Voltage question

[vendettav]

Hey guys i was wondering. when you're troubleshooting and asking for voltages across the tranny/ic pins.. how does it work. like how dya know if the voltage is wrong or something??? like dya look at the datasheet or something? 

[R.G.]

Hey guys i was wondering. when you're troubleshooting and asking for voltages across the tranny/ic pins.. how does it work. like how dya know if the voltage is wrong or something??? like dya look at the datasheet or something? 

It's - both complicated and simple at the same time.

First the simple stuff.
- there ought to be some non-zero voltage at almost every place except ground and pins that are not (by design and intent) connected to anything
- there ought to be the same voltage at every place connected together by a line/dots on the schematic; if you find different voltages at places that should be connected, then they are not really connected
- if you find exactly the same, identical voltage at two places, and they are not connected by a line on the schematic, then chances are they are accidentally shorted

The complicated things, and what you were actually thinking about is "how do you know what voltage should be there in the first place?"
These actually also divide into simpler (although more complicated than the first simple stuff) and more complex.

The simpler-but-somewhat-complex stuff is as follows:
- Bipolar transistors simply must have certain voltage requirements be true; the base-emitter junction must be in the correct direction, and must be about 0.45 to 0.7V or the transistor simply cannot be amplifying, although it may be being used as an on/off switch; also, the collector to base and collector to emitter must have enough voltage across it to be able to amplify. These things come out of understanding the device physics and current flow, but thankfully are simple enough to not have to memorize the device physics to use. If these sloppy guidelines are violated, chances are very high that there is a flaw in soldering/wiring/pinout/orientation, etc.
- JFETs must have their gate-source reverse biased by at least a little ( as little as 0.1V for some of them) to a lot (up to 15V for some of them) to be working right. They have huge variation, so this is harder to call than bipolars. But if the gate-source is forward biased, it cannot be working right as an amplifier. Like bipolars, the drain/source has to have some voltage across it to have something to amplify with. If these sloppy guidelines are violated, chances are very high that there is a flaw in soldering/wiring/pinout/orientation, etc. just like for bipolars.
- opamps can be used for amplfying or switching. If used for amplifying, the output MUST be between the most positive power supply pin and the most negative power supply pin; moreover, the voltage on the output MUST be the same as the + input within a few millivolts, or it is not amplfying anything. So must the - input pin. If these are not true, the opamp cannot be amplfying.
- logic circuits must have all inputs and outputs either at a valid high, a valid low, or actively switching, or they are not working as logic circuits.

The complex-complex stuff is what you asked - how do you know what the voltages really have to be? The short answer is, you go do enough thinking and learning electronics to work it out. It's like "how do you play a Mozart aria on guitar?" First you learn to read music and play the guitar. Then you look at the aria.

[vendettav]

thanx R.G. you're the best!
answered a lot of question and might be of some nice use really

now im really struggling with the pedal again.. i guess my adapter accidentally shorted and fried something.
still i got it all in the other thread http://www.diystompboxes.com/smfforum/index.php?topic=90026.20

if you could help, man, i'd be so thankful!