Learning to troubleshoot (EQ circuit)

Wondering if you guys can help me to learn how to troubleshoot a solid-state EQ circuit?

I posted to the build thread but suspect it may get lost there...

https://groupdiy.com/index.php?topic=29759.msg884882#msg884882

Replies here would be helpful for general solid state troubleshooting advice/pointers. I'd like to learn to fish instead of being given fish. =)

Thanks,
Chris

fazeka said:

I started trying to signal trace using my 'scope from input, comparing to the known good channel. I get to pin 2 on U3 and see a signal that's not there on the known working channel pin 2 U3.

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Pin 2 of a TL071 is + which is grounded in that circuit. So if you're seeing signal there, that means your ground is bad / floating. Check all ground points for continuity and bad solder joints.

Correction: U3 is either toast or it's connections are bad. If it's socketed, try swapping it out.

squarewave said:

Pin 2 of a TL071 is + which is grounded in that circuit. So if you're seeing signal there, that means your ground is bad / floating. Check all ground points for continuity and bad solder joints.

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Pin 2 of a TL071 is a -ve input

squarewave said:

Pin 3 of a TL071 is + which is grounded in that circuit. So if you're seeing signal there (at pin 2), that means your ground is bad / floating. Check all ground points for continuity and bad solder joints.

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(typo above corrected)

Either U3 is faulty (maybe only opamp pin bent inwards, not connecting to its socket) or feedback resistor R28 doesn't connect for whatever reason. With EQ set to bypass (SW1 as shown in schematic), the summing amp U3 is a unity gain inverting voltage follower (-1 * R28 47K Rfb / R43 47K Rin). A working opamp will do its best to keep both +/- input terminals at same level and as U3-pin3 (+) is hard tied to 0V reference voltage, the summing node U3-pin2 (-) will be at the same 0V potential (virtual ground).

This guy is trying to learn to fish. Does he know the basics behind the op amp he is probing?

Does the OP know what a differential amplifier is? How inverting and non inverting op amps work and how to configure them? That's fishing material to me..

Harpo said:

(typo above corrected)

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Oops again! The first graphic that comes up in Google images for "tl071 pinout" is wrong. Apparently some stuff on the Internet is incorrect.

Yeah, I would say U3 is just busted then.

buildafriend said:

This guy is trying to learn to fish. Does he know the basics behind the op amp he is probing?

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A little.

buildafriend said:

Does the OP know what a differential amplifier is?

How inverting and non inverting op amps work and how to configure them?

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Not without looking it up. Good reference suggestions?

Thanks,
Chris

fazeka said:

...Good reference suggestions?

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Mancini - Op Amps for Everyone
http://web.mit.edu/6.101/www/reference/op_amps_everyone.pdf

I did some studying... found some electronics books on a torrent... there are a lot, FWIW. I would recommend it highly, no any one specific title so just go out and peruse...

And U2 and U3 were the culprits. I replaced them and now all is well. =)

Thanks,
Chris

The input to an op amp is a differential amplifier usually with a transconductance stage after it. Then lastly a class AB output stage to drive a load. The differential amplifier input amplifies the difference. This is set with the feedback resistor around the op amp and the input R, OR it becomes a gain of 1 buffer when the output is tied directly back to the input. Google buffer, inverting op amp, and non inverting op amp. Look at the arrangements. Understanding virtual earth summing points helps too but for speed and practical purposes this short text kind of explains a lot.. The little cap in parallel with the feedback R sets the f response.

Thats my general understanding without diving back into the text books..

Does that help for troubleshooting purposes?

https://en.wikipedia.org/wiki/Differential_amplifier