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PostPosted: Mon Jan 30, 2017 2:40 pm 
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Working on a friends of mine NES-101.

Story is he had a lightning strike in his neighborhood while he was playing the NES. After power came back on, NES only went to gray screen once he changed the power supply (original power supply was directly plugged into wall and has zero output now). He gave it to a NES repair guy who said it was the capacitors without even looking at it. The guy charged him $50 to replace the capacitors and it still is gray screening.

I step in at this point and start buy just doing a front to back eval of it. The caps the guy replaced are of the correct size and properly installed. The power supply section of the NES is functioning properly (tested both downstream and upstream of the 7805 regulator).

When I powered the board directly from my variable power supply (8.5v supplied directly to the input side of the 7805) it is consuming 1.2amps and the CPU chip is getting insanely hot (I shut it off at 109ÂșC and climbing on the CPU). PPU heated up and stabilized. I am leaning twoards a bad CPU, but I really don't want to just willy nilly go replacing things until I can nail down what is really bad.

Also, I was running these tests with no cartridge installed so as to see the function of the unit. Does the unit need the cartridge installed to test proper function of the CPU? I would think the CPU would go into some type of hold until instructed mode when no cart is installed, not just turn into a space heater.

Any thoughts or opinions would be helpful.

Thanks!!


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PostPosted: Mon Jan 30, 2017 2:49 pm 
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Location: Poland
If console is able to display after startup solid one-colour screen, other than gray then PPU is probably OK. CPU is definitely fried up. During lightning strike, the power supply might suffer surge, which resulted in much surge on its output (and 7805 INPUT), which also resulted in surge on 7805 output, which resulted of CPU broke.
One more think, which works when i'm fixing famiclones - run multimeter on diode-test and measure voltage drop between GND and VCC (did not remember + and - of multimeter to which), but should be not less than 450 mV - do it when console is not powered.


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PostPosted: Mon Jan 30, 2017 5:37 pm 
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You basically confirmed what I am sure happened. Voltage spike into the 7085 which bled into the CPU and fried it crispy.

Has anyone had any luck with this chip as a CPU replacement?
http://mikesarcade.com/cgi-bin/store.pl?sku=RP2A03

It is calling out the same code as what is actually on the chip on the NES I have. I am not really looking forward to de-soldering the chips from both boards (the cooked one and the one from the doner front-loader I have) and then soldering the chip back in.


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PostPosted: Mon Jan 30, 2017 6:15 pm 
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That's a 2A03 no-revision version. It's interesting, because it lacks the short-period mode of the noise channel in the APU. It was used in some arcade machines, so that's why it's sold there. I won't recommend it for the NES unless you want to hear the difference in sound. It is the right CPU in that sense, but you want the later revised version.

EDIT: Never mind - this CPU is in fact a normal NES CPU on MikesArcade, and it'd work totally fine. Those games did originally have the no-rev CPU, but this is one taken from an NES/Famicom, hence the compatibility list near the bottom. That's a rev "H" CPU and will work fine.

I've got a Famicom board with a good CPU and a bad PPU (works, but sprites are garbled and it gets very warm). If you're interested in that to take its CPU, let me know. It has the appropriate 2A03 CPU, Rev G I think.


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PostPosted: Mon Jan 30, 2017 6:28 pm 
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Being that this top loader already has a rev H in it, I might as well use the rev H from Mikes.

I would rather hand my friend back 2 working NES's than a working top loader and a junker. I like seeing these things lasting forever.


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PostPosted: Sun Feb 12, 2017 6:12 am 
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Anyway to avoid this other than not play on a storm?
Would a fuse before the 7805 or after it do any difference in a case like this?


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PostPosted: Sun Feb 12, 2017 1:09 pm 
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Location: Seattle
Often, the 7805 is a fine sacrificial input stage. Even though it's not really designed for it.

My understanding is that "low voltage" appliances usually don't have/need real surge suppression, assuming that the power supply is supposed to prevent such a need.

There's a bunch of "easy" overvoltage protection schemes:

Code:
input --- fuse ---+--- load
                  |
                 MOV or DIAC
                  |
input ------------+--- load


Metal-Oxide Varistors (intentionally) get a lower and lower threshold voltage each time they're overvoltaged, but they can actually work multiple times.

The fuse could be replaced with a self-resetting fuse; those (also intentionally) get a lower and lower trip current each time they're overcurrented.

Another option involves a zener diode and Silicon-Controlled Rectifier:

Code:
AC --- fuse ----+  +-----+---+-------regulator---
                |  |   zener |           |
              rectifier  +--SCR          |
                |  |         |           |
AC -------------+  +---------+-----------+-------
(This is called a "crowbar")

Each of these are a different speed, which changes how suitable it is. I forget how they stack up, though. Commercial surge suppressors in the US "just" use a MOV and a thermal fuse.


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PostPosted: Sun Feb 12, 2017 3:00 pm 
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Cool!
I gotta implement the first option on my consoles.
Thank you!!


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PostPosted: Tue Feb 14, 2017 9:12 am 
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I remember seeing such a crowbar circuit on the original U.S. made Apple II power supply.


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PostPosted: Wed Sep 20, 2017 2:33 pm 
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Was there ever a resolution to this?

I've got a toploader NES with the same issue, grey screen. It's got a ton of corrosion, mostly on the rf shielding, traces seem fine, and I've replaced the capacitors. If the CPU replacement fixed it, I'll harvest one of a frontloader for now (I've got a bunch of non-working units)


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