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Where to find 341S0851 (c) 1990 APPLE - LCIII boot issues

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Hello All,


Anybody knows where to find this MCU which has Apple programming on it? I am working on a very tired board which suffered from battery damage etc. - no boot of course. Aside from all sorts of issues and a need to replace dead CPU (yup-it looks like) it probably has bad 341S0851 chip. I am after either finding a replacement or establishing if it is bad or something else is. Easiest would be just to swap it... but I have no other board/replacement chip and LCIII boards are short in supply those days (at least in Europe).


Perhaps others know better exact behaviour of the chip. My issue - it keeps reset LOW so CPU & Integrated controller - Sonora are kept in reset state all the time. Not that they would boot anyway but I see no life with oscilloscope in the system at all.


This is what I suspect:

> It links with some kind of power management chip also apple branded 0120 (wow you can get a lot from that!) which monitors voltages including PRAM battery. Holder, battery and traces are all refurbished and good voltages. Same as PSU - recapped and load tested with electronic load, all voltages are good.

> It provides connectivity to keyboard & mouse

> provides various reset signals (i.e. power on reset) as well as interrupt priorities (IPL0-2) to CPU


Lifting 15th leg up clears active low on CPU & Sonora - confirmed with oscilloscope but I see no other activity on any other of pins of the chip. Is it waiting for something from the CPU after setting interrupts at


I have checked for known crystal issue and yes it suffers from it but after heating and confirming good oscillation with oscilloscope still no life in the chip. Reset is kept low.




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  • 2 weeks later...

in fact just a generic Motorola MCU but with specific programming so true, only another board semi-dead or dead for another reason. Good to know usually they suffer from failing quartz crystal which is easy to fix.

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