AT&T 4425 (Teletype 56D) repair

tony duell ard at
Wed Apr 6 23:25:54 CDT 2016

> The "SL0227 / 404780" part appears to be a Zilog Z80 CPU of some kind,

The '404780' number looks a bit like a Teletype part number...

IIRC the Z80 has very odd positions for the power and ground pins, you
should be able to verify that it is a Z80 from that. Then stick a 'scope
or counter on the clock input pin to see how fast it is being driven, that
will tell you what type (plain, A, B, H) is needed.


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