BTT SKR E3 Mini 3d Printer Board - all 3.3v shorted to ground - Page 1
Forum section: electronics / repair

author: hicsvtdracones
author_rank: Newbie
timestamp: October 17, 2022, 08:28:57 pm
content: So this is my second SKR board I have killed... was doing somethign that required rewiring, and while looking at the board and a online schematic, I got turned around and reversed the voltage polarity. Not sure how I manage it, but doesn't help that SKR keeps swapping which is positive and which is ground on each board,, I double check it, and somehow, I am still wrong each timeI plug it in, and dead board instantly, just refuses to boot up.So, this time I decided to try to fix the board as I can't buy a replacement til end of month. I found out that everything on 3.3v is shorted to ground, all the caps, resistors, and obviously power pins. I assumed it was the 1117 3.3 voltage regulator, as those tend to die easily when you reverse the voltage polarity. I took off the regulator, and still, same issue. So, it has to be a different component, though not really all that much on 3.3v, other than the processor, which I don't think is the cause, but could be wrong.I tried finding the short by checking for heat, but my FLIR multimeter died over a year ago, and no flux on hand. I tried using alcohol, but it all evaporated at the same rate off several chips. No canned air eitherI doubt it is a cap also, but guess it could be. I was wondering if anyone has experienced this before, or might be able to pint at a component that is the most likely culprit?I have attached what I could find. I haven't been able to trave the exact circuit, but all 3.3v pins are shorted, the 1117 3.3 output pin is shorted to ground, and the capacitors nearby seem to be shorted also. The 100 resistors at the stepper drivers are also shorted.The picture of the board has red dots on every short to ground I found, not sure i got them all, and some could be intentional I suppose.Thanks for any help.

author: kripton2035
author_rank: Super Contributor
timestamp: October 17, 2022, 08:58:19 pm
content: build yourself a shorty-with-display device.it is an invaluable tool to find this kind of short circuits on a board...http://kripton2035.free.fr/Projects/shorty-display.html

author: hicsvtdracones
author_rank: Newbie
timestamp: October 17, 2022, 11:37:55 pm
content: That is an idea, I will have to look at what is required, I have everything but the AD8628 and MCP3421, might be able to use a different amp or opamp in place of teh AD8628, but as far as the mcp.. hmmmm, wil have to look at some dead boards. These capacitors are really close though, hmm. Lost my debit card, and have to wait about 2 weeks for a replacement, which is why I decided to try a repair, rather than waiting til the end of the month when my card comes in and just buying a replacementWhen it comes to the Shorty, I have been thinking about buying another FLIR DM, I had loved mine, pretty sure it had milliohm for finding short circuits, which I think is all a shorty is, a milliohm ohmeter,,, but also had the thermal camera, perfect for this stuff. I had it for about 2 years, then it just died one day, and FLIR was of zero help as I wasn't a corp or govt customer, I thought they had a lifetime warranty, but finding the dept that would honor it was impossible, and i had bought it directly from FLIR's website, so debating if the $250 or so is worth it, I am obviously a bit sour over the last one,,, My current multimeter is a $9 special, and amazingly, I have had it for about 8 years, and still works, though it isn't too trustworthy for anything other than verifying a shortAnyways, wil see how it goes. Was hoping someone might identify one of the components on the 3.3v circuit as prone to shorting when subjected to reverse polarity

author: MikeK
author_rank: Super Contributor
timestamp: October 18, 2022, 01:43:48 am
content: Quote from: kripton2035 on October 17, 2022, 08:58:19 pmHow is that supposed to measure milliohms without Kelvin probes?

author: kripton2035
author_rank: Super Contributor
timestamp: October 18, 2022, 06:17:23 am
content: Quotebecause this is using 4 wires probes... look at the schematic.the absolute value is not very precise because there are no precision resistors involved,but what we need here is a precise relative value of all components around the shorted path.

author: DavidAlfa
author_rank: Super Contributor
timestamp: October 18, 2022, 08:43:48 am
content: Yep, just connect some external 3.3V power source, limiting the current to 1-2A and find the part heating up.Luckly, it'll be just an infamous ceramic capacitor.If you don't have a an adjustable PSU, you can still do it in a poor man way from a computer ATX PSU, the orange wire is 3.3V (NOT yellow or red!) and put something in series to limit the current.Not having low value resistors? You can get there with a few meters of thin wire, measure about 1-1.5ohms.Before testing on the actual board, connect the wire to 3.3V measuring the current with the multimeter to ensure you didn't made some measuring mistake and it's throwing 20 amps.If the short is "very shorted", it might require more amps until you can notice some heating.Even with 5 amps, that's just 16W, a wire of 2m would barely get warm, just don't pack/coil it tightly to prevent it from building heat.Don't get too short on the wire or it will catch fire, if you need a lower resistor value, run another wire in parallel.