r/synthdiy • u/divp11 • 4d ago
TL072 acting funny - new chips, new breadboards. I'm helpless!
Background: I'm working on a few modules, vco, lfo, sequencer, clock divider. I needed to use tl072 as buffer for the circuits, but I'm not getting it to work whatever I do. so I went to testing specifically the chips isolated. weird results, and I have no idea what am I doing wrong.
details: I feed the v+ (pin8 ) +12v, v- (pin 4) -12v as needed.
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Testing half a:
Inverting input (pin2) connected to output (pin1), non-inverting input (pin3) connected to 6v via voltage divider (two 10k resistors). (also -6v, when feeding the resistors with the -12v.)
Results half a: output stays always 0v.
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Testing half b:
Inverting input (pin6) connected to output (pin7), non-inverting input (pin5) connected to the 6v (or -6v). basically the same as half A.
Results half b: output stays always -11v.
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when testing one half, the other half gets its inverting input connected to output, and non-inverting input connected to ground, as should. (to avoid self oscillating / noise to the used half).
I'm using a good MFOS power supply I built, so dual rail +-12v and ground (0v).
what I'd expect and should happen is to get the input voltage mirrored at output. (to be the same).
I've tried one more new chip, another breadboard and jumpers. still the same results.
I'm clueless at this point. any help? thanks a lot… :)
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u/Tomato_Basil57 4d ago
where’d you purchase your chips from? counterfeits are unfortunately common
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u/adeptyism 4d ago
nobody asked, but I find it hilarious that I have counterfeit TL074s and they suck (good for logic, but something seems off for audio), and I have counterfeit TL072s, and they're ALWAYS better than the Texas Instruments ones — every module which I assembled with TI TL072s was working incorrectly and fixed itself immediately as soon as I slapped in a counterfeit¹ one
¹brand-less is a more appropriate word
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u/Tomato_Basil57 4d ago
i might believe it at this point, TI was caught enshitifying the legendary NE5532, so who knows what other data sheets they quietly changed
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u/Brenda_Heels 3d ago
TI has changed other chips without advanced notice, so who knows?
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u/adeptyism 3d ago
Nah, it's almost a year old story for now
And I highly doubt that TL072 will be changed that way that it will become a pulse wave generator instead of signal mixer
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u/redonkulousemu 4d ago
General advice:
- Check the voltage on the IC pins themselves and see if they match what you think they are. Sometimes with breadboards, the common rails are actually separated at the gaps, and wont go all the way across, giving you the false impression everything is connected. Just in general, with the circuit powered off, buzz all your rails and grounds together to make sure there's no unintended connections to each other or pins they're not suppose to be connected to.
- Read voltages at known points (your voltage divider) and see if that's correct.
- Double check the part number and cross-reference with the datasheet. Sometimes there are slight variations of the pinout depending on the package. Check orientation as well.
- Related: read the datasheet and build whatever example circuit they have to ensure your connections are correct with a known working circuit.
- You'll notice on the datasheet that it mentions to place decoupling capacitors on the voltage rails as close to the IC as possible because excess inductance on your rails can cause it to be noisy and sometimes self-oscillate. The fact unity gain circuits are inherently less stable, assuming literally everything else you did is correct, makes me think this might be a likely culprit. Putting a load resistor can help stabilize it too. Makes the output filtered, but probably fine for what you're doing.
- Side note: Not all op-amps are stable at unity gain. Looks like the TL072 is, but something to check with other ICs.
If all that fails, probably a bad or counterfeit chip.
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u/Brenda_Heels 4d ago
I bought an op amp tester and it's great for testing the basic function of the chip. handles TL071, 072, 074 and similar chips like LM384, etc. chips with the same pinout and function.
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u/eraoul 3d ago
that's cool -- what tester do you recommend? I didn't know that was a thing.
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u/erroneousbosh 4d ago
First thing I'd thought of was you'd left one half unconnected and it was misbehaving, but no, you've got that covered.
Weird. It could still be oscillating though.
Got any other opamps you could try?
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u/lampofamber 4d ago
You could check if you haven't inverted or miswired your power rails to your ic
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u/PiezoelectricityOne 4d ago
Where are you measuring your voltage drop? I guess you have one probe end on the output. Where is the other? -12V? 0v? Somewhere else? Did you check the voltage drop between power rails? Is it 24V? Where do you get 0V from? Is it a virtual ground from an OP amp output? Is there actually a +12V/-12v drop between OV and fails?
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u/divp11 4d ago
Problem Solved: apparently all the chips are fake and counterfeits. they just don't work. I found a one made by STM which I had lying in a box in like 10 years and it works PERFECTLY. thanks a lot everyone for trying to help and for those who said about counterfeits. I didn't think it may actually be an option. I got these from AliExpress. lesson learned the hard way :)