I play the strings very hard, I do get some noisy tone.

Here's what I tried:

• I suspected it was a dead battery so I replaced it with a new one, still no sound. I also checked that the voltage from the battery can be measured at the preamplifier.
• I checked with two different amplifiers and doublechecked the cable.
• Knobs (volume, balance, bass, treble) are all in middle position. In the meanwhile, I took out the balance pot and verified that it shows the correct resistance. I haven't put it back in yet but instead clamped the signal of one of the pickups directly to the preamp input (I tried both pickups).
• I checked that all pots, all Gnd pins on the preamp, the cable, the bridge and the strings are all grounded.

I doubt that the problem is in the pickups because I see no reason why they should die at the same time.

My best guess currently is that the preamp is somehow broken but I don't know how to check this.

How can I further debug this?

Answer 1

I would say your best guess is probably correct: dead preamp. Which is strange because a properly made solid-state preamp should easily work without flaw for many decades, but that doesn't mean they all do. Be prepared that you'll have to replace it.

However since this has both active PUs and a preamp, it's definitely worth making sure the PUs aren't actually the problem. As you say it's highly unlikely that both PUs failed simultaneously, but what isn't so implausible is that they both don't get the correct operation voltage anymore. Have you checked that?

Regardless, I would also try tapping out the signal of one of the PUs directly to your amp. This sort of test can be best done by connecting the cable only for grounding, and taking the actual signal with a crocodile-clamp probe at various points in the circuit. I tend to put a ~470 nF capacitor in the probe to avoid biasing issues, though that actually shouldn't be necessary because the amp already has one.

Speaking of biasing – your description actually sounds like that might be the underlying issue: some connection is supposed to be at 4.5 V, but is actually at 0 V or 9 V instead and thus in the saturation region of the OP-amp it's feeding, unless you hit the strings hard enough to get some peaks into the linear region. If you want to go really nitty-gritty, you could use that knowledge to figure out what's wrong with the preamp circuit (assuming it's not encased in epoxy).