What is wrong?

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By Wierzbowsky

Guardian (3448)

Wierzbowsky's picture

03-07-2018, 01:01

It's pretty simple and doesn't require any tools. Works for sure with the 4116 chips, but may work with bigger ones as well.

You need to take a wire around 1mm or less in diameter, remove insulation on both ends and connect one end to computer's ground. Then you need to power on the machine, make sure there's some text on the screen and briefly touch pin 14 (DataOut) on each 4116 VRAM chip. Every time you ground the data output, check the screen. If the VRAM chip is working, the screen will change one way or another. If nothing happens when you ground the data pin, then the chip is 95% faulty. I fixed a few Russian and Japanese computers this way.

WARNING! You are using this method on your own risk! Please don't blame me if you ruin something in your MSX!

By OmaiGodman

Resident (59)

OmaiGodman's picture

03-07-2018, 09:25

I believe with Commodore 64 machines you can "piggyback" RAM chips to check if a chip is faulty, so you just take a chip that you know works (of the right type of course) and you put it on top of the chip on the motherboard. You then test whatever wasn't working because of the memory issue - if it works now then the chip that's being piggybacked is the faulty one (from what I gather the chip on top will simply "take over" the functions of the faulty chip). Not sure if this would work with an MSX machine but if the RAM chip's completely dead I can imagine it working.

I tried this method once because I thought my C64's RAM was dead but it turned out to be a faulty CIA chip instead LOL!

By Wierzbowsky

Guardian (3448)

Wierzbowsky's picture

03-07-2018, 10:37

If the broken chip pulls one or more signals down, piggybacking may not work. On my CBM the VRAM chips were pulling all 4 data outputs down, but I could only see that with an oscilloscope.

By Philip

Champion (380)

Philip's picture

03-07-2018, 12:42

Alexey wrote:

Every time you ground the data output, check the screen. If the VRAM chip is working, the screen will change one way or another. If nothing happens when you ground the data pin, then the chip is 95% faulty.

Ah, because dataout is active-low it will enable the chip so it will send data to the vdp and thus it is working... That makes sense, thanks for explaining!

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