[MZ-80 K / MZ-80 A / MZ-1200] DRAM failure: how to replace 4116 by 4164

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hlide
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Joined: Thu Jan 25, 2018 9:31 pm

[MZ-80 K / MZ-80 A / MZ-1200] DRAM failure: how to replace 4116 by 4164

Post by hlide » Wed Jul 31, 2019 11:42 am

French source: Remplacement de la RAM du TO7.

4116 chips are not reliable DRAM as they tend to be fragile and die probably because they need three voltages: +5, -5, +12.

Not only that, they are more and more difficult to find and not that cheap. On the other hand, 4164 are easier to find, less expensive and less fragile. And they only need +5V.

As a fact, MZ-80 K and MZ-80 A are using 4116 and I had to replace some defective DRAM on my MZ-80 K. At that time, I had to buy 24 x 4116 chips to be sure to cover all the needs.

So there is no reason we cannot do the same DRAM replacement as TO7 for our favorite MZ.

Here is the schematics about the differences between 4116 and 4164:
dram_pinout.png
dram_pinout.png (43.05 KiB) Viewed 51 times
Note that 4164 are 64Kbit and not 16Kbit so the MZ will use only a quarter of memory. For this, pin 9 (A7 which doesn't exist in 4116) is set to Vcc. We also need to disconnect pin 1 (Vbb -> NC) and pin 8 (Vdd -> NC) from the motherboard so there is no -5V and +12V rampaging around the DRAM:
preparation_support.jpg
preparation_support.jpg (127.94 KiB) Viewed 51 times
Then to connect the pins 8 and 9 to Vcc (+5V) so pin 8 can get +5V:
preparation_ram.jpg
preparation_ram.jpg (129.91 KiB) Viewed 51 times
If you don't want to modify the sockets, you can simply bend the pins 1 and 8 of the DRAM chips so those pins won't be inserted in the socket, making them not connected.

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