TL072 drift?

[ITAM805]

hi all,

A little background info first, I have a Fender Super Champ XD in for repair. These are known for becoming doorstops due to the fact that the DSP goes south - and takes all controls, volume, EQ, channel switching and FX with it. The DSP board is unobtainable and unrepairable according to those in the know, so one dodge is to bypass it all together and turn the brick into a very nice sounding little valve Champ.

Quite simply, the DSP board is bypassed and the o/p of the input TL072 (pin 7) is diverted to the post DSP buffer and thus onto the valve o/p stage, 2 pots are re-purposed as basic volume a treble cut

So I tried my own version of the hack and it worked well, or so it seemed. I felt putting the volume pot on the input (as per the schematic) wasn't a good idea so what I did is move to the o/p (pin7) of the TL072, via a non-polarised 10uF cap to prevent DC problems. However that is exactly what is happening, pin 7 is swinging from +8V to - 12V and then ceasing to work?

So I swapped out the original chip for a new one but the same thing happened, so I tried a 3rd chip, but the same thing again

I have tried disconnecting the pot but the thing drifts wildly. I also tried swapping the 1uF cap (C42) between the two opamps but to no avail

Has anyone an idea why this might be happening please?

Many thanks

[KeithsTV]

Where is pin 5 (the +input of the 2nd amp) connected to? I suspect this is floating which would cause the output to wander. Assuming the op amp has both a positive and negative supply, try connecting pin 5 to ground via a 10k resistor.

Keith

[ITAM805]

Thanks Keith, good point ! I'll try that and report back

[ITAM805]

Keith, you're a wizard sir ! I added a 10k and the offset is now a rock solid 7mV.

Many thanks

[KeithsTV]

No problem, glad to have helped. that's what the forum is all about.

keith