Stupid questions about op-amps

[gjsmo]

I'm building a multiRat right now, and I'm getting... nothing. I went through it with my voltage meter, and found 5.2 volts on pin 3 of the op-amp... with nothing connected to that pin  .
I thought it could be related to the inverting input, so I looked at that measured that... with both pin 2 and 3 disconnected, I had 6.7 volts at each.
Correct me if I'm wrong, but isn't it bad for an input to be PRODUCING voltage?
Lastly, I disconnected pin 7. No voltage on anything except pin 6... which is the output of the non-powered op-amp. I didn't disconnect it from anything else, but that would also mean that there's a voltage from past the op-amp.

Is this normal? Or should I take a good look at my wiring?
Is it possible the chip is bad (it's actually an NTE938M, but the writing is still visible underneath, and it says LM308N)?

[R.G.]

Is this normal? Or should I take a good look at my wiring?

You should ***always*** take a good look at your wiring.

Chips are made in highly specialized billion dollar facilities which employ highly trained and experienced people whose sole job it is to make sure that the parts that go out the door work, every last one. And they practice this, and work on it every day.

Yes, there is always the chance that a bad chip gets out, or that one has been damaged on the way to you, but it is hugely, vastly, incredibly more likely that your wiring and/or soldering has a problem. This gets said here in this forum over and over. It's still true. Each new batch of newbies pretty much universally doesn't read the previous postings of the question, and asks it anew for themselves.

[gjsmo]

Like I said... STUPID questions about op-amps.

Anyways, I just disconnect the thing from my circuit entirely, and hooked it up only to power and ground. I got...
Pin 1: 7.70 V
Pin 2 & 3: 6.72 V
Pin 6: 0 V
Pin 8: 6.83 V

Currently my battery is at 8.21 V.

So... there's still voltage on the inputs. With nothing connected to them. Like I said, should this happen? Or is this just standard behavior of an LM308, and should I move along to the rest of the circuit? BTW, I built the circuit twice, and it hasn't worked either time.

[amptramp]

The LM308 datasheet is here:

http://www.national.com/ds/LM/LM208.pdf

The LM208 is the industrial temperature range version of the LM308.

The device appears to be behaving as expected.  Pins 1 and 8 are compensation pins that have voltages specific to that part.  The pin 6 output is set by the offset if there is no connection to the inputs, so it will go either to the negative rail or within a volt of the positive rail.  The pin 2 and 3 inputs are biased through R3 in the LM308 internal schematic, so they sound right.  Each input is a bipolar transistor which will draw base current through R3, so you will find a voltage on the input pins.  The LM308 and LM308A are similar with the same schematic but better offset and drift characteristics in the LM308A version.

According to your voltage readings, the LM308 seems to be behaving.  Definitely check your wiring - usually the thing that is most obviously right turns out to be wrong.  Take a copy of your schematic and colour the connections in as you check the circuit against what you built.

[PRR]

> found 5.2 volts on pin 3 of the op-amp... with nothing connected to that pin.

It is NOT likely to work, or even make sense, with nothing connected to the input.

DC verification of the multiRat is simple. The junction of the two 100K resistors and 1u cap should be 4.5V (even with chip out). Pin 3 also 4.5V (or a wee bit less, 4.1V), even with chip out. With chip IN, pin 6 should come to 4.5V and pin 2 should come along, be 4.5V.

> So... there's still voltage on the inputs. With nothing connected to them. Like I said, should this happen?

We really do not care. You NEVER use an op-amp with nothing connected to it.

The concept is: you "tell" it what to do (by connecting voltages and parts) and it does that thing (if possible). If you don't tell it what to do, it is free to do whatEVER it wants.

Guessing from the published schematic: Q11 Q27 pull-up R3 Q3 Q4. Q3 Q4 are, in NORMAL operation, protection diodes. The Q1 Q2 pair are very delicate; Q3 Q4 absorb voltages that Q1 Q2 can't handle. In any normal operation, whatever voltage YOU put on the Q1 Q2 inputs is mirrored around Q5 Q6 Q27 R3 and holds the Q3 Q4 pair near the input voltage, so near that Q3 Q4 are "off" and do not affect operation.

Hoffman's Law. R.G.'s Law. Most "don't work" are wiring errors, NOT part failures. ASSUME parts are good.

[gjsmo]

3rd build... doesn't work. I should note that each build is COMPLETELY separate... I ripped out every single component between builds...
New battery, putting out 9.57 V. Power supply seems fine., 4.74 V at the 4.5 point.
So... I checked each resistor as I went along (although, as you say, assume they are good). I built this for the THIRD time, using a different layout on the breadboard. I eliminated the rotary switch, figuring that if it worked I could always use a jumper wire, and the volume control.

I get what you're saying about "it's your wiring", but I find it remarkably hard to believe that 3 different layouts/builds are all wrong... with the same effect. It's like burning a CD three different times and getting a buffer underrun each time, or re-wiring a Strat and wiring all 3 pickups backwards. I'm honestly trying to find a mistake, but I'm not finding it. I looked over my wiring every time. It always appeared correct in general.

I checked the cords too, in case you're wondering.

The only thing I can think of that's different (were you waiting for this?) is that I used polypropylene and tantalum caps, instead of ceramic and electrolytic. Maybe I should have said this earlier., but it seems to me that tantalums are just better electrolytics.

[R.G.]

I get what you're saying about "it's your wiring", but I find it remarkably hard to believe that 3 different layouts/builds are all wrong... with the same effect. It's like burning a CD three different times and getting a buffer underrun each time, or re-wiring a Strat and wiring all 3 pickups backwards. I'm honestly trying to find a mistake, but I'm not finding it. I looked over my wiring every time. It always appeared correct in general.

This is from your eighth post to this forum. If you'll check the archive, you can find many examples of people who find it remarkably hard to believe that their build, even multiple builds, are not working. I can't remember an instance where one of these turned out to be a bum component, although there may have been one or two.

There is a confluence of human traits that make this happen. You built this, using your learning and skills and made each decision along the way based on that set of learning and skill. If you learned something that is wrong, or there is a skill level needed that you're not aware of, you will, using the very best you can do, make the same decisions - and mistakes - again, perhaps many times. It's called being too close to the problem.

Fixing it takes either a break from it, a new person checking it out, or new learning/skills.  Worse yet, the way the universe works, which I personify as "Mother Nature", is not going to accept "in general" anything. It has to be correct in every detail to at least some minimum standard or it won't work right. Period. You cannot convince Mother Nature that you've tried hard and it's really, really right. She knows whether it is or not.

For a circuit that's known to work beforehand, if it doesn't work when you try it, then you didn't get the build sufficiently close to the previously known-working circuit. There is something wrong, by definition. Even if you convinced us that you really had done everything right, if it didn't match Mother Nature's Rules about electricity, it won't work. Our word doesn't count with Mother Nature either - it either matches The Rules, or it doesn't work.

The only thing I can think of that's different (were you waiting for this?)

Frankly, yes. Sorry, I don't mean to be snide, but it's no surprise.

Maybe I should have said this earlier.

It doesn't matter. Mother Nature doesn't care if you ever say it.

but it seems to me that tantalums are just better electrolytics.

Maybe. It all depends on how you define "better". If you'll check the archives you'll see a lot of typing about that too.

"Better" is meaningless without some reference. A fashion model may make a great uptown girlfriend, but she'll be a lot more expensive overall than the farm girl you met in a small Kansas town. The model is better for the glitzy life, the small town girl might be better for a long term relationship. Or not. Differences, even small ones, matter a lot and this is a highly multivariate problem. "Better" is almost meaningless here.

Tantalum is indeed an electrolytic cap. It's a hair-trigger kind of thing. Get them reversed in the circuit and they don't just reverse conduct and vent like aluminum. If the power supply is a burly, high current one, reversed tantalum explodes and/or bursts into flames. It's also more expensive. Better? You better need that lower ESR and smaller size, and be willing to pay not only for the initial expense but the increased manufacturing cost of making sure every stinking one is inserted correctly, as well as the warranty costs.

This kind of post appeared here often enough that I wrote down a guide book for the issue. That sticky marked "Debugging: What to do when it doesn't work" that you have to ignore and skip past to get down into the forum to see if there's a reply to your last post is worth reading. It presents an ordered way to get past "I'm certain I wired it up right," and actually find out if you did or not.

It makes good reading. You might want to have a look.

[stringsthings]

...."Better" is meaningless without some reference. A fashion model may make a great uptown girlfriend, but she'll be a lot more expensive overall than the farm girl you met in a small Kansas town. The model is better for the glitzy life, the small town girl might be better for a long term relationship. Or not. Differences, even small ones, matter a lot and this is a highly multivariate problem. "Better" is almost meaningless here.

Tantalum is indeed an electrolytic cap. It's a hair-trigger kind of thing. Get them reversed in the circuit and they don't just reverse conduct and vent like aluminum. If the power supply is a burly, high current one, reversed tantalum explodes and/or bursts into flames.....

does this analogy apply to women exploding when you say the wrong thing?   

[R.G.]

does this analogy apply to women exploding when you say the wrong thing?   

Depends on the chemistry of the insulation...

[amptramp]

...."Better" is meaningless without some reference. A fashion model may make a great uptown girlfriend, but she'll be a lot more expensive overall than the farm girl you met in a small Kansas town. The model is better for the glitzy life, the small town girl might be better for a long term relationship. Or not. Differences, even small ones, matter a lot and this is a highly multivariate problem. "Better" is almost meaningless here.

Tantalum is indeed an electrolytic cap. It's a hair-trigger kind of thing. Get them reversed in the circuit and they don't just reverse conduct and vent like aluminum. If the power supply is a burly, high current one, reversed tantalum explodes and/or bursts into flames.....

does this analogy apply to women exploding when you say the wrong thing?   

They call these women tantalizing.

[gjsmo]

I've built it a fourth time (it doesn't work, of course), and I MAY have found a discrepancy.

The multiRat schematic lists a BF245A (which is what I'm using). It's datasheet says that pin 1 is leftmost on the flat side, and the pins are 1:Gate, 2:Source and 3:Drain.
Others list a 2N5458, and that datasheet says that pin 1 is leftmost on the curved side.
I really should trust the datasheet, right? Or can a turn a transistor around and not expect it to melt?

[R.G.]

They call these women tantalizing.

They call them other things when they marry these "tantalizing" women. 

The multiRat schematic lists a BF245A (which is what I'm using). It's datasheet says that pin 1 is leftmost on the flat side, and the pins are 1:Gate, 2:Source and 3:Drain.
Others list a 2N5458, and that datasheet says that pin 1 is leftmost on the curved side.
I really should trust the datasheet, right? Or can a turn a transistor around and not expect it to melt?

What matters with JFETs in applications similar to this one is
(a) the JFETs are similar in characteristics; similar Vgsoff, Idss, Yfs, etc. They don't have to be identical but they ought to be similar.
(b) the pin, whatever number it is, that is the gate function is poked into the hole in the PCB; likewise, the drain-function pin in the drain hole, and the source-function in the source hole.

Datasheets are (nearly) always right about which pin is which, as long as the device you have was made by the company that released the datasheet. I looked up Fairchild Semi's datasheets for the BF245 and the 2N5458. If you orient the device with the flat side toward you, pins down, the BF245 pins are, from left to right, gate, source, drain. If you orient the 2N5458 the same way, the pins, from left to right are drain, source, gate. That is, they are reversed compared to one another.

In many JFETs, the source and drain are interchangeable. This is stated on the datasheet for the 2N5458. However, the gate is NOT interchangeable with the drain, which is what happens when you get the package backwards.

So I think you have some detective work to do. What device goes in there and the orientation depends on the original documentation.

By the way, in "Debugging: What to do when it doesn't work", I started with the following:

A full set of information about what you built
1.Name of the project: Example: Neovibe, from General Guitar Gadgets.
2.Links to the source of the project web site with the schematic or project, layout and wiring diagram. Example : Neovibe
3.Some candid admissions about how closely you followed the suggested layout, if any. If you just perfboarded it, say that, and if you can, include a picture of the board. Scanners do a GREAT job of making pictures of PCBs, by the way.
4.Any parts substitutions or modifications you made to the original. If you didn't have any 10uF caps and used 22uF, say that. If you had to use a 2.7K resistor instead of a 2.2K, say that. If you used a BC109 instead of a 2N3904, say that. By the way, if you substituted any transistors, it's a sure bet that the first question you get asked will be "Did you check the pinout of that transistor?" Delight the questioner by saying (truthfullyâ€) "Yes, I did a google search on that part number and check the pinout against how I soldered it in." If you can't say that, chances are good that you've already found your problem, all by yourself.

Because to take the next step in helping you fix it, I'd need to find the schematic for the circuit you're building, and worry about the physical layout, which seems to be at issue here. Those links to the circuit and layout help us to find the things you actually used without going on a search spree, and maybe finding the wrong ones to lead you down false trails with. The relationship of the schematic, which tells you the functional connections of pins, and the physical layout which shows you the mechanical connection of the pins, is what is at issue.

[gjsmo]

The schematic is just http://www.diystompboxes.com/pedals/ratmod.gif <<<this one.

I'll post some pictures later, but it's breadboarded, and I've got untrimmed leads (no, none of them are touching, I checked), so it might be hard to just look at it...

[drk]

you could try the "divide and conquer" approach. Try with just the op-amp, or the fet, and see if you can make it working. It's easier when you know that only half of the circuit doesn't work :p

not sure how you're building, but you could also consider trying 1st the circuit in a solder-less board, because its much easier to find the mistakes.