Dear all,
the small preamp in my Ibanez BTB-405QMTK bass guitar is giving me a hard time. The problem is, that I get a +9V offset on the output where it should be 0V.
The preamp circuit is a Vari-Mid 3B (IIIB), Ibanez and Meinl who are the official distributor and service center for Ibanez here in Germany refuse to send out schematics for this board.
They play the same f’d up game like Apple and Ersa do: No repair for you, just switch out the complete circuit board at your own cost.
The strange thing is, that the circuit itself is working without problems. Just the massive offset on the output is driving me nuts -> you can guess what happens if you plug it into an amp before turning down the main volume :/ The same is true for switching effect pedals with the bass connected of course.
The circuit is powered with 18VDC from two 9V batteries wired in series, however I am seeing more or less exactly half the supply voltage on the signal output (pin C1). This is true with everything wired in place (some voltage drop through the volume pot) and also with only the bare board connected to power.
Can anyone give me some pointers on where to start looking? Currently I am trying to RE the schematic, but it is quite a task with all the traces running under the SMD parts.
The SOT-23 transistor seems to be a
KTC3875, marked ALY and checks out OK.
The two SO8 op-amps are
TL062C.
Pin C5 is VCC (+18V)
Pin C4, C2 GND
Pin C3 Signal In
Pin C1 Signal Out
Every tiny pointer is highly appreciated!