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PostPosted: Tue Oct 10, 2017 9:12 am 
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I am a 'lapsed' electronics engineer - I had a short career in electronics design but my career moved on quickly and it's now been over 10 years since I last did any design/test work. I haven't even kept up with electronics as a hobby, but I have an issue I've just started working on and I wonder whether anybody here can offer any guidance.



I have access to a multimeter but that's it. No scope, not even a logic probe, so my testing will be fairly limited.





Here is the story so far:



I acquired a NES from somebody who said they had no cables to test it and so "assumed" it was faulty. As soon as I plugged it in I knew it was faulty as it displayed a solid grey screen, sometimes with a couple of vertical black bars.



(Completed) Step 1: clean cartridges. No improvement. I had two game cartridges (SMB/Duck Hunt and Marble Madness), I can't test either one as I don't have a working NES but I think the chance of them both being faulty is highly unlikely.



(Completed) Step 2: disassemble NES. Knowing that this could be related to the 72pin I disassembled the case and cleaned the 72pin connector and motherboard edge connector with isoprop. I noticed straight away that somebody had taken this thing apart in the past - two case screws were completely missing, as was one of the screws securing the cartridge carrier and 72pin to the motherboard. At this point I assume the person who gave this to me wasn't being entirely honest. After cleaning there was no improvement.



Suggested additional step (2.1): boil 72pin connector, reconnect and power up for test.



(Completed) Step 3: Reconnect 72 pin and check continuity. I checked the continuity between all 72pins of the connector (cartridge side) to the tabs on the motherboard and confirmed that there is definitely continuity when there is no cartridge in the slot.



Suggested additional step (3.1): I guess another logical stage will be to remove the cartridge board from its case, insert it into the 72pin and check for full continuity between the cartridge board and the motherboard tabs. therefore checking that the motherboard SHOULD be able to read the cart. I assume that doing this will remove the need for step 2.1 above.



(Completed) Step 4: Inspect motherboard for damage and investigate. I noticed that there was corrosion on some edge connector tabs and some of the traces and via holes connecting the edge connector to the WRAM chip. Using a multimeter I confirmed that there was still continuity between the edge connector tabs and the legs of the chip, so I don't think the damage is the cause of my problem. Checked Caps - all looked good. No obviously missing components.



(Completed) Step 5: Check GND and VCC continuity for all chips. I found that all chips appear to have power and are grounded to the backplane.



This is when I noticed that somebody had previously cut pin 4 on the lockout chip. It has been cut right by the package rather than at the board and so it would be difficult if not impossible to restore continuity to the motherboard.



Step 6: Check continuity for all other chip pins to their relevant locations on the motherboard, and to other linked components. I am still in the middle of this stage and so cannot report anything substantial yet but will aim to update this post as I progress.







Before I go any further with my testing do you think it's possible that the grey screen could be caused by CIC pin4 being disconnected? It seems unlikely to me. As the person doing the work didn't desolder the pin I think we can rule out heat damage to the chip.





I'd like to think that my approach so far has been logical given the limitations of not having decent test equipment, but I thought I'd ask the experts...have I missed something? Are there any other common issues that could be investigated straight away to avoid wasting unnecessary time fault-finding the rest of the board?



Is there something more I could do with the limited kit I currently have?


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PostPosted: Tue Oct 10, 2017 12:00 pm 
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Joined: Sun Apr 13, 2008 11:12 am
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Location: Seattle
CharlieMike wrote:
As soon as I plugged it in I knew it was faulty as it displayed a solid grey screen, sometimes with a couple of vertical black bars.
In order:

* Not blinking — CIC is disabled (usually desirable)
* Solid grey screen — CPU is not successfully executing code
* Vertical black bars — PPU might be damaged

Quote:
Before I go any further with my testing do you think it's possible that the grey screen could be caused by CIC pin4 being disconnected?
No.

If the CIC were responsible for the CPU not executing code, then the PPU wouldn't be able to draw any contents on-screen

Quote:
I'd like to think that my approach so far has been logical given the limitations of not having decent test equipment, but I thought I'd ask the experts...have I missed something? Are there any other common issues that could be investigated straight away to avoid wasting unnecessary time fault-finding the rest of the board?
Not really.

At this point, I'd check whether the CPU's address and data lines have stopped at a constant value, and figure out what it crashed on. The CPU or CPU's RAM are the two likely culprits right now.


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PostPosted: Tue Oct 10, 2017 5:05 pm 
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Buying a new connector to help ensure good connection is really the only logical first step IMO. Can't trust an old connector that hasn't proven itself yet regardless of how much effort is put into restoring it. Also look over the board for corrosion or damaged tracks.

EDIT: also I wouldn't be so quick to assume whoever passed it on to you had anything to do with its state. The number of NES consoles which are in the hands of the original owner and never been opened before is very small. It would be more surprising if you somehow knew it was never opened at this age.

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PostPosted: Wed Oct 11, 2017 1:50 am 
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Joined: Tue Oct 10, 2017 7:38 am
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Thanks for your input guys.


So it looks as though the CPU or RAM could be the issue here. I will concentrate on checking them tonight to see if there are any issues there. I know they're both connected to GND and VCC because I checked that previously so there are no supply issues, but as you say, if I could confirm whether the CPU is operating or if it's stuck what the values on it's pins are then I'll be a little step closer to understanding what's going on.

As for the PPU, well the vertical black bars only appear sometimes. not all the time. I guess the PPU could be at fault but I'll focus on the CPU/RAM first.


INL, I understand why you want to rule out the 72pin connector buy replacing it with a new one because there are many, man instances where that has been the route cause, but I think I have already ruled it out by carefully checking continuity in a number of different ways. I will however boil it just in case that makes a difference because it's quick and cheap to do and there don't appear to be any downsides. I would even consider wiring a cart direct to the NES board before buying a new connector. It might sound drastic but I'm reluctant to spend more money than I need to on this unit incase it's not salvageable. Wiring it up is a free option, won't take long, and if the fault persists it will definitely rule out the 72pin connector.


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PostPosted: Wed Oct 11, 2017 12:25 pm 
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So while checking out the cpu i found it got very hot over the course of 5 minutes so there is a problem there.

I found that when powered the pins had the following.

/RST = 5v
A13 = 1.5v
M2 = 2V
OE1 & OE2 = 5V
/NMI1 = 5V
/IRQ=0.6V

All other pins around 0.1 to 0.2v.

I couldn't tell anything else with only a multimeter.


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PostPosted: Wed Oct 11, 2017 1:03 pm 
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A13 is awfully suspicious. I'd try replacing the 74'139 (because it's cheaper and easier than the CPU)


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PostPosted: Thu Oct 12, 2017 12:53 pm 
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I ordered a new ls74139n chip because it's so cheap.

I also boiled the connector and cleaned it again but it made no difference.

I looked around for a RP2A07 but can't find one anywhere. If i order a pal famiclone cpu (UA6527P) do i also need to order a famiclone ppu? Or would the famiclone cpu play nice with the original ppu?


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PostPosted: Thu Oct 12, 2017 1:37 pm 
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As far as I know, the 2A07 and 2A07A are the only two Nintendo-original PAL CPUs.

Installing just a PAL famiclone CPU will produce something that's in a magical in-between land that's not quite Nintendo PAL NES and not quite PAL famiclone. Some games will work, some will be out of tune, some will crash in weird ways. But it's certainly more functional than "not at all".

Of course, we don't know whether the PPU works yet, either.


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