Electronic Circuits and Projects Forum

I'm new to this forum, so I'm not sure whether I'm better off replying to a thread that's been dead for some time or starting a new (and potentially redunant) one, so I've gone ahead and done both. Anyway, I built a circuit based largely on Post #4 in "Inverting amplifier," and it's not working. It looks like it's probably a problem with the chip, but I'm hoping maybe it's something I've done wrong that I can fix this weekend.

I've got 12 V, 2.5 V, and ground in all the right places, but with either a 5 V or 0 V input I'm getting a constant output > 7 V. When I adjust the potentiometer that I'm using in position R2 the erroneous output is unchanged, which is what leads me to believe that the chip must be at fault. Any thoughts? (For what it's worth, the chip I'm using is an NTE7144, which is the cross-reference Mouser gave me when I searched for CA3140; as far as I can tell this is a totally legit cross-reference, but since most of my electrical engineering knowledge is strictly theoretical, I'm curious if I've inadvertently made some kind of op-amp cross-referencing faux pas.)

Thanks in advance,
Joe Elliott

Originally Posted by ericgibbs

hi,
How is the Strobe pin on the NTE, pin #8 connected.?

It's not. Pins 8, 5, and 1 are floating. Is that my problem?

(And yes, I've verified that the pinout is as I expected.)

Thanks again.

Originally Posted by ericgibbs

I would try pin 8 via 1K to +V and check the output, if the same try, pin via 1k to 0V.

I don't have any 1k ohm resistors available here (I'm visiting family for Thanksgiving--it's my brother's electric car that's supposed to have a working fuel gauge before I leave tomorrow evening), but I tried this with 10k ohm resistors, and output was unchanged.

Originally Posted by ericgibbs

Hi,
If possible post a clear picture of the pcb, front and back, we can check your wiring.

The circuit in question is on the lower half of the board in these photos. From top to bottom the four external connections on that half of the board are +12 V, output to instrument panel (5-0 V = 0-100% state of charge), input from battery monitor (0-5 V = 0-100% SOC), and ground. IC mounting is a little unorthodox because (if you couldn't guess) in my hurry to get this built in time for the Thanksgiving holiday, I designed the PCB with the IC mounting points backwards. The unused pins in the IC socket are meant to be used to add resistors (or in this case a plane wire to eliminate variables for troubleshooting purposes) in parallel with the potentiometer on the output (which is there for fine tuning, since bench tests of the gauge indicated that it'll read 100% at more like 0.5 V than 0 V). (The other potentiometer is in place of R2 in the aforementioned schematic, to fine tune feedback resistance.)