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PostPosted: Mon Mar 19, 2018 8:05 pm 
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Greetings! Been lurking for a bit on these forums, finally decided to register! :D

A while back I acquired a toaster that was producing some minor graphical artifacts; not scrambled graphics but rather some glitchy graphics (Unfortunately I don't have any pictures of what it looked like, but I can take some shots of it when I get the chance) After troubleshooting it a bit I figured that the PPU or SRAM was likely the source of the problem.

I found a UMC UA6538 on AliExpress which I proceeded to buy as a replacement PPU. I desoldered the original chip and soldered in a used 40 pin socket that was lying around. Everything fit together, but when testing the 6538 with an SMB cart I got some interesting results. To begin, the picture appears to be a very flickery monochrome mess, where absolutely nothing can be made out. (I will post pictures soon.) It looks like something is wrong with the sync as well, the picture is wavy and jumps all over the place. When testing a cartridge, oddly enough, the music and sounds are noticeably slowed down. Not in pitch, but only in tempo. Everything else seems slower as well, things like Mario's jumps.

My guess is that this is a clone PPU possibly made for PAL consoles? I'm not sure but that might explain the slower timing and picture issues. I was also able to test the original PPU in the socket, which still produced scrambled graphics. (They've gotten much worse, I'm assuming that desoldering it may have caused some damage to the PPU.) However, the sound is normal and the picture is displaying fine.

I'm wondering if anyone can help me diagnose the issue here. I will post pictures ASAP. Thank you!


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PostPosted: Mon Mar 19, 2018 8:35 pm 
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If you're seeing a "flickery monochrome mess", it's probably because your television doesn't understand how to decode a PAL video signal - not only does it run at different frequencies, but it also encodes color in a different way.

The fact that the audio tempo is slower (but that the audio pitch is unaffected) serves only to reinforce this.

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PostPosted: Mon Mar 19, 2018 8:36 pm 
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Joined: Sun Apr 13, 2008 11:12 am
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Location: Seattle
Dr1nks wrote:
My guess is that this is a clone PPU possibly made for PAL consoles?
Correct. (see also)

A "normal" PAL PPU should end up running at 40Hz vsync (compare 50/60) and 12.6kHz hsync (compare 15.7k), due to the incorrect crystal for this PPU.

Oddly, I think color would actually work (I mean, there would be color, not that it'd be the correct color) if the hsync rate were correct, because for all versions of the PPU it generates the color modulation frequency by dividing the master clock by six.


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PostPosted: Mon Mar 19, 2018 8:38 pm 
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Joined: Sun Sep 19, 2004 11:12 pm
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Location: NE Indiana, USA (NTSC)
You are correct that the UA6527P+UA6538 chipset is made for PAL consoles that play NTSC games, such as the Dendy. The UA6538 isn't identical to the official 2C07, as the UA6538 has a later NMI for compatibility with Famicom games with mappers that contain a CPU cycle-counting interval timer. Games whose mapper has a timer like this (FDS, FME-7, VRC4/6, etc.) expect NMI to occur 20 lines before the pre-render line (or, equivalently, 21 lines before start of picture).

To replace 2C02, try a UA6528.


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PostPosted: Tue Mar 20, 2018 11:01 am 
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Joined: Mon Mar 19, 2018 7:23 pm
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Oh gee. I can't believe I ordered a PAL chip. I guess the possibility just went right over my head.

Welp, I appreciate all the helpful replies. I've ordered the right chip this time. Guess I won't be needing to post pictures then.

As for the sync, IIRC my monitor doesn't display colour when there's no sync. I was able to get colour on a consumer set though.


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