Reviving an ST M48Z35 battery backed RAM chip

The information on this page also applies to M48Z18/M48Z08/M48Z35Y/M48T08

There’s nothing we love in our equipment more than a chip with a non-replaceable battery which eventually runs flat, and costs a fortune to replace, assuming the part is even still available.

Today’s culprit is from ST. In this case the part is still available and not as absurdly expensive as some of the rarer varieties but I’m not feeling patient enough to wait for a replacement.

Let’s go out to the workshop and mill this bugger open…

Can’t see anything yet. Time for another pass…

Ah there we go. A small coin cell battery. There were some sparks and smoke when the mill punctured it (which I was expecting).

Next job is to clear up and attach some wires in place of the old battery. Be careful about where the battery matter ends up as this stuff is poisonous.

The battery is located at the bottom end (pins 14-15) of the chip.

I had a little bit of trouble persuading solder to take to the shell of the old battery, but it got there eventually. Inevitably the mill severed the negative terminal, so I had to dig into the potting compound to reveal enough of it to solder to.

And here’s the end result. The new battery is mounted in a Keystone 1061, but eh. Anything will do. I will never have to replace that chip again.

Posted in Repair / modification

One thought on “Reviving an ST M48Z35 battery backed RAM chip

  1. Neat! I’m going to have to resort to these procedures in the near future whenever Analog Devices decided to stop manufacturing the Dallas clock chips!

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