How to check an OP AMP in circuit

[johnnydiaz]

I have a few OP Amps still in circuit and I'd like to check them. How can I do that? They are TL072CN & 5532DD Basically I have a preamp where one of the master outputs is not working so I'm assuming that its the op amp. So I basically want to test it. What else do you think it could be?

Thanks,
Johnny

[budm]

Re: How to check an OP AMP in circuit

Do you have diagram of what you are working on? The TL072/5532 are dual OP-AMP, You should download the data sheets so you can see the inputs/outs pin of the IC's. You can start by checking the DC voltages first, Black probe on the metal chassis, Red RED probe to pin 8 (B+ supply for the Op-amp), then check pin 4 (B- supply for the Op-amp).

[johnnydiaz]

Re: How to check an OP AMP in circuit

I don't have a diagram for this circuit. When you say dual op amp do you mean two outputs? I'm looking at the pinout of the tl072cn and have a few questions? Are pins 1-4 are for output 1? Are pins 5-8 for output 2? Since one output is working and the other isn't is it possible that half the op amp has gone bad or does the whole chip go bad?

In order to check voltage on pins 4 & 8 I will have to reconnect and power unit correct? What should the voltage be at these pinouts?

Sorry but I'm a newbie and appreciate all the help.

[budm]

Re: How to check an OP AMP in circuit

Dual op amp, mean one package has two operation amplifiers inside the package, each amp has two inputs (1 Non inverted input, 1 Inverted input), and 1 output. The power supply voltage can be as high as 18vdc. See the spec sheet (TL072 has the same pin out as 5532).
Pin 1 is the out put for amp#1
Pin 7 is the output for amp#2
You will check the voltage with power applied.

You can see example of the audio circuits in the datasheet.
If you want to learn more about audio, check this site out.
http://sound.westhost.com/
check the ARTICLES button.

[johnnydiaz]

Re: How to check an OP AMP in circuit

OK so I tested the voltage and on pin 8 I got 15.60vdc & pin 4 -15.35vdc. I'm assuming this is within spec. What would be the next step?

[budm]

Re: How to check an OP AMP in circuit

Ok, the voltages are OK. To find out where you lost the audio, you will need an oscilloscope to trace out the signal, but I do not think you will have one, it will be hard to do.
So this tool will be your audio tracer. Do you have something like a PC powered speaker of some sort? If you do, you will have a make a small probe, that can be attached to the powered speaker audio input, the input will have left and right channel and one ground. The ground wire will be attached to the metal chassis of what you are working on, then you can use either the left or the right channel wire (you will have to cut the audio cable up to attach the home made probe to this cable). Just to be safe, you can attach 4.7K~10k resistor in series with the probe.
On the amp you are working on, you will have to feed it with some audio to the input. Then you can use the probe to look at the signal output of the IC and see which output is bad. You will have to adjust the volume on the PC speaker to the desired level.

[johnnydiaz]

Re: How to check an OP AMP in circuit

I actually have a Tektronix 4 channel oscilloscope but doesn't know how to use it that well. I have a powered speaker which has a single input which I can use. I will have to find a resistor of the value. May I ask what the resistor will do in the circuit? Which pins are the signal input?

[Radio Fox]

Re: How to check an OP AMP in circuit

What is the make/model of the equipment you are working on?

What Tek scope do you have?

I have a 2445 and a 2445A

Before I could afford my first scope, I used to use a Micronta (Radio Shack) Transistorized Signal Tracer. It is just a high gain amp in a box powered by a 9v battery. Very useful for fault finding amplifiers. I still have it on my bench.

http://www.eham.net/classifieds/detail/306963

[johnnydiaz]

Re: How to check an OP AMP in circuit

The make is an generic mixer I don't know. I have a Tektronix 2465 oscilloscope.

[cadiman]

Re: How to check an OP AMP in circuit

If you are chasing audio signal any scope will work. A few beers and about an hour and you will be able to chase an audio signal.

As said above a scope will make life really easy for checking op amps.

What happens in audio is the input signal is small and any chip failure can just short out the signal and you won`t see anything.

I have removed the op-amp and check for signal in at the board if you don`t see anything.

[smason]

Re: How to check an OP AMP in circuit

Before I could afford my first scope, I used to use a Micronta (Radio Shack) Transistorized Signal Tracer. It is just a high gain amp in a box powered by a 9v battery. Very useful for fault finding amplifiers. I still have it on my bench.

http://www.eham.net/classifieds/detail/306963

I have one of those. Used it about a month ago.

[999999999]

Re: How to check an OP AMP in circuit

You don't necessarily need an o-scope for initial testing, simply apply power, supply input audio signal, measure for AC voltage at the opamp input and whether there's AC voltage at the output pin(s). If signal is getting to the opamp but not coming out, odds are it's the opamp to blame (or else a solder break but that's sort of rare for low powered circuits).

Similarly, all along the audio signal path you can probe for AC voltage with a multimeter to find where it stops.

[999999999]

Re: How to check an OP AMP in circuit

^ When I wrote the above, I took output not working to mean it's silent, not noise. If there is noise, measuring only an AC voltage won't tell you much, except whether it has the right voltage gain relative to the input based on the gain the opamp subcircuit sets it to have, if any.

[Radio Fox]

Re: How to check an OP AMP in circuit

Whatever instrument you use to measure the signal, the good thing about one half of a stereo amp not working, is that you have the working half to check against.

By putting the same signal into both channels, you can check the same point in the circuit for each channel to see where you are losing the signal.

[johnnydiaz]

Re: How to check an OP AMP in circuit

I finally changed the OP amp and now I hardly get any volume out. When I start to increase the volume I start to hear a few pops and static. I know my op amp is good what else could it be that is causing this?

[rievax_60]

Re: How to check an OP AMP in circuit

When everything is working right, feedback from the opamp output to the inverting input via feedback resistors, act to keep the 2 opamp inputs at the same DC voltage.
The opamp output should sit half way between the + and - rails.
With a split rail supply, +15V 0 -15V, both input pins and output pin should usually all measure 0VDC.

[S2udio]

Re: How to check an OP AMP in circuit

Check all the dc blocking caps in the signal path...as you have a scope should take
no more than a few mins to suss where the dead end is !