Basic beginners troubleshooting question on chasing a short on a PCB - Page 1
Forum section: electronics / beginners

author: pizzigri
author_rank: Regular Contributor
timestamp: September 16, 2025, 01:28:31 pm
content: So, this is what is frustrating me for the last week.I got a vintage computer logic board (an Apple PowerMac 6500). It had cap jouice on some places and would not power on. So I cleaned the board with a mix of demineralised water and IPA, let it cook at 40° Celsius for two hours, give it a full rinse in 100% IPA and then recapped using tantalum caps from Mouser. Done many many boards like this in the past.Moment of truth, I connect it and give power. Nothing, the softpower does not turn on the ATX PSU. Weird, OK, lets swap boards and the system boots just fine.OK, I check the rails and - there! On the 3.3V rail, there's a short. Well, the whole rail measures 0.06 Ohm between GND and +3.3V.The other rails measure between 2MOhm (+12V) and 440 Ohm (5V). all measurement done on a static and unconnected board. So, I have this dead short, how to chase it? I have a good lab PSU, an Agilent E3631A. Dial in 2V, and 1A on the primary output. use good leads that measure less than 0.03 OHm and crocodile to feed this to the shorted power rail, under the assumption it will heat up where the short is.So, what I get is the full amp (the PSU is set at CC) and well the board absorbs only 0,04V initially and slowly drops the voltage down to 0,009V. The board does NOT get warm, I am observing it with a thermal imaging camera of mediocre to decent quality, a Hik micro E20 Plus.there are NO hot spots or anything like that, and although I have a good DMM, an Agilent 33410A, the resistance is the same everywhere, almost as if the whole board is shorted everywhere at one... which would be crazy as it looks pristine - except for the area of about 1x1 inch that had the worst leaking caps with some not too horrible corrosion and that I tried to clean up as much as possible.What should I do?

author: themadhippy
author_rank: Super Contributor
timestamp: September 16, 2025, 02:08:31 pm
content: QuotePah thats a measly 2 watts,give it some welly if ya wants to see the smoke signals

author: ahsrabrifat
author_rank: Regular Contributor
timestamp: September 16, 2025, 02:45:42 pm
content: Old cap electrolyte and corrosion residues can remain slightly conductive even after rinsing. Use flux remover / PCB cleaner (commercial, low residue) or an ultrasonic cleaner with de-ionized water followed by 99% IPA, then bake at 50–60 °C.

author: snarkysparky
author_rank: Frequent Contributor
timestamp: September 16, 2025, 03:06:10 pm
content: put a current limited supply on 3.3 to ground. Set its voltage so if the short clears it won't exceed 3.3.now get an accurate DMM and put the ground lead on where the ground supply enters the board.With the pos DMM probe find the point on the board where the 3.3 rail voltage reads the highest. This will be the nearest point to your short.It only gets you close but its better than nothing

author: MathWizard
author_rank: Super Contributor
timestamp: September 16, 2025, 04:44:04 pm
content: One time I was chasing a short on a PCB, with current injection. I was cleaning places with IPA and could see what looked like sparks and the surface glare of the liquid change, and it really was arcing. I had to use a 30x loupe to get a good view of it. In that case, one of the problems was too much aggressive flux residue got under pads and traces.I think I found a shorted SMD ceramic cap that way too, I could see flashes of light, and with IPA I'm sure I could see surface glare changes in the IPA, and still see it sparking under it. I wish I had a decent camera to record such things. But hey I actually still have the 1st problem PCB.

author: pizzigri
author_rank: Regular Contributor
timestamp: September 16, 2025, 08:32:19 pm
content: Thank you everyone, great advice - Ill immediately test it.

author: pizzigri
author_rank: Regular Contributor
timestamp: September 19, 2025, 09:37:37 pm
content: So, an update. The logic board I am chasing the short on does NOT get hot anywhere - except on the crocodile that connects the GND terminal of the PSU. That's the only spot that gets hot, the body of the crocodile...It draws full 2A but still at a measly 0.235V, and i cannot get the PSU to stay on to CV as it switches to CC automatically since it draws all the available current under the limit I set. It's an E3631A on the 6V output. I have a thermal camera and it's incredibly frustrating.Alternatively, wtih the PCB disconnected, I tried measuring the resistance using the 34410A, which does go to millivolts - range 100,0000 Ohm - but it gives me 63 milliohm everywhere on the board. Aaargh.

author: PeteH
author_rank: Unknown
timestamp: September 19, 2025, 10:26:32 pm
content: You want to find the lowest mV reading on the board (not the highest).That is the closest to the short location.Inject enough amps to not cause further damage but enough to walk the meter probe towards the component... Measure rail locations with one probe tip and you'll narrow in on it.

author: xavier60
author_rank: Super Contributor
timestamp: September 20, 2025, 12:09:18 am
content: Looking for the lowest voltage point on a shorted rail can be difficult if the rail can't be easily traced on the PCB.For tracing shorts on laptop boards, I used to look for the highest voltage reading on the ground plain.Best done with a DMM which has 1uV resolution.

author: Simmed
author_rank: Frequent Contributor
timestamp: September 20, 2025, 12:56:19 am
content: i found some schema6500 is 3.7mb looks legithttps://www.macdat.net/repair/apple_schematics.htmlthis site is gold