SNES Console Issue - 5V shorted to GND - Page 1
Forum section: electronics / repair

author: Buckethead
author_rank: Newbie
timestamp: August 25, 2019, 10:26:00 pm
content: I have an issue with a PAL SNES where 5V is shorted to Ground. I'm a electronics newbie, so I'm struggling to figure out where.I attempted to install a SuperCIC modchip. When I powered the system on for the first time after installing it, I got nothing - no output, no led light. At this stage I didn't do any problem solving because I had a bit of a rats nest of wires and wanted to do a cleaner install, so I just de-soldered everything and started again. This time I decided to completely remove the original CIC chip. I left it on previously, just lifting the required legs.I tried to power the console back on again after the 2nd install, still nothing. So I started checking things out with a multimeter. The voltage regulator was still receiving around 8.5V at it's input, but it was 0V at the output. The heatsink was also getting extremely hot very quickly. Shortly after the console had been on (a couple minutes), I was no longer reading any voltage at the regulator input either, so I presume the fuse had blown.I then discovered there was continuity between GND and 5V at the voltage regulator. Similar story between any other 5V and GND points after that. For example there is now continuity between contacts 9 & 18 where the original CIC chip used to be. (9 should be GND, 18 should be +5V).I have since completely removed any wiring for the SuperCIC modchip. I've removed the voltage regulator (KA7805) and the bridge rectifier (RC203) from the PCB as I wanted to rule out any problems with them. I've cleaned up all the contacts where the original CIC chip was. The continuity issue between 5V and GND still remains. There are no physical signs of damage on the PCB at all as far as I can see.TLDR: 5V is shorted to GND and I need help figuring out how to find out where. Presuming (hoping) the console isn't already fried beyond repair...

author: helius
author_rank: Super Contributor
timestamp: August 25, 2019, 11:08:32 pm
content: You say 5V is shorted to GND but what resistance do you measure exactly? 1 ohm? 100 milliohm? 5 milliohm? This makes a big difference.Finding a short on power to ground is not that easy because there are dozens if not hundreds of paths between them on the board. A common diagnostic technique is to first, attach a current-limited power supply to the power and ground rails (with current limit set fairly low, at 0.5A or thereabouts), and watch to see what part of the board heats up the most using a thermal camera. You have a low-resolution thermal sensor in your fingers that can be pressed (har) into the same job, and some techs also use an alcohol swab, watching where it evaporates fastest.Other ways of finding board shorts are the Polar Toneohm, or an inductive probe, or desolder everything and use elimination.

author: Buckethead
author_rank: Newbie
timestamp: August 26, 2019, 12:02:47 am
content: I just have a cheap multimeter so I'm not able to accurately measure very low resistance values. All I can say is on the 200 ohm range I'm measuring the same resistance between 5V and GND as I am touching the probes together (0.1 ohm).This is sounding like it's a much more difficult problem to solve than I expected!

author: narkeleptk
author_rank: Contributor
timestamp: August 26, 2019, 12:34:56 am
content: I'd start by removing the regulator you said was getting hot. If the Vi for it comes back then you can spend most of your time the Vo side of it looking for the short. It sounds like the vr is dead now anyway.

author: james_s
author_rank: Super Contributor
timestamp: August 26, 2019, 05:50:52 am
content: Was it working properly when you started? If so, are you sure there isn't a solder bridge somewhere?

author: Buckethead
author_rank: Newbie
timestamp: August 26, 2019, 05:56:25 pm
content: Quote from: narkeleptk on August 26, 2019, 12:34:56 amI did try removing the regulator to rule that out. I don't think the voltage regulator is dead as the 1.5A fuse blew before it. It seems the short is definitely on the Vo 5V side, as Vi still read around 11.5V (until the fuse went). I've since temporarily bridged the points where the fuse was with a wire and very briefly turned on the console to confirm Vi is still receiving an expected voltage - It was.

author: Buckethead
author_rank: Newbie
timestamp: August 26, 2019, 06:01:01 pm
content: Quote from: james_s on August 26, 2019, 05:50:52 amYes it was working without any issues before this. I really don't think there's a solder bridge anywhere, but I'll give the PCB yet another check tonight to see if I can spot anything. I've wicked away all solder from where the voltage regulator was and also where the original CIC chip was, so I can't really see where else there could be a solder bridge.

author: zzattack
author_rank: Regular Contributor
timestamp: August 26, 2019, 06:14:32 pm
content: If there's no obvious short then check the ppu/cpu chips for a solder drip. Definitely sounds like a leftover somewhere.

author: Gyro
author_rank: Super Contributor
timestamp: August 26, 2019, 06:56:55 pm
content: Is there a tantalum bead capacitor near the output of the regulator? If so, that might be a sudden failure candidate. A bit of a coincidence though, unless the console has been sitting unused for a long time.

author: Buckethead
author_rank: Newbie
timestamp: August 26, 2019, 08:26:45 pm
content: Quote from: zzattack on August 26, 2019, 06:14:32 pmYes, this is exactly what's happened! You can tell I'm a beginner for not previously noticing or thinking to check there..On PPU1, below the lifted leg 24 I soldered to, a little drip of solder has bridged legs 25 & 26, which happen to be connected to GND and +5V (see attached schematic).Now, my next question is - Have I done any permanent irreversible damage to the PPU1 chip? If not, any advice on how to best remove this solder bridge? Just go in with some solder wick? I'm worried about making things worse.

author: zzattack
author_rank: Regular Contributor
timestamp: August 26, 2019, 08:34:28 pm
content: Just apply some flux, touch the pads with your iron, then drag the iron away from the pins and the bridge should solve. Make sure you heat both bridged pads simultaneously, and the solder should follow your iron. Look at some drag soldering videos on youtube to get an idea of the process.No, most likely no permanent damage. These things can withstand quite a bit of tinkering.

author: Buckethead
author_rank: Newbie
timestamp: August 26, 2019, 09:22:38 pm
content: The drag soldering looked a bit daunting, but turned out to be easier than expected. I no longer have a short to GND!Now to re-install the SuperCIC mod and hope that everything still works. I'll report back once done. Thanks for the help.

author: Buckethead
author_rank: Newbie
timestamp: August 27, 2019, 01:07:39 am
content: SuperCIC installed, everything working fine. Great result, thanks!