@fredrik has mentioned fake ICs a few times and my stash of TL074’s turns out to definitely be. The first sign should have been when I found a single LM324 mixed in, but I didn’t put it together at the time.
I built a PM Foundation State Machine and it wasn’t working. The gate just stayed high.
I’m usually hesitant to contact support for anything, but after a lot of troubleshooting with no luck, I contacted the manufacturer and I got expert help testing and calibrating everything. One of the very first things he asked was if I was using genuine TL074s. The charging circuit is picky.
In any case, the module is now behaving nicely and it’s calibrated better than it would have been without the help. Ergonomics are obviously terrible, but it’s a very powerful module so I’ll just live with that.
Charging in an ADSR?
Is there a schematic available, I’d be curious to understand what that means.
Was the problem maybe if the TL074 is used as a comparator to detect the end of the Attack phase and the voltages are too close to the power rails? (The PMf SM user guide I found talks about calibrating to detect the envelope voltage reaching 10V which is very high if it isn’t divided before comparison.)
Also, where did you get your suspect TL074s from?
Are there any cosmetic or other indication that they are not genuine?
Genuine parts can also sometimes be defective, especially if they are not new. I have gotten some genuine, but clearly used, parts from AliExpress before.
Do the suspect parts still act like a quad opamp?
Do they work in other less sensitive (better designed?) circuits, or do they just seem dead?
Predictably, AliExpress. The giveaway is the printing and the case. Of course, real ones have been around for a long time, so they can look a lot of different ways depending on their vintage. I think getting them from a reputable supplier is probably best. Would have saved me some money to just cough up the 59 cents each they cost at Anchor.
I just switched in a genuine one in the splashback delay and it all of a sudden sounds much better. Before, the feedback would go into garbage mode super easily and now I can get really nice controlled feedback sounds out of it. Here’s a comparison shot, though the genuine one (top) is likely pretty vintage:
Do the legs look tinned? That’s often the best sign that things have been “recycled” (see the video I posted earlier). It doesn’t really feel like TL074s are expensive enough to warrant outright fakery, but you never know, and TBH I don’t recall seeing any chip with three indentations before so won’t even guess what I’m looking at here.
But note that driving a TL074 to 10 V with a ±12 V supply may take you out of the supported range, so if you hadn’t mentioned Aliexpress you could just have been unlucky:
But, yeah, driving it that close to the rails is probably why the circuit is so picky in the first place. I did try a bunch of the fakes with the same result and two different genuine ones that worked.
And not surprisingly, Googling for “GK43903X” brings up a bunch of people selling “ST” chips that look exactly like yours, with the same production codes… Actual ST chips use a different, rather distinct typeface (but that may have changed over the years, I guess).
Top is genuine. I know because it came from a reputable source, the others are fake. I guess the third indent is the tell tale sign is the third indent in this case, but also the performance.
I don’t get this ‘third indent’ idea. As far as I have seen videos on evidence of fake components they were rebranded and a new type number was printed on them. But what would cause a third indent on them is unclear to me. That would also be a dead giveaway so making sure the paint sticks and the correct character type is used would be a waste of time then.
They probably repainted an even cheaper quad op amp, so the third indent is an indication that it’s a different chip simply because genuine TL072s don’t have it. Not saying this is a reliable method or definitive test, just that the two kinds of fakes I have both have it.
It’s hard to tell by looking at the packaging. Reputable manufacturers have multiple places that package their parts and each place can produce parts that look a little different. It is an indicator though if you know a part is counterfeit, then maybe most of the counterfeit parts also came from the same packaging house.
An electrical test is the best way. A quiescent current measurement may be enough to tell you if it is the right part. Then check output swing, slewing, offset, and input impedance. If those match then it should work OK.
There is also the “4th shift” effect where a contractor will use your production line with inferior parts and produce counterfeit products during off hours.
I’ve been bitten by crappy fake opamps, you used to be able to tell the proper ones from fake by comparing photos of the chips. Got a couple of genuine OPA1622’s winging their way over to me… when the post catches up!
Hope you didn’t waste too much time with the fakos…
Another less elaborate way I saw in a youtube video is to use some solvent and see whether you can wipe away the ink of the markings printed on the packaging and maybe see what the original lettering lying underneath says.
Most markings are laser etched into the epoxy. It is a dead give away that there is something wrong with the part if you can wipe off anything. Ink marking was used prior to laser marking so watch out for “vintage” parts.
Some counterfeit parts are not really counterfeit they are factory rejects that didn’t quite make the test limit. Sometimes the manufacturers themselves will sell these parts under a different part number or grade.
To be clear, selection or binning is not always the difference in grades. Sometimes the higher grade parts means that the part went through more rigorous time intensive (expensive) testing and the other grades did not. The different grades may be exactly the same part but you pay extra for the higher grade to get the assurance that the part has been tested and works at the more extreme limits.
