Fixing Rocktile GB-15 bass guitar combo

I bought a damaged super-cheap bass combo for practicing at home. Description provided by the previous owner mentioned that signal overdrives and that it sounds unpleasant.

After turning this on for the first time I noticed that:

• there is unwanted overdrive/distortion
• distortion changes over time
• after couple of second there is no sound

As always I checked supply voltages first - they were fine. Power stage built around TDA2030 worked too so I concentrated on the preamplifier. Replacing opamp didn't help so I had to understand how the circuit works. There was no circuit diagram available so I reverse-engineered it from the PCB:

After connecting a signal generator to the amplifier I noticed that it operates normally for a while after touching pin number 2 or 3 of the 4558 operational amplifier. Using oscilloscope I observed that voltage on the pin number 2 of the 4558 is slowly rising and when it saturates the circuit doesn't amplify anymore:

I knew that this has got to do with an input bias current of the operational amplifier. In one of my favorites books "The Art of Electronics" I read that input bias current problems become visible some time after turning the circuit on, when impacted capacitors charge, for low input bias current amplifier it can be even minutes after turning a circuit on.

According to circuit diagram every input pin of the operational amplifier has got a DC path designed for input bias current. I measured R7 resistance on its pins and it appeared to be fine but by measuring resistance between pin 3 of the 4558 operational amplifier socket I noticed that the R7 is not there. I inspected the PCB and discovered that there is a crack on the PCB between the R7 and GND. On the movie below you will see (sound is important for that movie).

After fixing that PCB crack with a drop of soldering tin the amplifier started working

One more note here is the fact, that the input jack socket shorts input to GND when there is no plug inserted. It is so to avoid noise when there is no guitar connected. But to connect a signal generator to the PCB you have to be aware about that and to plug something in to open the input socket.