Sadowsky Preamp only works in Passive Mode

The layout of the circuits and the fact it works in bypass mode make me think the culprit is somewhere on the treble-bass boost knob, in the middle right of the arrangement as pictured. This is, for most practical purposes, "the preamp"; everything else has PCBs purely for the solderless connections. You can double-check those, it certainly wouldn't hurt, but assuming all passive controls work in passive mode you've eliminated most potential issues with three out of four knobs of this preamp kit.

Diagnostic time. Plug a cable into the bass, then grab your multimeter (if you don't have one, Extech makes decent ones for not a lot of money, and you really want a multimeter for pretty much any wiring work you do), set it to voltage mode, and touch the red battery lead's solder point on the treble/bass board with one probe, and with the other probe, touch any green or black wire terminal other than the one right next to the red wire. At virtually any ground you choose, you should read a voltage, ideally 9v but anything north of 5 is a "pass" on this test. If you do not get voltage, you either lied to us and the battery is dead, or there is a break in the battery connection somewhere that you will have to suss out.

If you did not get voltage on the first test, two more tests can quickly point you in the right direction. Again in voltage mode, probe the red wire's solder point and the point of the black wire next to it. If you do not get voltage, the problem is somewhere between the battery and the preamp board connection, most likely a broken or bad solder joint within the battery connector. I can't tell you how many snaps I've had to quickly fix by opening up the vinyl on the back and resoldering at least one of the wires. If you have more time than soldering skill, just replace the whole connector, you can buy them cheap on Amazon or at any electronic hobbyist store, though you might have to splice the wires to reuse the connector to the preamp board.

If the battery connection is fine, switch to resistance/ohmmeter mode and probe all combinations of three solder points; the black battery lead, and the green and black wires going to the jack. Make sure the cable is still plugged in to the bass for this test. Whichever two wires you choose, the meter should read less than an ohm; if the meter reads an "overload" or discontinuity, that's your problem, and the prime suspect is the solder point between the green wire and the sleeve terminal of your output jack, which will show a discontinuity between the green and black jack wires. Resolder the green wire and test the continuity between the black and green wires at the preamp board again.

Now, if back at the very first test, you did read a good voltage between the battery positive and a grounded point, then the preamp board is getting power. Pretty much anything that could be wrong at this point is very likely on that board. What you can fix depends on what's wrong, and your skills in finding the problem and potentially in your small-electrical soldering skill. Simple problems include a short-circuit between legs of a transistor or other component, caused by a stray piece of copper or bare wire. More likely, something got an overvoltage at some point, released the magic smoke and/or the magic resin, and you will need to replace at least the failed component. In such situations, I wouldn't trust anything else on the board, so you might as well replace it. This is the worst-case scenario, and one of the less likely unless you really abused the components getting them into the control cavity.

 

How is the battery being switched on and off?

- John

 

Same as most battery-operated audio circuits; the jack is a TRS jack, where the ground is connected to the sleeve, and the negative power rail back to the battery is soldered to the ring terminal. Plug in a typical TS instrument cable and you complete the connection between the negative rail and ground. Remove it and you disconnect the negative rail so no current can flow.

 

If you are measuring voltage across the battery connector at the board but not with reference to ground elsewhere in the circuit, then the battery isn't getting turned on. I.e., the negative lead connected to ground.

In your picture I can't tell where the green wire connected to the Ring connection on the board (just below the battery negative) goes.

- John

 

This is either a discontinuity of circuit, or a short circuit. Either way, most voltmeters will register little or no voltage differential. To confirm which it is, you can probe the same points in resistance mode on the multimeter. I would expect a fairly high resistance if not discontinuity, as you're basically probing through the FET boost circuits with the transistors' gates closed or near closed. The exact resistance would depend on the circuit design (and the position of the boost control pots; you might actually get relatively little resistance if the treble and bass boosts are at minimum, as the design might just shunt most of the amplified signal to ground). If the resistance is relatively low (I'd say anything south of 1Kohm would be concerning, though again I haven't seen a schematic of this board so I can't be sure), that means your 9V rail (red wire path) is shorting to ground somewhere after the battery terminal but before it can do its job with the boosts. Again, check for any unintentional metal on metal contact with any part of that PCB.

I didn't expect that at all; the green to ground connection could have been nonzero (but low) due to the physical connection through the plug sleeve, and that would also make the connection between the two black wires through the green nonzero. The connection between green and battery negative should, if the circuit makes any sense, be a direct connection through the PCB, linking the ring terminal and the battery negative, so to hear that these two points have the only measurable load is really weird.

Just to confirm that reading, if you unplug the battery and jack plugs and directly probe the BAT- and GRN terminals, that should yield the same reading.

That's entering my thought process as well. You're right, for a preamp with so many pieces the wireup is stupid simple, so if it doesn't work, and there isn't an obvious discontinuity or short-circuit, there isn't a whole lot that could be the problem that is "user-serviceable".

 

You do seem to have an extra hole in your control cavity. The biggest problem with series/parallel is going to be what to do with the blend control. That's where the current circuit combines the signals of the pickups, and it does so in parallel. A switch to series arrangement, with the blend pots still in the circuit, would make the blend for the second pickup in the chain into a master volume. So, it makes more sense to cut the blend completely out of the circuit if you switch to series, and that means the switch not only has to change the destination of the first pickup in the chain, but the second, which IMO will requiring the switch to be a 3PDT at least).