NES-001, fried 7805, possible Vcc overvoltage. - Page 1
Forum section: electronics / repair

author: BrokenYugo
author_rank: Super Contributor
timestamp: November 26, 2021, 11:12:41 pm
content: Went to fire up the NES a couple days ago, totally dead. Power brick putting out expected AC, console smelled like something was getting hot, tore down to inspect.Nothing I could see looked burnt, so I applied power and measured Vcc at like 2 volts.Figured maybe I'd get lucky and it's a bad rectifier or 7805 or something, so I pulled the 7805 (a reputable modern part I installed as part of bulletproofing the thing like a year back) and put 13V into it as the NES does, and got like 9V on the output and 100ma quiescent current! Checked with my scope and saw no signs of oscillation on input or output, put a 1k resistor between output and ground and saw very little change.Feeding the board 5V with my bench supply gives a draw of about 1.3A, too much for something that runs off a 1A regulator. I measure about 10 ohms Vcc-GND with the power removed, so it's an active problem.That's about as far as I got so far, any pointers moving forward? My fear is the 7805 went and overvolted the thing until one or more chips failed (hopefully not the two more or less irreplaceable ASICS) and dragged the rail down.Did I mention this is the second NES to fail on me like this? First one I gave up on easily because I had this formerly spare board that I knew just needed a jellybean 74 series logic chip replaced, which I pulled from that first board as it tested ok (so far as a TL866 tests anyway) and I hadn't considered it had perhaps been overvolted (I figured the short killed the 7805 in that one).

author: Rich S
author_rank: Newbie
timestamp: November 27, 2021, 02:08:50 am
content: A modern LM7805 regulator IC (from reputable manufacturer) is fairly robust, has protection from overheating and short-circuits, can source up to 1.5A, and can accept input voltage as high as 35V.https://www.ti.com/lit/ds/symlink/lm7800.pdfThat said, its not bullet-proof. Hit it hard enough, or long enough, without enough heat-sinking, it will destruct. So Its good you check it out of circuit with a known-good power supply and resistor load.So check the power brick: is it still putting out its rated voltage at that 1.3A? And is it delivering "clean" DC (no ripple)? (do you have DVM--set to AC volts-- or a oscilloscope to check this?). If no, then something inside that brick has failed.BTW, dropping 13V down to 5V, at 1.3A, means that IC has to dissipate (13 - 5)*1.3 = 10.4 wattsSo it should be on a heatsink - something to keep below its shutoff temp. Like, at least, 10-deg-C/W heatsink.good luck - RS

author: BrokenYugo
author_rank: Super Contributor
timestamp: November 27, 2021, 06:27:19 pm
content: Nintendo was a little weird and used a AC brick (just a 9V transformer) with the bridge and big filter cap inside the system, the 13V DC doesn't get used for much and I have confirmed that it's there and clean. The 7805 just has a little folded aluminum heatsink which normally doesn't get too hot, the system shouldn't draw anywhere near 1.3 A, that's a sign of a problem. I've not yet checked to see if it's at all running in this state (doubtful) or checked if anything is heating up abnormally.I guess my primary questions are has anybody else seen a linear regulator IC fail and destroy a logic board like that? Or am I wrong and it failed after something on the board went and overstressed it?

author: james_s
author_rank: Super Contributor
timestamp: November 27, 2021, 06:30:44 pm
content: I have seen linear regulators fail and destroy 5V logic on multiple occasions, although I have never seen it happen in a Nintendo. I'm not sure why you replaced the original regulator, they are not known for being unreliable and the upgrade has apparently caused the system to fail and be damaged rather than prevent it.

author: Martian Reality
author_rank: Newbie
timestamp: October 04, 2022, 03:27:25 pm
content: I don't know how much help it is, but I have recently aquired a NES,which seemed to be not working at initial testing and inspection.It appears to only give blue screen. The CIC is operating correctly, aparently though.The cart connector is in realatively excellent condition, not bent or scraped by any evil Game Genie (with its overly thick pcb and nails on chalckboard death grips). Only the fingers on the motherboard to the cart connector shows any slight deformities (a small fraction of what old 8-bit pc or colecovision board traces look like, and what seems to be a thin bit of oxide film on part of just a few of the motherboard's fingers for to the cart connector).I did a mild clean with 99% iso-propyl alcohol and a soft toothbrush, but didn't notice too much dirt or grime generally, the most of the main board looks good, was a bit of tin/oxide around one or two through hole solder blobs, but I brushed at it and finished it of mostly with my fingernail to be gentle on the board.Nothing on the mainboard smells funny.The RFMod/Power Module definately has a some noticeable pungent chemical odour.I began to worry it could be a fault in the PPU or CPU, but after much searching forblue screen repairs online and finding none of them useful or relevant, and justpoking the multi-metre at all the chip powers and grounds and the reset and mainpower routes and the 7404 inverter and the CIC and resigning to order a cap kit,my brain started working on it's own...I recalled one of the many repair videos I watched mentioning a slow bootup,and (either the same or another video) saying if left on after a while it would boota game.So I decided to just for kicks, play around with waiting, powering off a while and powering on and just leave it alone, or different on/ off/ wait times between.Wouldn't you know it, it's been able to boot, with all the sound and graphics working fine (at least with RF-out so far, haven't lucked into it booting with A/V-outs yet)I have noticed the 7805 and it's heatsink are burning hot-ish, fairly hot, not impossible to touch, but longer than a few seconds might result in a mild burn.Also the 5v rail is closer to 6v or a hair under 6v. My meter has a slightly low battery,but this seems like it could be high enough to be glitching out all the 5v logic, probably not enough to damage anything, but plenty to produce unreliable operation, here manifesting as unerliable bootup. The CIC operation is alway reliable nevertheless.I haven't taken a scope to anything yet. But it seems like probably the 7805 maybewas damaged slightly. I purchased this NES recently with a lot of other consoles with mild use from sitting in a closet since kids moved out years ago. The NES and 1 cart have evidence of being from Value Village (long before price gouging). The NES didn't come with a power adaptor, neither had the slim ps2 in the lot, but there was one oddball 12vdc higher current switching chinese made power.My guess is maybe a higher voltage was fed to the 7805, which increases the heat dissipation, since they "burn off" the excess voltage as heat to regulate the 5v.I haven't desoldered anything in the NES yet, but suspect either caps and or the 7805. There's a handful of surface mount caps and resistors on the underside of the RFMpd/Power Module board, frightningly the caps appear to be of various shades. My past experience lead me to think they should all be the same colour and that heat will darken them, or blacken them when they burn out.I think I need a schematic of the RFMod/Power Module, hopefully it's not too difficult to recreate one from scratch. I have other NES consoles which seem only to be cursed by Game Genie'd cart socket connectors (ie no other anomalies).