Troubleshooting woes "correct voltages?"

[endorphin]

I was wondering how is it you guys can tell whether or not a transistor or IC has correct voltage readings?

Are these things you can tell right away or do you calculate using formulas, or is it the datasheets?

This puzzles me, as all I can seem to find for most transistor datasheets are max voltages, pinouts, and dissipation stuff that I don't understand. 

Perhaps i am not looking through enough datasheets.

[armdnrdy]

The power pins of a transistor or IC should be evident....but many of the other pin voltages will depend on the circuit.

[Kesh]

The circuit around the transistor will mostly determine the voltages by biasing the transistor a certain way. The transistor's hfe will play a part, and also whether the transistor is germanium or silicon. Perhaps read up on biasing the two most common transistor circuits, the common emitter and the emitter follower.

[LucifersTrip]

I was wondering how is it you guys can tell whether or not a transistor or IC has correct voltage readings?

a good build report or schematic will have working voltages listed...so you need only compare.

before you build anything, you should search the forum & web for the working voltages for what you intend to build

there're also some resources that give more general tips
http://www.geofex.com/fxdebug/bias_prob.htm

[ashcat_lt]

Opamps will have the most positive power supply voltage on the V+ pin and the most negative on the V-, to pass audio we usually want to see all in and out pins halfway between the two rails.  For most stompboxes this means 9V, 0V, and 4.5V respectively.  These usually wont be bang on exact, but major deviations will indicate a problem somewhere.  You need to look at pinout/datasheet to know which pins are which, of course.  I cant say for sure what kind of voltage you should expect on compensation pins...

[R.G.]

I was wondering how is it you guys can tell whether or not a transistor or IC has correct voltage readings?

Are these things you can tell right away or do you calculate using formulas, or is it the datasheets?

As noted, it's not the transistor or IC necessarily, in analog circuits. Things are different in digital.

The real bottom line is you have to understand the circuit to know if the voltages you measure are reasonable. There are a few fundamentals, like the fact that bipolar transistors must have at least one forward biased junction (usually the base-emitter) and one reverse biased junction (base-collector) to be amplifying; and for JFETs, the gate-channel junction must be reverse biased.

But even this is gleaned by understanding how the device works. You have to know, for instance, that a forward biased diode junction has a voltage determined by the semiconductor material. Silicon diodes are 0.45 - 0.7V if there's not a large current flowing through them. LEDs are forward biased by 1.2V to 4V, depending on the color, which is obtained by using different semiconductors.

It's much like the song, "The Gambler". You have to know when to hold them, know when to fold them. Building up that knowing takes time.

[R.G.]

I was wondering how is it you guys can tell whether or not a transistor or IC has correct voltage readings?

Are these things you can tell right away or do you calculate using formulas, or is it the datasheets?

As noted, it's not the transistor or IC necessarily, in analog circuits. Things are different in digital.

The real bottom line is you have to understand the circuit to know if the voltages you measure are reasonable. There are a few fundamentals, like the fact that bipolar transistors must have at least one forward biased junction (usually the base-emitter) and one reverse biased junction (base-collector) to be amplifying; and for JFETs, the gate-channel junction must be reverse biased.

But even this is gleaned by understanding how the device works. You have to know, for instance, that a forward biased diode junction has a voltage determined by the semiconductor material. Silicon diodes are 0.45 - 0.7V if there's not a large current flowing through them. LEDs are forward biased by 1.2V to 4V, depending on the color, which is obtained by using different semiconductors.

It's much like the song, "The Gambler". You have to know when to hold them, know when to fold them. Building up that knowing takes time.

[PRR]

You can say that again.

[R O Tiree]

You can say that again.

Sometimes you have to say things twice before learning occurs. Perhaps RG was just cutting out the "misunderstanding" phase in the middle

[Jdansti]

If you have a specific circuit you're working on, follow the instructions here: http://www.diystompboxes.com/smfforum/index.php?topic=29816.0 and post your results back on this thread. The folks here will be able to help you figure out what it all means and make some suggestions to help.

[endorphin]

Thanks guys for putting me in the right direction. 

I've just always wondered how you were able to come to such conclusions whenever someone posted their readings.
I can build a lot of things and make them "pretty" and stuff, but I am limited when it comes to troubleshooting and that is not satisfying.

I've revived some projects to troubleshoot recently, but in the meantime some reading to do first. It is interesting going back to them with a little bit more understanding each time.

More questions to come....

[Jdansti]

Some folks here are electrical engineers and have studied this stuff until they know it frontwards and backwards.  They've also had careers working with much more complex stuff than fuzzes.  Others have absorbed some of the theory over time and have done self study. Some like me have just piddled with it for a long time and know just enough to be dangerous

Having said all of that, the underlying principles of electronics are found in the science of physics. Fortunately, smart people over time have provided us with formulas for calculating the effects of applying voltage to a transistor or adding a resistor to a circuit, etc., but don't expect to learn all of this overnight.

As you mention, once you learn the mechanical skills and can read schematics, it's not difficult to build 99% of what you find on the forum. It's good that you're trying to learn the "why" in addition to the "how". Keep asking questions and learning. Try research your question beforehand because it will help you understand the answers that the forum provides and so the folks trying to help will know that you're doing your part in trying to learn. 

[PRR]

> how you were able to come to such conclusions

You need a 1000 pound piano on the second floor. You tie a pulley on the piano, run a rope from 2nd floor to piano-pulley and back to you on the 2nd floor. How hard must you pull and how much rope will you pull?

Train A leaves NYC going west at 40MPH. At the same instant, train B is 100 miles west, heading east at 60MPH. Where will they collide?

Electric circuits can be solved with similar thinking. Often there is more than one train, more mental juggling.

[R O Tiree]

To continue PRR's analogy, knowing where trains A and B will collide is the art of knowing what should happen and it is essential to the art of debugging:

What if neither train moves out of the station?

How about train B moving really slowly while train A goes far too fast?

What if train C comes out of its siding too early? Maybe trains A and B will never collide, because trains B and C collided first...

So, knowing (even roughly) what should happen, together with careful observation of what's actually happening will enable you to zoom in on a small clutch of components and then zoom in further to identify the fault. Sometimes, the fault is right at the beginning of the signal chain, so nothing appears to work but that, in itself, tells us where the problem lies. Sometimes, an audio probe reveals that everything is hunky-dory up to a certain point, etc, etc.