It is currently Fri Nov 16, 2018 1:19 pm

All times are UTC - 7 hours





Post new topic Reply to topic  [ 13 posts ] 
Author Message
PostPosted: Mon Feb 05, 2018 4:32 am 
Offline

Joined: Mon Jan 11, 2016 11:39 pm
Posts: 21
Hey guys, so I've been repairing consoles to convert them to RGB and I happened upon a particularly disgusting system for eight dollars.

This thing had tons of (probably 30 year old) dead bed bugs and insect droppings inside, was rusted out on the RF box and shield, didn't power up at all etc...

I cleaned the shit out of the shell replaced the caps (which were ALL totally bursted) and promptly redid the RF section. No dice, no power even though the regulator was seeing 5V if I shorted pins 1 and 2... Then I realized that the LED pad had come up and and needed to be manually wired over its trace.... Then I realized that even though I got the LED to light up again while RF pins 1 and 2 were shorted, the power switch wasn't working and needed to be totally cleaned out (emery board on the contacts)... Then I was getting a "pop" while turning the power on from the audio jack, but no video. Then I figured out the trace had broke from the 330 pf ground cap to the video pin (I have NO idea how this got cut).

So after all that and cutting the lockout chip I have ended up with a white screen and a pop upon turning on the system. I did the following... swapped out 72 pin for one I know works, filed down corrosion off of the motherboard side of the 72 pin, and scrubbed thoroughly with isopropyl alcohol til all the black had left. I swapped out the rf section with a USB clean power and simple video amp one I made a while back. I noticed the green trimmer had a shitload of rust in it and returned a "2 pf" reading on my meter so i replaced that with 2 15 pf ceramics in parallel.

Just for shits, I reconnected the lockout chip and it seemed to approve of my cartridge connection. I reheated many solder joints in tribute, but the solder gods did not yield a plentiful harvest.

So here's my theory. All the caps were popped... even ones that were underneath the shield which seemed to be untouched by the water damage. That leads me to believe that maybe something bad happened as far as power was concerned for this thing. I'm thinking maybe someone plugged in a tip positive supply after one of the electrolytics had died open which caused 13 volts over the wrong side of all of the other electrolytics. Must've been some fun smoke. Then once it broke they didn't give a shit about the system and left it in their garage or flooded basement or something. Somewhere where bed bugs wouldn't mind living.

Anyway, I noticed some extremely faded IC's on this guy. Two of the 74 series thingies have very dark and faded text and both of the RAM chips in front of the CPU and PPU are very faded as well. I'm thinking that those may have gotten hit sorta hard during this thing's capacitor apocalypse. The CPU and PPU themselves have a few legs with a tiny tiny bit of corrosion on them but otherwise look totally fine and aren't like heating up or anything. Actually nothing is heating up and everything is getting the proper 5 volts as well.

I'm not awful with analog electronics but I don't know shit about digital. Usually I just use one of those arduino powered "IC Testers" where you pop the chip in and it tells you whether or not it works so you can pop it back in the socket. Unfortunately, the NES is not socketed.

Does anyone know a way I can test these chips without doing the awful job of desoldering? I got a scope an oscillator and a fluke 87. Anyone thinking of anything obvious that I might be totally missing?

Thanks!

-Dave


Top
 Profile  
 
PostPosted: Mon Feb 05, 2018 10:26 am 
Offline

Joined: Sun Apr 13, 2008 11:12 am
Posts: 7715
Location: Seattle
Given that you're getting a solid white screen, you know the following work:
* 21.5MHz crystal, driver, and clock source
* parts of the PPU

Given that there's an audio pop, you know that ... well, at least one transistor and capacitor in the audio path work.

For me, the next steps would be checking the voltages on the pins of CPU. See also this thread


Top
 Profile  
 
PostPosted: Mon Feb 05, 2018 3:10 pm 
Offline

Joined: Mon Jan 11, 2016 11:39 pm
Posts: 21
Well I skipped the part where I tested the entire video and audio paths. Everything works fine on that front.

Ugh so looking like it's the SRAM. Better get some desoldering tips. I hadn't even considered that it could be the RAM in front of the PPU either. Well I'll take out my donor board, swap out each chip one by one in a crap shoot and let you guys know what it was for documentation's sake.

By the way I also painstakingly tested for continuity from the connector to each ram and CPU point. All perfect.

I would check the CPU pins (it's AC right?) but I would have absolutely no idea what to do with that information or what I'd even be looking at. Like I said I have not a clue when it comes to digital electronics.


Top
 Profile  
 
PostPosted: Mon Feb 05, 2018 3:25 pm 
Offline

Joined: Sun Apr 13, 2008 11:12 am
Posts: 7715
Location: Seattle
Use the pin out on our wiki and just measure whether pins are high, low, or changing.


Top
 Profile  
 
PostPosted: Mon Feb 05, 2018 4:25 pm 
Offline

Joined: Mon Jan 11, 2016 11:39 pm
Posts: 21
PIN number:

1: 1 mV
2: 1 mV
3: 1 mV
4: .690 V
5: .650 V - .716 V (wasn’t consistent)
6: .740 V
7: .650
8 .650
9: ~.650
10: ~.650
11: ~.650
12: ~.740
13: ~.740
14: ~.650
15: ~.600
16: ~ .450
17: starts at ~30 mV and goes to a steady 8.2 mV
18: starts at 10 mV and sinks down to 2.7 mV and stabilizes
19: .650 V
20: goes to zero

Here's the first half. Most of them were between .590 V and .750 in any given measurement. No actual rhyme or reason. Between pins 16 and 18 is where things started getting weird.


Top
 Profile  
 
PostPosted: Mon Feb 05, 2018 4:29 pm 
Offline

Joined: Sun Apr 13, 2008 11:12 am
Posts: 7715
Location: Seattle
Why is pin 3 low? Pin 3 is driven directly by the CIC.


Top
 Profile  
 
PostPosted: Mon Feb 05, 2018 8:06 pm 
Offline

Joined: Mon Jan 11, 2016 11:39 pm
Posts: 21
I don't know. There is continuity between pin 9 of the CIC and pin 3 of the CPU. Looking at the schematic that's where the direct connection seems to exist. I don't know if it's supposed to be that way or not, or why it would or wouldn't, but that seems to have to do with the reset circuitry largely. I don't know how NOT gates actually function or what exactly happens between pin 7 and pin 9 of the CIC where it seems there's a connection to the RESET button and then to the CPU.

Are you sure there's even supposed to be a voltage there?


Top
 Profile  
 
PostPosted: Mon Feb 05, 2018 8:39 pm 
Offline

Joined: Sun Apr 13, 2008 11:12 am
Posts: 7715
Location: Seattle
Yeah, /RESET is definitely supposed to be high (≥3V above ground) for anything to work.

On both the PPU and CPU.


Top
 Profile  
 
PostPosted: Mon Feb 05, 2018 9:22 pm 
Offline

Joined: Mon Jan 11, 2016 11:39 pm
Posts: 21
Well I mean, that pin gets 5V DC if that's what you mean.


Top
 Profile  
 
PostPosted: Mon Feb 05, 2018 9:42 pm 
Offline

Joined: Sun Apr 13, 2008 11:12 am
Posts: 7715
Location: Seattle
Wait, what were the numbers you posted above, then?


Top
 Profile  
 
PostPosted: Mon Feb 05, 2018 10:22 pm 
Offline

Joined: Mon Jan 11, 2016 11:39 pm
Posts: 21
Oh that was AC. Like I said I know nothing about digital electronics. I presumed that since people always talk about looking at CPU's with a scope that all this stuff is transmitted through square waves or something.

I can give you the DC values of just about everything but I already took out one of the sram's so I gotta replace it first.


Top
 Profile  
 
PostPosted: Mon Feb 05, 2018 10:29 pm 
Offline

Joined: Sun Apr 13, 2008 11:12 am
Posts: 7715
Location: Seattle
OH. Ok, those AC voltages are fine.


Top
 Profile  
 
PostPosted: Tue Feb 06, 2018 12:06 am 
Offline

Joined: Mon Jan 11, 2016 11:39 pm
Posts: 21
Yea, I realized that it would be much easier to just insert a new component than steal one off another board. I also inadvertently just got my donor board working so that would've been dumb.

Ordered some SRAM and some sockets. I'll replace any component that looks like the ink has been melted off of it and update in a week or two simply for documentation's sake in case anyone else has a grey screen and similar issues. The RAM and sockets cost a total of 8 dollars most of which was shipping.

I am trying these puppies out: http://www.buyicnow.com/it.php?i=640


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 13 posts ] 

All times are UTC - 7 hours


Who is online

Users browsing this forum: No registered users and 5 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
Powered by phpBB® Forum Software © phpBB Group