LM741 testing

[RoGeorge]

On 2 of the 741's with 20V input, I get 9.97V on both pins 3 and 6.
On another 741 I get 1.9V on pin 3 and <1V on pin 6
On another 741 I get 10 V on pin 3 and 3.14V on pin 6

So 2 look ok while the 2 do not?

Indeed, the first two 741 seem OK, the last two are not good.
Well done. 

[RobH]

1st picture of the faulty 741 as requested

When I grounded pin 3 on a working 741, after removing the x2 100k resistors , supplying 20V, I got 18.20V at both pin 6

[magic]

New and clean package straight from the factory and a shapeless blob vaguely resembling Texas Instruments logo. This is 100% guaranteed Chinese fake.

See my own collection of fake opamps from AliExpress
https://www.eevblog.com/forum/projects/whats-inside-the-cheapest-and-fakest-jellybean-opamps/

When I grounded pin 3 on a working 741, after removing the x2 100k resistors , supplying 20V, I got 18.20V at both pin 6

As long as there is connection from pin 6 to pin 2 it shouldn't be doing this, the output should be about 2V. It doesn't even matter if pin 3 is grounded or completely disconnected, it should be 2V.

In reply #5 in my thread I have shown how they wire the left half of a dual opamp to work like a single opamp. I suspect that your "good" chips are this exact thing. There is an easy test: DMM in resistance mode, black probe on pin 4 (ground), red on pin 1 or pin 5 (offset adjustment). Real 741 has about 1kΩ in both cases. (My sample measures: 1250, 1300). Fake chips will show open circuit.

As for the bad ones, there is a chance that they are dual opamps with fake markings. Apply power to pin 8 instead of pin 7 and test them again, taking output from pin 1 instead of pin 6. So connect pin 1 to pin 2, connect pin 3 to the resistors, see what's on pin 1.

[Terry Bites]

OK OK OK, they're 324s!!!

[RobH]

@magic,

oops panic over, as I didn't ground pin 4, only pin 3, Doh!
With pin 4 grounded as well the out put is 2.2V on pin 6.

The good ones show a resistance of about 1.4K between pins 1 and 4, while the dud ones shows almost open circuit.

Thanks for that.

[magic]

This may be a good 741, then
I suppose it also looks differently from the specimen you posted?

The one with fake TI logo is most likely not a 741, not even a damaged one. You could check if it's a fully functional dual like LM358.
I recall somebody posting on this forum that he found NE5534 (another single opamp) which actually was RC4558 inside.

[RobH]

@ magic,
At the risk of being cheeky, how would I check if it is a 358. I can't find a circuit, yet, that I can put together to find out.
Thanks

[magic]

It's an opamp, so most of the same circuits will work, mostly in the same way, sometimes with small differences.

You can start with what you have, the voltage follower.
But now the output is on pin 1 instead of pin 6, so connect pin 1 to pin 2 and disconnect pin 6.
And the power supply is on pin 8, instead of pin 7, so connect 20V to pin 8 and disconnect pin 7.

There is also a second channel on pins 5,6,7 on the other side of the chip. Full pinout:
https://components101.com/ics/lm258-op-amp-pinout-datasheet
or simply download LM358 datasheet.

[RoGeorge]

LM358 has two opamps inside a single chip, so it's a different pinout, see the datasheet:
https://www.onsemi.com/pdf/datasheet/lm358-d.pdf

https://www.electroniclinic.com/lm358-ic-pin-configuration-working-lm358-circuit-examples/

Make a similar circuit as you made when testing the 741, a voltage follower, as magic said:

As for the bad ones, there is a chance that they are dual opamps with fake markings. Apply power to pin 8 instead of pin 7 and test them again, taking output from pin 1 instead of pin 6. So connect pin 1 to pin 2, connect pin 3 to the resistors, see what's on pin 1.

[Zero999]

And what would it tell you about the chip?

It will tell you whether it works or not.

Yes please if you will.

Use the 741 to build a Schmitt trigger oscillator to flash an LED.

[RobH]

Thanks for the information on a LM358, and I have now connected the fake 741 up as a 358  using a voltage follower.

Results
With 20V input, I get 12.6V on pin 2 and pin 1.

That doesn't look right to me, so I guess it's not even a 358??

[EPAIII]

Err, sorry but the DIP-14 was one of the packages for the 741. I have three of them sitting in front of me right now. And they came in an 8 pin TO5 which, IIRC had the same pin order as the DIP-8. Yes, I have one of those too. It was a very popular chip. Still is, I believe.

So, what package does the OP actually have?

<pedant> No it isn't - those have too many pins to be 741 chips </pedant>

[EPAIII]

A full test of a 741 would take a lot of time and equipment. But you can improve on that circuit you found (in the original post) by using a higher supply Voltage (20 Volts would be good) and instead of R1 and R2, add a 10K pot with a 1K resistor from each side of that pot to the + and - power rails. Then run the wiper of the pot to the non-inverting pin (usually pin 3). The rest of the circuit is the same. Connect a meter or scope to the output (usually pin 6) and turn the pot up and down from one end to the other. If the output Voltage goes up and down from about 2 Volts to 18 Volts, more or less in step with the pot's position, the chip is OK. If it does not then the chip is bad.

This is not a complete test, but it should be good to catch most bad chips.

You do know that 741s only cost less than half a US dollar in quantity of ONE from a reputable dealer. It goes down from there. The shipping should cost more than 10 new, known name brand chips. And they ARE available.

[Ian.M]

@EPAIII
You've got me there.   Its a *LONG* time since I encountered a 741 or similar single OPAMP in anything other than a DIP-8 package.  I *think* I've got a couple of TO-5 package OPAMPs gathering dust in a draw somewhere but haven't set eyes on them in years.

OTOH Terry's picture:

is very unlikely to be a board full of OPAMPs solely because there are insufficient passives.  If it was a modern board they could be SMD hiding on the underside, but the DIP packages and close numbering of through hole capacitors make that vanishingly unlikely.

@All,
Assuming a scope is available, what are your thoughts on using the OPAMP in a Wien bridge oscillator circuit to functionally test it?  The major issue would be AGC to stabilize the oscillator at unit loop gain, but for a simple OPAMP tester, a soft clipping circuit should be sufficient.