mita wrote on 2024-03-05, 17:00:
One more note: do not jump into a deep hole overanalyzing the post codes. Post codes can point to a given direction suspecting one element, like a keyboar controller. If you do not have full documentation about the chipset - most of the case it is true - than do not spend too much time measuring signals because it is unlikely to find the culprit. I think most of the case there is a broken wire somewhere. If it is not blocked by a component than you can get result quicker to check the traces via microscope.
Yes, POST codes can be quite precise, when you have the reference for the exact BIOS version for the exact board from the manufacturer, but that only happens about one time in twenty or less. Frequently you can't even find the code table for the specific revision of the BIOS, and many manufacturers modify them a lot, so you are comparing between a couple of adjacent code tables. At that point it's more a Magic 8 ball kind of indicator "signs point to maybe..." and you have to take a broader "something probably to do with X" kind of view. Of more use sometimes is knowing which parts it has passed, so you can eliminate things to look at. A lot of times it's "wiggle stuff, make number go higher" since bad connections on old hardware are very common.
Optical inspection is often neglected, really detailed inspection can find stuff a lot quicker than probing it, as you say. A particular trick is to make use of light reflections at different angles, you see a line of pins on the edge of a QFP chip, shine the light off them, they all shine bright and one is dull, because it's slightly tweaked, it has lifted off the board, or is twisted and fractured or something. Then also reading the shine off packages can be useful, the surface can sometimes tell you whether it has burned out, by a duller brown reflection when light hits at an angle, where as straight on, it looks fine, just black. Sometimes this is in color or texture, but oblique light shows it up. Bad solder joints often show dull also.
Temperature is also a useful tool, in the 90s I only had finger prodding, then IR thermometers got cheap, but have a large cone of detection so gotta point it near and accurately, some guys have got thermal cameras to help them out, but I haven't sprung for one yet. Anyway, if things are getting too hot too quick, they either have a problem, like wedged on DC, no clocked signal, or their support capacitors etc for oscillation have failed or gone out of spec. So you backtrack from there or suspect the device if it's all good upstream. Then also you can use freezer spray or inverted can of air for "would it work if it was colder" to see if semiconductors are still working at all, or heat gun for "would it work if it was hotter" mainly useful for marginal caps. Also "frosting" the board can let you see if some devices are completely inactive, making no heat, or some are making heat way too quickly, by where it melts off real quick or doesn't melt off at all.... you don't want to do this on high humidity days though or you will have too much water condensation.
Anyway, those are relatively fast ways to hunt out problems before you hook up test leads and start obsessing over exact voltages and ohms readings. That is, as if you say, any references are available. Sometimes service manuals are a mixed blessing... the procedures are designed to find faults in normal failure modes at EOL of the most likely to fail components. If however something else happened, End Of Life components already replaced, and you're getting into decade plus long term component wear that was never anticipated, or something as simple as mechanical (or chemical/corrosion) damage, that was a little too subtle to spot on an inspection, then those procedures will have you going round and round in circles... yes, they are from the manufacturer, but it's not the entire scope of everything that could ever go wrong.... just the low hanging fruit. So maybe go through them carefully once, then forget it, they don't apply, you are off script, do the things you do when you haven't got a manual.
Unicorn herding operations are proceeding, but all the totes of hens teeth and barrels of rocking horse poop give them plenty of hiding spots.
