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Donkey Kong Jr - Mostly graphics problems


SNESNESCUBE64

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So this was a boardset that I got from 8_bit_guerrilla as he no longer needed it. When he bought it, he got it from a seller saying that it had minor sprite issues. I think the seller lied a bit, because it was a bit more than minor. Essentially, the positions of sprites were all goofed up. On top of that, the original braze kit (an old one mind you) also no longer worked.

The first step I followed was finished converting it back to stock, which was a simple as removing the leftover bodge wires (8_bit already removed most of it). the only thing to note was that Vpp had been cut on the 7C ROM on the video board. This was because the ROM was replaced with a 2732, which is the same pinout, but with an extra address pin where Vpp was for the braze kit mod. Vpp is supposed to be tied high, so I fixed that issue.

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Once the bodge wires were fixed, I hooked up both the CPU and video boards to be greeted with garbage on the screen, no booting. This was simple enough, it turns out ALL of the sockets were nasty and corroded. I ended up replacing each one, that solved the issue, the CPU board was now able to boot just fine. I tested it with a known working video board, and everything worked perfectly. 

The next step was to test the video board. The issue here is that colors and positions were totally goofed. The first suspect was the ECL RAM. The ECL RAM on Nintendo arcade boards is used for color mainly because these RAMs are incredibly fast, especially for that time. The first one was straight up missing the cap, exposing the die (shown in the bottom picture), I figured that would be a problem, so I replaced it with a known good ECL RAM. That brought back some of the color, but sprites were still messed up. 

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The thing about sprites on DK Jr board, is that they are normally stored in the 2148 static RAMs, but I come to realize that this wasn't the issue. I ended up testing most of the chips in that circuit that could potentially cause an issue like that, but to no avail. So I figured that maybe for some reason, the ECL RAM might be bad, as it controls the color. It made sense as from a logical perspective, the game was running just fine, sprites were internally kept where they were supposed to be. But the thing is color and color position are calculated and done completely separate from the 2148s. And with colors still being messed up, there was a chance that it could have still been it. After replacing it, it brought all the colors back, and everything was positioned where it was supposed to be. EXCELLENT.

With the game boardset running, I could do a burn in test. This proved to be problematic, as I started having sprite issues again, this time it was a logical problem as sprites were kind of split up. Essentially it was a horizontal position counting error (sorry, I didn't take a picture, didn't think to). It ended up being a 74LS367 driver chip at 1P. This made sense as passed on the horizontal pixel count, eventually ending up at the sprite RAM's address lines. Replacing that fixed this issue, the old chip would just become flaky after warming up.

One final problem that I encountered was that digital audio was distorted and quiet. This screamed the MB3614 OP-AMP at 7K. These kind of OP-AMPs are a failure point in the circuit. Really, there are only three real points of failure with digital audio. The 8035 MPU that is used to generate the sound (of which I replaced with a NOS one), the DAC that converts it to an analog sound, and the OP-AMP that makes it loud enough to make it to the actually amp in the cabinet. Being it was distorted and not just complete garbage and not working at all, I figured it to be the OP-AMP. I replaced it with a LM324 as that was close enough to the specs of the original. This ended up fixing it.

After all of that, I now have the boardset working. It was a bit more challenging for me, especially the ECL RAM part as I've never worked with it before. What was nice about this experience as I got to learn a lot about how video is generated on DK and DK Jr boards, which I will take with me to the future as I don't think these are gonna be the last ones I do.

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Edited by SNESNESCUBE64
Fixed broken images

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