The cartridge connector can probably be refurbished, but you can also still get new ones from China, which are supposedly not great, but will often work better than not-refurbed ones.
I will put the NES with case back together, the other board will be used as a spare, I guess. Both boards are from @elosha@chaos.social
Weird stuff happening on PA13, which can come from the CPU, but also on the cartridge connector.
So I replaced both the WRAM and the cart connector -- in hindsight I should only have replaced the cartridge connector. But I didn't see this issue before. So I am not sure if the WRAM really was faulty, and since I replaced a narrow for a wide SRAM chip, I can't swap them for testing purposes...
Every once in a while I get a garbled screen, but all in all the CPU gets stuck very quickly. The bus turns to static data, and that's it. I assume that maybe again the WRAM is broken? The levels look fine this time, but as there is not much activity, my guess is that the CPU locks up so quickly that the SRAM reads out broken values. I don't see ANY CS on the PPU.
And there is another broken NES. This one got a replacement UMC PPU, and for good measure I also tested both with an original and UMC CPU. However even with those two replacements it won't run.
#nintendo #nes #retrogaming #letsfixit
As the cartridge is fine, the only other component that is questionable would be … the SRAM! Also there are not many other components left over and the SRAM is connected directly to the CPU so it COULD trash the bus.
But something else is wrong… I have an original RP CPU and a clone UMC CPU. With the UMC the data pins of the CPU look broken as hell. The CPU is good, but this looks like some bus conflict — something is pulling the signal down HARD.
It turns out: if you single shot the scope the CS signal will be generated for a while and then stop. So something IS executing and trying to program the PPU, but then ceases to work. I simply didn’t see it during operation of the NES. Only when doing a single shot and then resetting the NES, I could see this behavior.
What I noticed: on the Video bus side of the PPU there is ZERO activity. And no CS for the PPU. So I suspected the LS139 which generates the CS. But minipro says: all good!
@tauon@chaos.social jo. Man muss bei dem schon ordentlich handeln. Aber bei einer 10€ cart 25€ als Initialpreis zu verlangen, nee… ist vielleicht sein Stil, aber törnt mich ab.
The clock signal at one side of the capacitor looks slightly wonky. On the other side of the capacitor it is completely gone. As there is nothing else between it and the PPU I think we have a dead PPU, pulling the signal down. What a bummer.
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So what's our score?
The cartridge connector can probably be refurbished, but you can also still get new ones from China, which are supposedly not great, but will often work better than not-refurbed ones.
I will put the NES with case back together, the other board will be used as a spare, I guess. Both boards are from @elosha@chaos.social
And yup, now the machine works. That's two for two.
So now we have a nice NEC SRAM chip and a different cartridge connector!
Weird stuff happening on PA13, which can come from the CPU, but also on the cartridge connector.
So I replaced both the WRAM and the cart connector -- in hindsight I should only have replaced the cartridge connector. But I didn't see this issue before. So I am not sure if the WRAM really was faulty, and since I replaced a narrow for a wide SRAM chip, I can't swap them for testing purposes...
Every once in a while I get a garbled screen, but all in all the CPU gets stuck very quickly. The bus turns to static data, and that's it. I assume that maybe again the WRAM is broken? The levels look fine this time, but as there is not much activity, my guess is that the CPU locks up so quickly that the SRAM reads out broken values. I don't see ANY CS on the PPU.
And there is another broken NES. This one got a replacement UMC PPU, and for good measure I also tested both with an original and UMC CPU. However even with those two replacements it won't run.
#nintendo #nes #retrogaming #letsfixit
And there we go! All that was wrong was the U1 SRAM. Another NES brought back to life! It seems a dead SRAM can pull down the signals to bad levels.
Now the old SRAM is out, a Motorola part. And the new SRAM is in. The waveforms now look MUCH better.
As the cartridge is fine, the only other component that is questionable would be … the SRAM! Also there are not many other components left over and the SRAM is connected directly to the CPU so it COULD trash the bus.
For comparison: this is what the same pin looks like using the original CPU. Much cleaner signal! Not perfect, but solid logic levels!
But something else is wrong… I have an original RP CPU and a clone UMC CPU. With the UMC the data pins of the CPU look broken as hell. The CPU is good, but this looks like some bus conflict — something is pulling the signal down HARD.
It turns out: if you single shot the scope the CS signal will be generated for a while and then stop. So something IS executing and trying to program the PPU, but then ceases to work. I simply didn’t see it during operation of the NES. Only when doing a single shot and then resetting the NES, I could see this behavior.
What I noticed: on the Video bus side of the PPU there is ZERO activity. And no CS for the PPU. So I suspected the LS139 which generates the CS. But minipro says: all good!
I have now a replacement CPU snd PPU. But even with those I still get a grey screen.
@dentaku@fnordon.de definitely!
@tauon@chaos.social jo. Man muss bei dem schon ordentlich handeln. Aber bei einer 10€ cart 25€ als Initialpreis zu verlangen, nee… ist vielleicht sein Stil, aber törnt mich ab.
The clock signal at one side of the capacitor looks slightly wonky. On the other side of the capacitor it is completely gone. As there is nothing else between it and the PPU I think we have a dead PPU, pulling the signal down. What a bummer.
The oscillator circuit is not that complex. The 51pF capacitor on the right goes off to the PPU while the left transistor drives the CPU.