The Pictorial C64 Fault Guide

General tips - RAM (U9 - U12, U21 - U24)

It's not uncommon to see a bad RAM chip in a C64. Micron RAM chips ("MT" letters printed on the chips) are known to fail more often than the other types used in C64s. According to my personal experience, there may also be some correlation between long periods of inactivity and failures. The maximum voltage of the RAM chips is only 0.5 V higher than the normal 5 V, so they are also among the first to go in case of overvoltage.

The most common failure mode seems to be a normal blue screen but with "Out of memory error in 0" displayed in place of the normal startup text. Often there will also be some incorrect characters on screen, which can help pinpoint which RAM chip is faulty (see here). If a RAM chip is destroyed by overvoltage, it will more likely result in a black screen. In this case the faulty RAM chip(s) may also get hotter than normal, power up the machine and test them with your fingertip to check this. They normally run pretty cool so any that get hot are likely dead.

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