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Troubleshooting Top Loader Issues with power and PPU
http://forums.nesdev.com/viewtopic.php?f=9&t=17501
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Author:  datsunhls30 [ Fri Jul 06, 2018 2:45 pm ]
Post subject:  Troubleshooting Top Loader Issues with power and PPU

A year ago, I bought a top loader that appeared to be in good shape off eBay. My goal was to build a little composite video amp and give it as a gift. I'm just a hobbyist in electronics stuff, I have a lot of experience soldering/desoldering/troubleshooting but have no formal training.

Initially the NES seemed to work, but was intermittent. From power on, it may take several resets before I'd get a game to load. Additionally the 7805 was pretty hot to the touch, even on the heat sink. I did not notice excessive heat from the CPU or PPU. When I measured the +5v rail, I would get something closer to +3.6v. I decided to replace the large electrolytic capacitor, still got 3.6v. I took the 7085 out of circuit and it produced +5v, but under a ~200mA load it would get very hot and the voltage would drop. I believe that chip should handle more than 1000mA without a voltage drop.

Easy fix, right? I replaced the 7805 with a brand new one, and suddenly my +5v rail was +5v (and the regulator seemed to run much cooler). Also, the unit would load the game every time. I was pretty pleased, I fixed it!

On to the next problem, jail bars(or at least reducing them). I started replacing the other electrolytic caps(C1, C3, C11) around the board with new ones, and testing the extracted ones in the process. All the caps were in spec, but here's where stuff went bad:

I turned on the top loader when at least one of the capacitors was out of circuit (I think it was C1, the .01uF cap near the cartridge slot). The unit produced solid color video, but a ton of audio noise and a big horizontal black line in the center of the screen. I put the cap back in circuit (the original one, with the correct orientation), but the broken video/audio persisted.

At this point I'm looking around trying to figure out what I bumped loose and I can feel heat coming off the board from a foot or so away on my face. I touched the CPU and the PPU, and the PPU was burning hot. Needless to say I was pretty sad.

So kinda long-winded to this point, but I'd love to know what I did wrong.
- did I cause a failure by operating the top-loader without one of the electrolytics in-circuit?
- was the PPU failure imminent from the way the old 7805 was loaded down/failing, but replacing the 7805 was able to supply the extra current to finish bricking the PPU?


I'd still like to repair it, but I'm hesitant to sacrifice a working NES to get a PPU for a top-loader that may burn that PPU to a crisp, too.
- I just ordered an UA6538 on order from aliexpress to see if I can at least get video without killing a real PPU. I must sheepishly admit that I ordered a UA6527P a few months back thinking it was a PPU swap when in fact it's the CPU. I realized my mistake writing this post. (needless to say, it didn't work as a PPU)
- Anyone have experience with throwing caution to the wind and just replacing the PPU and it working, or is my unit a possible serial PPU killer?

Thanks in advance, I've learned a lot from reading past posts here and was impressed by how helpful you all can be.

Author:  lidnariq [ Fri Jul 06, 2018 3:59 pm ]
Post subject:  Re: Troubleshooting Top Loader Issues with power and PPU

Console5 says that C1 is 100µF, not 10nF?

Without better pictures of the mainboard, it's hard to say anything useful? It looks like C1 is one of the large bulk capacitances on the mainboard (between 5V and ground), but I don't know how it differs from the 1.5mF capacitor.

The PPU is likely toast; artifacts like that imply a digital failure. But a picture would be nice anyway.

I don't think it likely that this mainboard would destroy another PPU.

Author:  datsunhls30 [ Fri Jul 06, 2018 7:52 pm ]
Post subject:  Re: Troubleshooting Top Loader Issues with power and PPU

You're right, C1 is 100uF. I was posting this from work and looked at a pic online and I read the value of the little ceramic next to it.

Here's the current video output. That's the stable video picture, not missed scanlines from a photo of the screen.
Attachment:
File comment: Video output
IMG_2853.JPG
IMG_2853.JPG [ 595.39 KiB | Viewed 576 times ]



Also, I really got some of the details wrong. Both the CPU and PPU heat up. Right now, the resistance between the 5v rail and ground is around 12 ohms with the original PPU RP2C02 soldered to the board and the clone CPU UMC UA6527P socketed. The only way I get any video out is by socketing the UMC UA6527P clone cpu, the RP2A03 has failed short too and produces no video output when socketed.

Attachment:
File comment: Main board
IMG_2860.JPG
IMG_2860.JPG [ 466.72 KiB | Viewed 576 times ]


Visually theres no other damage to the main board. My next step will be to pull the PPU and check the resistance from the 5v rail to ground.

So you sorta answered my question/concern, that the existing mainboard probably won't kill good chips that are put in it, but is anything known about *why* these NES chips fail shorted? It's the main failure mode as far as I've read. I really got spooked when I was just poking around on my victory lap of repairing the 7805 problem earlier and whatever I did caused (apparently both!) chips to catastrophically fail. I was frustrated enough that I'm finally coming back to it after about 6 months :)

I think the big 1500uf cap is to filter the rectified voltage on the input side of the 7805 and the 100uF would be decoupling(?) closer to the chips. My only theory would be that a spike out of the 7805, without the capacitor in place, could cause damage to the CPU/PPU, but that's not really how the 7805 powers up. I doubt they'd ever see greater than +5, just a "dirtier" power rail since it lacks that capacitor to smooth it out. I dunno.

Author:  lidnariq [ Fri Jul 06, 2018 8:03 pm ]
Post subject:  Re: Troubleshooting Top Loader Issues with power and PPU

datsunhls30 wrote:
My only theory would be that a spike out of the 7805, without the capacitor in place, could cause damage to the CPU/PPU, but that's not really how the 7805 powers up.
That is exactly my hypothesis.
Quote:
I doubt they'd ever see greater than +5, just a "dirtier" power rail since it lacks that capacitor to smooth it out. I dunno.
A lack of output filtering can cause output oscillations, which could cause a voltage spike that damaged things.

Author:  datsunhls30 [ Fri Jul 06, 2018 8:57 pm ]
Post subject:  Re: Troubleshooting Top Loader Issues with power and PPU

The answer I wanted to hear was that it wasn't my fault...guess in this case I might need to take the blame for killing a top loader :)

Thanks for your help lidnariq, if there's any other thoughts on the matter I'd love to hear them.

Author:  lidnariq [ Fri Jul 06, 2018 10:27 pm ]
Post subject:  Re: Troubleshooting Top Loader Issues with power and PPU

On the bright side, if it were your fault, then you know that it would be safe now that you've corrected it.

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