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Donkey Kong - No Boot/Resetting


SNESNESCUBE64

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Yet another Donkey Kong boardset that boots to garbage. This one was a bit more interesting as there were two main problems with this board. Whenever the game does not boot, I always check the ROMs and make sure that the game can read the ROMs if I can check that. The ROMs all checked out good using my programmer, so now it was time to check to see if the game board could read them. The way I check is by using a Fluke 9010a with a Z80 pod. What this does is it emulates the Z80 CPU and allows me to run troubleshooting, such as ROM and RAM checks. To check the ROMs on Donkey Kong, you are going to need to run checksums on the address range of 0000 to 3FFF. Attached are what the checksums should commonly return, a different rom set will return different values, but these are the most common for this type of board.

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After verifying all of the ROMs and that they could be read with no issue. The next check was to check the CPU RAM. There are six 2114 SRAMs (1kx4), resulting in three pairs of two. The way they work on Donkey Kong is that they are set up in pairs to form a full 8 bit bus since the 2114 only stores words that are 4 bits wide. They are separated into the upper and lower bits, the lower being D0 through D3 and the upper being D4 through D7. Often times when these fail, the data will be messed up for only one. 

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While running through my RAM checks, I found that there were two RAM chips that had actually failed: 3C and 4B. After replacing these, it now passed all of the RAM tests, but it still wasn't booting. It looked like it was actually resetting while trying to display part of the title screen.

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What was weird about it was this is that nothing looked out of place with any of the control signals, so I ended up doing some research and found that the TKG4-14 has a watchdog circuit (I believe that's what it is). One way to disable this for testing is to cut the jumper CR3. To my surprise after cutting the jumper, the game booted up! After probing around, I found that pin 5 was actually floating. It ended up being a problem with C164, that cap had tested open, the watchdog was always activating. After replacing it, the CPU board worked exactly as it should.

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One new capacitor and two new RAMs and this game is now back in working order, on to the next DK!

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