What the books don't tell you?

[guitjr]

There are lots of books (and web info) that address basic troubleshooting and test meter use.

But there seems to be a lot of practical debugging information these sources don't cover.

The best analogy to my electronic debugging questions I can come up with is this:
I CAD-drew (from the ground up), and then constructed the two level house in which I live. It was the first time I'd ever taken on a task of this sort. I did everything (frame, exterior, sheetrock, interior, electrical, plumbing (not drains or gas)) except the pier foundation. I knew that everything else would depend on it and so I wanted it to be perfectly done—the pro who did that had to redo it when I (CORRECTLY) insisted that a 1/2" discrepancy on the diagonals of the rectangle on which this house would stand was not up to standards. Now wood is a forgiving medium. The biggest problems I constantly confronted and continue to confront had purely to do with experience: How much shorter (or how tight) need I cut a piece of wood?; How much extra wire should I leave in each box and how much extra wire should I be feeding the framework between boxes (the wire is never pulled straight); How hard should I tighten a plumbing connection? How large should I drill the holes in the frame that hold electrical wires or that hold flex water pipe (how large a hole would compromise the piece of wood)?; How many times do I need to feather the joint compound to ensure a smooth interior wall?

Is there a book or webpage(s), that address:
NOT what a resistor is and how to measure resistance across a single in-the-hand resistor...
but questions such as the following:

Using a DMM (a meter that I don't have to worry about pinning the meter!)

And relative to each of the following measurements: voltage,    resistance,    current,    capacitance,    frequency,    continuity and diode test, and    transistor test
- Do I ever have to worry about switching leads (if black isn't on ground will the reading always read negative)?
- What measurements can I do  in circuit with power ON?
- What measurements must I do  in circuit with power OFF?

With the following in mind:
- What measurement mistakes will result in damage to the circuit being tested? (yes, I understand that shorting wires—i.e., applying a single probe across two wires can be disastrous)
- What measurement mistakes will result in damage to the DMM?

Are there tricks that one can use in conducting measurements and in tracing circuits?

Component values:
What kind of (schematic-to-actual) fudge factors can one expect in component values or components one may need to substitute?
- Must every resistor be exactly the ohmage specified in the schematic?
- Must every cap be exactly the value and voltage?
- Must every <insert component> be exactly the value...?

[karbomusic]

Some of it is called experience and there is no replacement for it. That's why experience doesn't come in book form.. Many of the above are just questions one can ask as a beginner but at the end of the day, doing and learning from mistakes is where every ounce of value is. I'm also sure a good bit of the above can be found on the net in the form of beginner tutorials. Just my two cents.

[merlinb]

Virtually everything comes down to "understand how the circuit works". If you understand how something works, then you know what to look for and what to expect. When you measure something that doesn't make sense, you can then also hypothesise what could have caused it to produce that reading. Similarly, most of the other questions boil down to "understand how a DMM works".  So don't waste too much time looking for neat answers to 'what can you measure with a circuit on'? Instead spend the time learning how electronic things work- that way ALL your questions will eventually answer themselves.

Must every <insert component> be exactly the value...?

This one bothered me when I was a beginner, too. Eventually the answers came by learning how circuits work, and building up a collection of circuit building blocks, and learning to spot them within larger circuits. Most of the time it is not the specific component value that matters, but the ratio of some components that counts. For example, you might be able to double a resistor provided you halve a corresponding capacitor, so the cut-off frequency remains the same. But you still have to know how a circuit works and what it is doing to be sure. There are few cookbook answers to your questions.

Until then, the most useful learning tool I found was to watch somebody else design a circuit and explain the choices they make along the way. Or to watch someone experienced troubleshoot a circuit, and explain what they're doing and why.

[R.G.]

Using a DMM (a meter that I don't have to worry about pinning the meter!)

Read the books, but keep in mind that cheap meters are best considered an indicator, not a precision measuring device. If you get odd results, *also suspect the meter may be on the wrong scale, probes connected wrong, battery low, or it may have just been used to measure 120Vac voltage on the 20ma current scale and now dead*. Don't always trust the meter. Check the meter with other things to see if its readings make sense if you find something odd.

- Do I ever have to worry about switching leads (if black isn't on ground will the reading always read negative)?

A meter is a floating measurement device. If used to measure voltage, it does not know or care where you put the red and black leads. All it knows is what the voltage is between them. If black isn't on ground, it measures all voltages relative to where the common lead is connected, whereever that is. It may cause negative readings, but may also just cause funny readings. Know where your probes are connected.

There are times when you want to know just the voltage difference between two points neither of which are ground. The meter happily does that for you. But it's up to YOU to know where the probes are connected and what that means.

If you're measuring voltage or current and you have a DMM, swapping the leads on two points should only change polarity. However, this is not necessarily true in measuring diodes and caps with the resistance scales of the meter. In measuring resistance, the meter supplies a voltage/current through the leads and the voltmeter inside measures what that should tell you. Works great for resistors. But if you're trying to measure the polarity or forward voltage of a diode, then the voltage OUT of the meter for measuring resistance matters. It seems like all meters should make the red lead positive - but I have found some meters that don't. Check out your meter and find out what it does on diodes, etc,. that you already know the answers for.

- What measurements can I do  in circuit with power ON?

Voltage and, if you have set the meter up properly beforehand to let the current flow through the meter, current.

- What measurements must I do  in circuit with power OFF?

Resistance or continuity. You can't get good readings of component values in circuit because of the other stuff connected in parallel. All you get is indications, not accurate readings. That may be enough.

- What measurement mistakes will result in damage to the circuit being tested?
- What measurement mistakes will result in damage to the DMM?

As you noted, shorted connections, but also putting the meter on the wrong scale, as in measuring voltage on a current or sometimes a resistance scale. Mostly, never measure current, capacitance, or resistance in a live circuit, as the result may be unpredictable even if it's not damaging. And watch your voltage scale to be sure you're correctly on DC or AC, and set to a maximum scale voltage higher than the maximum you'll read.

Are there tricks that one can use in conducting measurements and in tracing circuits?

Too many to type.

What kind of (schematic-to-actual) fudge factors can one expect in component values or components one may need to substitute?
- Must every resistor be exactly the ohmage specified in the schematic?
- Must every cap be exactly the value and voltage?
- Must every <insert component> be exactly the value...?

No component is *ever* exactly the value in the schematic. That is the reason that there are tolerance values associated with components.

Skilled designers have been beaten about the head and shoulders with the fact that components have tolerance variation until they quiver in fear thinking about what tolerances adding up will do to their circuits. Fortunately, being forewarned, they are forearmed; they have tools and analysis skills to predict what component tolerance will do. Very skilled designers do designs in which any component within the tolerance band specified will make the circuit work just fine.

Novice designers are more or less aware that tolerances exist, and have no tools to cope with them or predict their effects.

In general well specified circuits will have something like "all resistors 5% (or 10%, or 1%) unless otherwise noted" listed somewhere on them.

[GibsonGM]

^^^This  (R.G. but esp. Merlin)

The reason I have, at times, come down kind of hard on the Vero folks whose circuit won't work is this.  If you're not learning how things work, it will be extremely hard to figure out why the build went wrong. And leave you with your hands up helpless the next time it happens, too.  And makes those helping you have to work much harder.  The 1st (and best, IMO) part of learning is to learn to read schematics!!  

Again, just reading all you can about various circuitry eventually builds up your memory of how things should work....I used to read old HAM radio books from the ARRL, esp. the yearly handbook, and drilled the fundamentals into my head.  Sooner or later, you start to be able to tell how/why something is an oscillator vs. an amplifier, and what things should clue you in that something has gone wrong!     Old US Navy training manuals have been neat to study, too (geek at heart, I guess).  

Drill yourself on Ohm's Law, as that IS in fact the MOST important tool you can have! You will use it CONSTANTLY to figure out circuit parameters, and POWER CALCULATIONS, which will tell you if your parts are going to go up in smoke unless you upgrade them....you've been warned, ha ha.  

Never measure resistance with the circuit powered up (disconnect the battery out/unplug wall wart).   Learn how your meter works, and what the current max is (and voltage max, too, on the leads!  For later, when you work with tubes).      I've burned more than a few fuses by going over the 500mA that my "COM" port can take - need to move the probe over to the 10A port for big readings - something to watch out for.

For component values...remember, we are mostly using 10% to 20% parts, so the way I look at is if I am within 10% or so of the nominal value, I'll generally use it (meaning next size up/down).  The actual measured value of a resistor, say - no worries.  Brown/black/red, call it 1K even if it's 991 ohms       If a circuit requires a tighter tolerance, usually it will say.   So, 1.2K would be ok instead of 1K.
The purists may argue, tho, that as you do this you're moving away from what the original item used, and you DO change things a bit this way.   Monte Carlo on purpose...  

[karbomusic]

The reason I have, at times, come down kind of hard on the Vero folks whose circuit won't work is this.  If you're not learning how things work, it will be extremely hard to figure out why the build went wrong. And leave you with your hands up helpless the next time it happens, too.  And makes those helping you have to work much harder.  The 1st (and best, IMO) part of learning is to learn to read schematics!!  

Here here!   I'm assuming that you mean those who find a vero layout and paint by numbers. I use vero but everything I do is a bread board, a schematic, lots of testing, scoping, measuring and drawing THEN I do my on darn vero layout from scratch every single time gosh darnit. IOW, I use vero to solder less jumpers but I typically have the circuit and what it does memorized frontwards and backwards by the time the soldering iron gets warm.

To the OP: Hope I didn't sound harsh, the idea was that no matter what you read, doing it and the mistakes that go with it is truly where success and answers come from, or at least how they get painfully burned in.

[GibsonGM]

LOL, people get mad at me because I can't tell a kid how to fix his thing as he cannot read the schematic!    As a way to make things easier/faster/cheaper, hey - I'm all for it....but if you don't want to learn, it's tough to help you out.   If you're not here to learn, maybe just buy the box from the store??   

ANY way that helps get you from A to B is cool!  Esp. if you are learning so that one day you can pass on that skill, which is becoming more rare as time goes by.

[Digital Larry]

You probably want to stay away from trying to make current measurements with your DMM unless you really know what you are doing.

#1 The impedance of the multimeter in current-measuring mode is nearly zero and can both short out your circuit and fry the DMM.
#2 You have to "open" the circuit to put the DMM inline.

When I first got into electronics, approximately 1967 (really) I may have had access to what was called a VTVM (vacuum tube voltage meter).  Later I got a Radio Shack analog multimeter which I may still have around here, broken handle and all.  In some ways I think these give a better "feel" for what is going on, and also you are not given an exact result which is just as well, as electronic engineering is not an exact art.

[merlinb]

I've always found it very odd that some people claim not to be able to "read a schematic", or ask "how do I learn to read a schematic?".
(I'm not talking about understanding how a circuit works, I just mean how to find a part on a layout and figuring out which symbol it corresponds to on the schem, or how to create a layout from a schem).

What is there to learn? Why isn't it obvious to everyone? All you need to know is that a physical component with two leads, or three leads, or eight legs, is represented by a symbol with two connections, or three connections, or eight connections. So what's left apart from memorising the handful of symbols and knowing how to find the pin connections on a data sheet?

It hurts my brain when other people claim not to be able to read a schematic. I mean, it's not like learning to read a book. It's just a few symbols that look more-or-less like the real thing does! So what's the difficulty?  
Heck, we did basic circuit diagrams with bulbs and batteries when I was in school, aged 10!  When I eventually got interested in electronics (aged 18) I remember having to look up the connections for an opamp or the resistor colour bands, every time, but it wasn't like the idea confused me.

[Fender3D]

lol
True, Merlin...

I think you misunderstood the purpose of Internet... 
I mean, the same concept inspiring someone downloads music or film or porn for free, in these dedicated forums sometimes inspires someone to get pedals or repairs for free...

For free intended as no money, or no sweating...

[karbomusic]

Why isn't it obvious to everyone?

Because all schematics aren't created equal and there is at least some small learning curve is my guess; maybe zero previous exposure. I agree though, since most debugging posts are from blindly building someone's layout having no idea. There are at least some ground rules on what the symbols mean. Luckily, I can read them and create them fairly well and can pretty much design from end-to-end (schematic, BB, Layout) etc. But it won't be obvious to a beginner because a beginner won't even know what the symbols mean. They do need to invest the time to learn those ground rules though because anything worthwhile requires at least a little effort. Because of that, I forced myself from day one to do it myself and not use someone else's layout which has helped me immensely. When I do a layout and build it now, I already know it is going to work most likely on the very first try and it usually does.

I don't know what I'd do if half of what I built had to be debugged post build, that would be extremely frustrating.

[Digital Larry]

When I got started, I built many many kits using pre-fab PCBs, making my own from patterns in Popular Electronics or even just the point to point paint by numbers approach of "make it look like this".  It was ten years before I really understood much of anything about how/why.  I don't mean that to discourage anybody!  I was just having fun making things and having them work for the most part.  I had a cousin who was an electronics tech and while he fixed a few things for me, he didn't try to teach me how things worked.

[Jdansti]

I recall as a kid being in bed with a flashlight under the covers with one of Dad's electronics books. I memorized component symbols and copied them with paper and pencil. I'm still learning how the components work.

Several people mentioned analog meters, and I still have my Radio Shack analog meter that my Dad gave me when I left home. You don't want to reverse the polarity of the leads when measuring DC voltage on an analog meter. The digital meters will show you the polarity on the display, but the needle on an analog meter will try to go backwards, and there's nowhere for it to go.

[disabled_shredder]

Merlin I understand where you are coming from with not seeing how people can't read schems. It's not the symbols always per se. I see them as a puzzle and that's what confuses me. I can read a schem and have no problem an I can look at a layout and be ok but when I go back and forth it's like thinking in 2 different languages. I always refer to the mxr dist+ there are about 3 or 4 1.5 or 1m resistors all going different places if you go from layout to schem for me at least makes things jumble up. I'm not sure everyone is like me but most of the time it's walking through the layout with the schem one or the other is easy but together. I still have trouble. But I can bias a tube amp and don't know ohms law. The math makes no since! And for what it's worth now I have designed a few layouts and have restored plenty of amps. It's important to read schems but not the end of the world. Although it will eventually leave you stuck bc you have to learn it to move past only what you know back and forth.

[PRR]

> Must every cap be exactly the value and voltage?

Cap value should be "close"; but in audio "close" can usually be 20% off, even 50%, and more.

Cap voltage is a Blow-Up value. You only need "enough".

My backhoe uses 3,000psi hoses, because pressure can exceed 2,250psi. If I used 30psi fuel hose, it would burst when I started the engine. If I used 1,000psi power-steering hose, it might hold for a while but burst when I grabbed some dirt.

OTOH... My truck has 13psi in its fuel hose. I "could" use the 3,000psi hose from the backhoe, but it is frightfully expensive. Better to use the 30psi stuff sold for fuel line.

"Mostly" no cap in the project will ever feel more than battery voltage. So a 9V pedal needs 9V caps. Usually the next-up rating is 16V.

Vacuum-tube amps may run on 400V, but cathode caps will "never" exceed 3V or 30V, whatever. 16V or 50V caps are called for.

Voltage boosters, such as a 9V to 48V converter for phantom mikes, obviously have some caps at the higher voltage. "Understand the circuit."

So you find a treble-boost cap that can never have even 3V across it. Value is 100pFd, and preferred type is Silver Mica. You look in the old catalogs and see the SM caps *start* at 500 Volt rating. Can't buy nothing weaker. It isn't (or wasn't) worth splitting a half-cent worth of mica into thinner sheets. It is usually no-problem to use a high-volt cap in a low-volt job, just a matter of cost. (And size: modern mini-toys need mini-caps so now you can buy dinky mica caps at 50V... probably mostly used in 3V systems.)


Similar for resistor ratings. Most 9V stompbox resistors won't dissipate 1/16th Watt. Historically 1/2W parts were cheaper than anything smaller. Later 1/4W became the cheapest size. Today 1/8W resistors are as cheap as any. However my fingers and eyes hate 1/8W parts and dislike 1/2W parts. My stock is all 1/2W except for special situations. The Designer might actually compute the hottest resistor, but then note "all resistors 1/8W" (even the ones that are 1/100th Watt actual dissipation). Or he may just say what he used, which for me would be "all 1/2W" (because my eyes are weak and I'm too lazy to check if smaller ratings would work).
_____________________________________

Many of your questions suggest you should TRY IT.

Leave your meter on DC Volts; 9V stomps won't kill a Volt meter. (Beware extreme tube circuits; modern meters don't like >300V or >500V, and some die.)

Knock-up a don't-care circuit to play with. Like an LPB. Not many parts, only a few cents, enough nodes to poke to keep you busy. If you want to do it right, hammer copper tacks into a board far enough apart that the part leads just barely reach. Now you can get into the nodes without shorting adjacent nodes, and make notes on the board.

Change a resistor to a value 30% off and see what happens to voltage and sound. (The LPB is in fact fairly critical about the Base resistors, and might best be tuned for the actual transistor in hand.)

[PRR]

> some people claim not to be able to "read a schematic", What is there to learn? Why isn't it obvious to everyone?

It is as easy as reading a map. Appleton connects to Lodi with this line marked Hoover St. Input connects to C1 along this line. Lodi connects with 4 other places; C1 connects to 3 parts R1 R2 Q1.

Ask the younger generation to read a map. Many of them can't. Like making-change, it is a dying skill. GPS, Garmin, and Siri have turned any voyage into a "turn here, go straight, turn right, stop" experience.

[GibsonGM]

I agree, it's a lot like reading a map, but if you go saying that Calais connects with Millinocket, or Skowhegan connects to the Forks, you're going to upset a lot of people - cuz ya can't get theyah from heyah, Paul - you KNOW that...    

Re:Schematics......A lot of this IS just 'newness', and you have to have a desire to learn.   Add to that the disappearance of "DIY" in many aspects, and you're left groping in the dark if you DO want to DIY.   It's a shame...in the old days of cars, people were expected to fix most stuff themselves.     Now, even putting up a piece of sheetrock is a "professional job" for a huge proportion of people.  It seems to me that there is a huge societal pressure to NOT learn new things, to color inside the lines, to just do what you're told.   BAH, heck with that.  Do like Paul suggests, do some REAL breadboarding !

What will the non-diy do if they can't afford people like me and the other guys who can 'do most anything'?  Not about pedals, but their homes and cars?   Repairs get expensive FAST.

[duck_arse]

I was lucky, as a kid, my brother's Electronics Today International magazines were the first circuit diagrams I ever saw, complete with "capacitor C1 connectects to PNP transistor Q1 base ..." in the how it works'es. and then I had Electronics Australia Magazine to compare, they just printed what they were handed, so I learnt how NOT to draw diagrams as well.

a couple of years of reading before I got my hands on any real parts. (I can read it, still don't understand most of it.)

[karbomusic]

I was lucky, as a kid, my brother's Electronics Today International magazines were the first circuit diagrams I ever saw, complete with "capacitor C1 connectects to PNP transistor Q1 base ..." in the how it works'es. and then I had Electronics Australia Magazine to compare, they just printed what they were handed, so I learnt how NOT to draw diagrams as well.

a couple of years of reading before I got my hands on any real parts. (I can read it, still don't understand most of it.)

When I was 7 dad found me standing in the room pressing the tip of a flashlight light bulb to the top of a battery with a confused look on my face. He said here... Took a paper clip and fashioned a connector that wrapped around the sleeve of the light bulb allowing me to spring load it onto the battery with the tip on + and the paper clip attached to -. More than one light bulb lit that day as I had just learned about electrons flowing. Pivotal moment in my life looking back.

Around 10 years old (early 70s) dad got me one of those Radio Shack 100 projects in one kits with the spring loaded connectors and components built into a cardboard type enclosure. I loved that thing, its where I first learned about oscillators and all kinds of cool stuff. No ICs if I remember.

[guitjr]

THANK YOU ALL!  I'M FINDING THIS THREAD (Istarted) fascinating, and chock full of useful information (including what-to-do-next suggestions).

I got involved in electronics in the early 60s (radio amateur—General license); became well acquainted with the schematics in the ARRL handbook (cookbook?); took apart TVs for parts; built a number of "rigs" for myself (cw); also a number of Heathkit kits (including a VTVM). Eventually I got a (B&K) scope...

in all this time, of building and using the items I'd built,  I never had the basic questions answered that I started this thread asking.

I remember there was a lot homebrew about the way I went around adjusting and testing equipment (quickly turning on and off the power supply when one of its tubes went too red; using light bulbs as antenna tuners; getting a few shocks along the way; etc.  Of course none of this could top the time I used a licensed electrician to inspect and sign for my electrical (not electronic) jobs who went to the waterlogged basement of a New York City building; throw a few planks across the pools of water; then gingerly lick his fingers as he deftly checked the fusebox...and lived to sign the job.