AT&T 4425 (Teletype 56D) repair

Seth Morabito lists at
Wed Apr 6 18:00:38 CDT 2016

* On Wed, Apr 06, 2016 at 03:11:54PM -0600, Eric Smith <spacewar at> wrote:
> SL0227 is a house number, and isn't likely to mean anything to anyone.
> It might be a Z80, but it could just as easily be a mask-programmed
> Z8.

This is good information.

I did measure a 3.3MHz clock signal on pin 6, which leads me to
believe it's a Z80 or derivative, but I will proceed with caution and
try not to make any assumptions.

Voltages look OK, I see the aforementioned clock, and my cursory
inspection for cold solder joints and broken traces didn't show
anything obvious, so next I'll dump the ROMs and see what secrets they

Seth Morabito
seth at

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