Fixing a DEC Letterwriter (LA100)

[Tony Duell]

> > I guess I'd start by checking all the PSU outputs. I doubt that's the
> > problem here, but PSU problems can cause the most amazing faults...
> >
> 
> That's indeed where I started and, according to the multimeter, all is
> OK. Could of course be something gruesome going on that I will only be
> able to detect when I take a scope to it...
> 
> > I do have the printset, but it would take me a little time to find it. As
> > ever I have no scanenr, so I can't provide a copy.
> 
> Don't worry - I have found the prints on Manx (they were "hiding" on a
> second page of search results that I managed to miss). For the
> archive: http://www.vt100.net/manx/details/1,4232

IIRC this unit is microprocessor-controlled 8085?) and the LEDs are under 
the control of that microprocessor.

What does it use for RAM? If 2114s, then the lucky-dip side of me says to 
check/replace them. I've had _dozens_ of that chip fail in all sorts of 
devices, with various odd results.

Other than that, I guess you start by checking the processor clock and 
reset signals, then see if there's any activity on the data and address 
buses.

-tony