Signs of a "bad" op-amp?

[scotsman]

I recently build up (another) GGG IC buffer - and ran into a problem.  It had been working perfectly until Sunday when I got home and fired up my "rig."  It started making loud hissing sounds!  Im guessing it's the op-amp but is there any way to confirm this? 

Any help is greatly appreciated!

[petemoore]

1 take bias voltage readings
when those look bad, try another opamp...at the risk of blowing it, too...OR
2 stick your suspect opamp in another opamp 'known to work' circuit and see how it performs.

[R.G.]

Good calls, both, Pete.

Another hint - suddenly starting loud "angry" sounding hissing is often a sign of RF oscillation.

[Sir H C]

For an op-amp the + and - inputs should have the same (within a few millivolts) readings.  If they do not then there is a problem, either the op-amp or whatever is closing the loop is bad.

[scotsman]

How does one go about taking a bias voltage reading?  What pins am I putting my meter on etc?  Thanks for all your help -

[Sir H C]

Depends on the op-amp, but the biasing is generally looking at the two input pins and the output pin and seeing that they all make sense.  First, as I said above, you want the + and - pins to be the same voltage ***for most applications***.  Some applications don't have these the same voltage, when using an op-amp as a comparator for example.  But usually they should be the same.  The output voltage should be somewhere between the rails, hard to say where without knowing the circuit.

Also for the input pins, with a single supply, measure versus ground and you usually will see something in the 3-5 volt range on the two pins.  If it is near 0 or + supply there is often a problem.

[grapefruit]

If you're using a single op amp like on GGG, Pin 2,3 and 6 should be somewhere around 4.5V with respect to ground. (If you're running off 9V). If not check the connections of R1 and R2 and if they're ok replace the op amp. If the op amp is in a socket, as pete said, just swap it for another one.

FWIW I'd have a current limiting resistor before the op amp non inverting input (pin3). I normally use 1k.


Stew.

[scotsman]

I replaced the op-amp in the "hissing one" - and it did it again.  So I decided to build another - same thing again...  Could my 1 Spot have something to do with this?

My voltage reading at

pin 2 = 4.75
pin 3 = 4.28
pin 6 = 4.75

[grapefruit]

Have you tried running it from a battery?
Also, there should be a cap accross the power supply pins, especially if running from an external power supply.