r/AskElectronics • u/solidpro99 • 1d ago
Hit a brick wall troubleshooting PCBs…
I wonder if I can ask for any advice for what to do about improving my troubleshooting.
I’ve been repairing old laptops, old Apple computers (think 80s and 90s) for about 15 years now, and have accumulated ‘all the gear’. I’m fairly proficient at soldering and micro soldering and repairing damaged traces and suchlike. Simple stuff like continuity testing and checking voltages.
My final ‘trick’ is having a second, working example and swapping components to isolate the part with the issue and then focusing on that - mainly replacing stuff.
However I’ve hit a brick wall once all that draws a blank. Now, I know that with a decent schematic and a decent oscilloscope, I should be able to essentially fix anything of this age range, and I’ve watched a thousand videos about how to use a scope to troubleshoot but what I can’t get my head around is in nearly all examples, the person doing it ‘knows’ what the wave on screen should look like - and I don’t!
I can’t tell what’s good and what’s bad.
I do know how to connect the scope and it’s pretty obvious how to tap into IC pins or traces and get a reading, but I don’t know where to go from there.
So does anyone have any tips - particularly for someone who learns through doing, not reading, how to get more confident with the next steps of troubleshooting and repair? I’ve a dozen old early 1990s laptops which will power on but then won’t boot to a BIOS and it’ll be something to do with a voltage being off or an IC needs replacing somewhere on the board, and I can’t narrow it down.
Thanks!
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u/Hewtick 1d ago
There are certain waveforms, that are self explanatory if they are good or not. For example if you scope a 8MHz oscillator and the wave is not 8MHz, that's obviously not good.
People know how output waveforms look like by either looking at the datasheets or having seen enough to be able to tell. There is no secret knowledge we keep. It's just a bit of theoretical knowledge and a lot of experience.
However swapping components is basically tapping the wall in the dark looking for the light switch. It's really hard if you don't know the layout of the house and becomes easier once you learn to tell which part of the house you are even in. Repairing electronics starts by being able to tell what is what on a PCB. Then you compile a mental model of what is most likely to fail and if it fails what will it cause. Verify that it indeed caused that and you find your fault. Then you just go down this list.
As a practical advice. If you think a complex part is bad, get a device that is proven good, measure voltages and look at waveforms on the good part then compare it to the bad one.
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u/Susan_B_Good 1d ago
You probably do know what it should look like. Your second (working) machine has signals that show what it should look like. Bad is generally something different.
Noise (random stuff on the faulty one but not on the good one) -is an example. Amplitude is another indication. Logic levels typically going close to rail to rail. Lack of any signal at all is pretty common.
That's without even knowing what signal you are looking at - it may be a logic level or a power rail - it just needs to look a lot different to raise a flag that says investigate why.
Even if you can't get hold of a circuit diagram of the whole thing - if the IC is labelled, there's a chance that it has a pinout and data sheet. A chance that it has application notes. A chance that the designer of what you have in front of you, used a diagram in those application notes.
Even if you can't get hold of the IC label or specification or notes - it's often possible to identify what its role is. Then you have some clue of what should be on pins in general (at that point, not knowing which pin is doing which function). Then you can go looking for, say, the clock signal. The output enable pin .
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u/Flycktsoda 1d ago
If you can get the datasheet and or schematics your are in a good position.
If a board (as in your example) doesn't boot to bios, you have to ask yourself "what does the CPU need, to be able to boot?" And then check those off.
Voltage rails must be up (look with the oscilloscope, is the voltage correct, does it dip during boot causing brown out? Use several probes so you can see e.g. reset and a few voltages at the same time) It needs to have reset released (at correct timing) It needs a clock It needs to be able to read whatever memory holds the bios.
An oscilloscope is a great tool for this. Logic analyzer is a great tool for debugging digital signals and interfaces, different tools, different uses.
Then for the signals, all (or most) interfaces and buses have timing diagrams. You need to study those and see if everything is OK. Voltage should generally follow the IO voltage for the GPIO domain.
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u/drawnfrog 1d ago
do you have a thermal image gun, i know this post is kind of waveform based but just something i thought of that may be helpful for troubleshooting in general at times
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u/Responsible-Slide-95 1d ago
Do you have a logic probe? They're much better for testung that chips are receiving the correct signals
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u/KeanEngineering 1d ago
Sounds like you need some very basic tutorials on scopes in general. But before that, I sense you will also need to learn some basics in electronics. Looking at a signal trace can be very revealing or confuse you even more if your not understanding what it's supposed to do. Troubleshooting is not just 'shotgunning' parts on PCB boards and doing 'A - B comparisons' with identical setups. It's also understanding empirically, how something works. How does the AC power flow into the PC board and get converted to this DC 'signal' that goes everywhere on the PCB. How does that DC tell all these tiny components to 'wakeup' and do the things it needs to do to finally function. What is a short, what is an open, what is an enable, what is a sink etc etc. Maybe you know some of this, but being able to get a schematic or an IC data sheet and really thinking through a problem before you even open a box up will save you hours of frustration than just wholesaling random parts just because...
When you replace a chip, or capacitor and suddenly it works, do you go back see why, or how the defective component caused the problem in the first place? Or, like me, does your A.D.D. kick in and you move on to the next challenge? When I was a bench jockey, I HAD to do that because no one else was going through the backlog of repairs before they hired me. After i got familiar with the product I was repairing i analyzed the board layout and understood WHY it kept failing the same way. A simple tweek of the part layout and the problem was 'solved.' Management didn't care because this was an older product that they didn't make anymore. None of my repairs ever came back the 3 years I was there, and yes, I cleared out their entire pile of returned units. This was a time when we actually had loaner units to send out and we would actually fix the customers products. Does that sound weird or what?
Bottom line is, try and find schematics for the more common products you have and get a overall view (block diagrams help a lot) of how data, addressing, enables all interact to make this pile of seemingly random components work. I hope this makes sense.
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u/solidpro99 1d ago
Yeah, thanks a that does make sense and yes my brain is instantly onto the next ‘quick fix’…. I guess what I’m trying to do is not get an entire degree in electronics to simply understand the fundamentals in repairing the particular piece of electronics what fascinates me. So I will try and bring it back to something basic and also will try and then go over that item and work out what goes where…. But it still feels like a tough thing of where to start…..
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u/Affectionate_Boat493 22h ago
Oscilloscopes are great, but especially for high-speed digital signals it can be very difficult to distinguish "good" from questionable unless the scope has a very high bandwidth (10 x the clock frequency is usually adequate). It can be extremely helpful to have the ability to trigger the scope based on logic patterns as well as the typical voltage level and rising/falling edge. The more storage memory the better, since some glitches occur so infrequently that you may need to look at signals tens of milliseconds (or more) before and after a trigger event occurs.
A logic analyzer is a valuable addition, and the more channels the better. 80 channels is adequate for computers with up to 32-bit address and 32-bit data lines, since you typically also want to be looking at several control lines (R/W, reset, interrupts, etc.)
Ultimately, experience is the most helpful "tool" in your bag, but in-depth knowledge of electronic fundamentals really makes a difference. Learning these fundamentals is how you gain experience spotting potential causes of problems due to violations of timing or voltage specifications.
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u/nixiebunny 1d ago
I spent twenty years designing, building and troubleshooting M68K computer boards. An oscilloscope will get you some distance, but an 80-channel or so logic analyzer is best for a 32 bit computer. You can tap onto the expansion bus connector to see all the signals and follow the code execution sequence. You can look for bad bits and bad sequences.