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?
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?