Op-Amp non-connecting pins.

[newfish]

Hi All,

I recently built a D+ clone-ish box, and had no problems with it.

Then I built another for a friend, but this one threw up some problems.

Both boxes were on a hand-drawn PCB layout - verified by the first build.

What I don't understand is how I got voltage (typically 4v) on pins 5, 1, and 8 of the Op-Amp (LM741 and its derivatives, socketed).

The socket is soldered to the board on all eight pins, but the copper pads for pins 1,5 and 8 don't connect anywhere - they simply form a mechanical joint.

I removed the Op-Amp, and checked each socket hole for continuity with my DMM - and got no reading for pins 1, 5 and 8.

I've been through the usual de-bug tricks (jeweller's loupe, razor blade around the traces etc) to make sure there are no hair-line traces / solder bridges, and there are none.

Oddly, this fault only happened when using an LM308N, not an LM741 in the socket (same pin-out, and the LM308N worked well in the first pedal).

On reflection, the LM308N may not have been pushed into the socket enough (despite hearing a soft 'click' as it went in), or there's something else going on that I don't understand.

Other than, "Make sure your Op-Amps are properly seated next time", can anyone offer any advice as to why this stray voltage was present?

Thanks in advance.

[Govmnt_Lacky]

Hey newfish,

Have you considered that the source of the voltage might be INTERNAL to the op amp itself?

The pins that you reffer to as "unused" may be unused in your circuit however, that does not mean they are unused in EVERY circuit.

Try looking at the ICs datasheet. It will have a schematic of the internal operation of the IC and may explain to you how the voltage is getting to the pins.

Good Luck 

[StereoKills]

Check out the LM308N's datasheet compared to the LM741's. I think you'll find that the pinout for the LM308N uses pins 1 and 8 for a compensation circuit with a capacitor, while the LM741 does not. The most common example I can think of is in the Rat circuit.

[newfish]

Data Sheets!

Data.   Sheets.

Why didn't I think to look there?

Cheers guys.  It had never occured to me that the OA itself might have some use for these pins.



Thanks once again.

[PRR]

> how I got voltage (typically 4v) on pins 5, 1, and 8

If you are doing some precision work, these pins let you diddle the inside of the opamp. Two will probably be near a supply rail, used for precision DC trim (utterly useless to us). One is to slow-down a too-fast opamp, and indeed that point may be half a volt off from the output voltage, which is 4.5V, so 4V would be reasonable.

741 may have the DC trim but no connection for slow-down compensation (it's stuck in low speed).

That's not your problem. 97% of the time, something aint connected like you think it is, or is connected when you think it aint.

[newfish]

Cheers, PRR.

Have now learned to double-check datasheets before diving in with both feet.

Am hardly doing 'precision' work - just a meaty-sounding overdrive...