How can I draw a schematic by reverse engineering a circuit?

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I ended up with a custom made preamp or compressor of some sort, but it doesn't really look quite like any other preamp or compressor I've seen, doesn't quite act like one, and has some really strange inputs/outputs. Audio inputs/outputs are a 1/4 "mic" input (with in/out switch and gain knob) and stereo RCA inputs and outputs. And it has 2 power cords AND a power outlet on the back! I opened it up and it's just unbelievable inside: the main PCB is done with components placed in IC sockets on the board, and then the points are connected on the underside by dozens of tiny, tiny wires! And they're not soldered; each end is tightly wrapped around the legs of the sockets!

So I'm trying to figure out what this thing is, and use it as an opportunity to learn about components, symbols, how schematics work, etc. I'm just tracing the wires from component to component and drawing it using the appropriate symbols and values, etc., but my "diagram" is getting really messy.

Are there any good starting points, conventions, etc. that might help me make a more sensible schematic? For example, everything that's connected to the black wire - can that stuff just go to the ground in the diagram, or does it have to go back to the power?

I suppose that most schematics are laid out with appropriate groupings, i.e., a timing circuit together, a filter circuit together, etc. Since I have no idea what I'm looking at, I can't really do that.

When I get this schematic done I'm going to post it, along with pictures of this thing. Hopefully by then I'll figure out what it's supposed to do!
 
It's called "wire wrapping", and it was used in computers way back when. I used it for a digital logic box I built a few years ago.

Usually a schematic from a mystery circuit takes a few revisions to get right, and to figure out the flow and organization. You could definitely start by tracking down the power connections, since those won't matter too much to the audio path.
 
twitchmonitor

You are getting cought up in the details.
Start putting ink on paper do not make it neat.
It takes me about 3 redraws to get the circuit laid out on paper
so it makes sense. Every time you do the redraw of a section block recheck the schematic using the circuit. By the third time most mistakes
are found. Just do it. Do small sections and take breaks so you will not go crazy.
 
Yea,

Adrian speaks the truth.

Just break it down into smaller chunks- start with the PSU. This gets it down to how many rails you're dealing with. If there are multiple 0V points as well, this is a great time to find out before you start attacking the main circuit!

Then get a big piece of paper (I'm talking A3 here) and draw the input and output jacks on opposite ends of the page. Then trace the wiring from the connectors backwards in both directions. When you get stuck on one side, go back to the other. After numerous cups of tea, the two sides will meet.

Be prepared to get through a few sheets though!

:grin:

If you like crosswords, you'll have no problem at all.

Mark
 
Do a block diagram first. These are just squares that say pwr supply, voltage amp, input, output, etc. Then break it down.

You can use a hi-liter pen to verify the final schematic.
 
I think I have it in shape, but I?m still ironing out a few details. There are some very confusing things going on inside this box. And I?m doing a ton of reading on all the various components so I have some idea of what I?m looking at, since I?m completely new to all this.

For example, there?s a relay which switches the signal going to the rectifiers (which control the gain of the variable gain op amps), but the relay is turned on/off by plugging in a short 2-prong power cable from the back of the unit into an outlet in the back of the unit! WTF? There?s also a pair of stereo RCA inputs and a TS ¼? mic input (with gain knob), but it seems the mic input only feeds the gain control circuit ? it never passes audio to the outputs! Some kind of crazy sidechain? And I can?t tell if the variable gain IC is acting as a compressor or expander based on the way it?s wired (it?s a ?compander? and can be hooked up in either configuration). The left input goes to the right output, and vice versa. There?s a sample and hold IC in there, and frankly, I don?t know what on earth it?s doing there. And on and on. But based on the apparent complexity of the thing (seems complex to me, anyway), and the obvious care that went into its construction, I?m sure there?s good reason for it to have been built the way it was.

Anyway, I?m going to keep trying to get this schematic into good shape and I?m hoping some of you folks might be able to help explain what?s up.
 
write the numbers down of the semiconductors and look up the specs and pinouts and print them out.

Look for app manuals or a schematic of a circuit like it This is big part of reversing things often people copy bits from the app manuals or other circuits or one of the cookbook books. Often the parts around the semi or tube etc are the same in the app manual.

Not much is different and new!

Block diagrams are a good way to start.

when drawing pots mark cw or ccw for the wiper movement on your drawing this will sometime help alot.

Google

Use a light table if you have one and a meter to trace pcb traces

make a list of the parts and then link the parts to your madeup number R1 C1 etc. This helps make the part count "addup" when you think you have the complete schematic.

Years of practice helps the most.

I this case some pictures will maybe help.
 
D'oh! Yeah, a vocoder would make a ton of sense. Don't know why I never though of that. I'll keep that in mind next time I'm working on this thing.

What's an "app manual?" Sounds like a good point of reference, if only I know what they were or where to find them...

I downloaded the spec sheets for all the ICs and I've used the pinout to draw the bulk of the schematic (there are 5 discreet units in all). Unfortunately, because I'm a novice I can't deduce how my thing relates to the various diagrams on the spec sheet. If it's not exactly the same, I just can't make sense of it, you know? But MAN is this a great way to get my feet wet!
 
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