Advice on troubleshooting TK Audio The Strip - Distorts after ~100sec

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Having trouble just bought a "TK Audio The Strip". Mic Pre, Compressor and EQ.

After about 60-120sec it starts to distort, don't have a scope here right now to troubleshoot but i put in a 1kHz test tone from my soundcard and then used the spectrum analyzer in my daw. You can clearly se the harmonics after a while.

I noticed a bulging cap near the input, it was a 50v 100uF and if i poked it with my finger i could hear noise. Replaced it with what i had, 50v 220uF. (you can se it because its standing upp and pointing towards a red elyt, gonna change it to 100uF thats why) Measured it and it showed about 60uF, to low. But that didn't help the distortion that sets in.

There is a class A transformer stage that i can engage, but it does not matter if i have it on or off. And i dont hear a change in sound when the class A is engaged, theres a carnhill output transformer in the circuit so i should hear something...probably. Cant even se a sign on the spectrum analyzer when running pink noise and engage/disengage the "vintage" circuit.

Maybe there is more caps that need to be replaced but i don't know if that explains why i get distortion, maybe its some kind of bias problem...

One thing that i should add is that its only the hi-z instrument input on the front that seems to have the distortion problem.
If i plug in a mic in the back i cant hear or see any distortion.

When using the hi-z instrument input and the distortion sets in it doesn't matter if i adjust gain, engage/disengage eq or comp.

There is a TL071 near the instrument input jack and next to the red cap a THAT1512.

I if i switch on the unit while measuring dc on the output of TL071 it goes from 0.2VDC to about -15VDC, measuring from chassi ground to pin 6. And AC decreases from ~0.67 to ~0.500VAC. When the DC on pin6 reaches -14VDC i can se distortion on the spectrum analyser.

Wonder if the TL071 is defect or maybe there is something else near it.


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I tried to draw a schematic of the instrument input section, hope i got it right.

Hm i disconnected the anode, positive leg, of the electrolytic cap.

The voltage on the output of TL071 still goes down to -15VDC.

How common is it for those TL devices to fail like this?

I cant se another possibility than a faulty TL.

Someone mig bounce some idéas here? :)
DC Voltage readings of TL071

pin1 -18,0 (offset null1)
pin2 -16,81 (inverting input)
pin3 -1,19 (non-invering input)
pin4 -18,23 (Vcc-)
pin5 -17,97 (offset null2)
pin6 -15,9 (output)
pin7 +18,06 (Vcc+)
pin8 0

Why is there dc on the input of the op amp without anything connected...
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Okay i found one TL072 and wired it to just use one OP.
Voltage readings now look better, and the unit sounds fine! :D

1 0,29 (1out)
2 0,26 (1in-)
3 0 (1in+)
4 -18,25 (Vcc-)
5 0 (2in+)
6 0 (2in-)
7 -16,85 (2out)
8 18,05 (Vcc+)

Pos Reg
1 16,82 (adjust)
2 18.07 (Vout)
3 24,35 (Vin) <— 0,142VAC ripple

Neg Reg
1 -17,0 (adjust)
2 -25,90 (Vin) <— 0,07VAC ripple
3 -18,27 (Vout)

Two things that i not like, the outputs have a 0,2V difference between neg and positive output.
This is a high end preamp costing ~13k.

Going to replace caps in power supply and check voltage divider resistors connected to the regs.
No ripple on the output of LM317 and LM 337, just a 0,2V difference.
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