I've thought about posting this question myself in the past, but now that you did, here are my tips: step 1: if not clear already, assign reference designators to each component step 2: make a netlist of the board step 3: create schematics from the netlist I've read in the past that Tony called someone a whimp cause he said he didn't dare to reverse engineer a 4 layer board or something like that. I wonder, if _that_ is easy, how does one solve problems like traces that run under components? If its a simple and cheap component, it could be removed and replaced later. Bert

I wouldn't do it that way. I ahve an aversion to copying things out -- firstly it takes up time that could be better spent hacking, and secondly it's a good place to introduce errors. OK, some tips... Ask if anyone has the schematic :-). Seriously, you don't want to spend time drawing out a diagram to find somebody has already done it. Of course you need to compare the time taken in drawing out the diagram with the time taken in getting a copy of one already drawn (in my case, it would often be quicker to do the former!). Make a list of at least the main components on the board. It's probably not necessary to list every resistor (at least not when you've got used to drawing out schemaitcs), but list every chip (or section of a chip), the main transistors, etc. Use manufacturers designations (U1, Q7, etc) if they're silk-screended on the board. If not, then either label things (You can get little numbered stickly labels about 8mm in diameter from stationery chops) on the board/components, or make a sketch (photo?) of the board and label that. You'll get a list like : U1 a b c d 74LS00 U2 Z80A-CPU U3 2764 U4 74LS138 U5 a b c d e f 74LS04 and so on. When you've drawn a component, cross it off the list. That way you don't keep on drwaing out the same thing... Identify (at least) the ground and prefereably power rails. Several times I've found unexplained connections between what should be unrelated circuits which turn out to be extra (low noise, etc) power rails!. In the case of SMPUs, remeber there will be 2 grounds -- the 0V on the output side and the 'hot ground' -- the -ve side of the mains smoothing capacitor (normally). Use different symbols and don't confuse them. Think what should be on the board, and try to identify the main stages before you start drawing things out. This might involve making rough sketches of part of the circuit. Identify external connections as far as possible You _must_ have data on all the components. Well, 'must' is a strong word, and I've drawn out boards containing custom chips that I had no data on. But working without data -- at least pinouts and prefereably application circuits -- is hard. The application circuit is handy, if, e.g. you have a chopper control IC that has an external RC network to set the frequency. It's a good bet that network is in your supply too -- find it. 'Open circuit' all components that would look like a dead short on your meter. This probably means removing all transformers and inductors (the ones in SMPSUs have resistances well less than 1 ohm!), sense resistors, fuses, swithces, relays, and so on. You will have to desolder the transformers and remove them totally, things like resistors are best lifted at one end only so you don't forget where they go!. Remove any daughterboarrds and handle them separately. You should be able to figure out the signals to the pins, for example. This, again, may involve desoldering. Transformers -- particularly multi-tapped ones -- are hard. You really want to indentify the sequence of pins on each winding. Sometimes carefully inspecting the component will help. Sometimes you have to deduce it based on your understanding of the circuit when you've drawn it! Complex components (e.g. microprocessors) are a lot easier than simple ones (e.g. resistors). The former are only used in one way. The latter could be almost anything. Assinging signal names is hard. You will want to name signals that go off-sheet, and maybe signals on the same sheet if they're key signals in the design. But to give a signal a name you really have to know what it's for. That means you have to understand the whole thing, And you should put titles on the main blocks of the schematic too, but again this means you understand what's going on. OK, ready to start. I would start at the mains input. Trace where it goes (probably through a filter, then a bridge, then to smoothing capacitors). You do the tracing with a _good_ ohmmeter, combined with visual inspection (where does the track seem to be going!) and knowledge (the emitter of the chopper transsitor probably goes to the 'hot ground' via a sense resistor). Find the chopper control IC, find how it's powered, how it drives the chopper, and so on. Then find the feedback loop (votlage regulation loop). You will _never_ successfully trace a schematic you don't understnad. You will find some suprises. I've had circuits that seem to make no sense at all, but do so after a lot of thought. If you get something like this, check and check again that you've not missed something... Personally I'd not pick an SMPSU as my first serious schematic to trace, but you can't always choose what you have to do :-) This is presumably on the mains side of the PSU. Can you find any dead shorts with an ohmmeter? Is there a dead short across the output of the bridge, for example. -tony