Fixing a DEC Letterwriter (LA100) - test-drb@ccmp.vtda.org

12 Jun 2008

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Subject: Re: Fixing a DEC Letterwriter (LA100)
   From: ard at p850ug1.demon.co.uk (Tony Duell)
   Date: Thu, 12 Jun 2008 05:00:42 +0100 (BST)
     To: cctalk at classiccmp.org

  I have since found the full print set on
vt100.net, so I'm all set on
 that front. I've started tracing back from the logic controlling the
 LINE and CTS indicators but haven't found anything obviously wrong as
 yet. I have however read some funny-looking voltages off the 7406 in
 this part of the circuit: 11.something volts while the datasheet says
 30. Could of course be down to my test equipment as I'm limited to a
 basic digital multimeter at the moment. 
The 7406 has what's called an 'open collector output'. The output pin is 
connected to the colelctor of an NPN transsitor (the emitter being 
grounded), but to nothing else inside the chip. What this means is that 
the output pin is either low (transistor tuend on) or floating 
(transistor turned off), it'll never be pulled high bu that chip.

The 7406 can handle and external voltage oof 30V on the output (that is, 
you could connect a resistor between the output pin and any voltage less 
than 30V, in shich case that supply voltage essentially becomes the 
'high' output voltage). But there's no reason why you _have_ to use 30V. 
Any lower voltage is fine. In particualr, using this chip (and its 
non-inverting brother, the 7407) to convert between TTL and 12V levels is 
quite common, ans what I suspect is happening here.

-tony 
Make sure the power supply is working.  It's a multiple output deal and 
it's possible or even likely one output has gone away.   Also that machine 
will error mode if any PS output fails.  I have two working LA100ROs here
and worked part of the program when I was at DEC.

FYI the datasheet for the 7406 only give limits not how it will be used 
in the circuit. The 11V sounds about reasonable.

FYI the common failyure modes for LA100 were Printhead, PS and drive mechanics
[Motor and the sprague clutch for ribbon feed]. Logic board failures were 
commonly RS232 related (usually form really nasty ground loops >25V).

The printer can be start in diagnostic and feature setting mode.  It requires
paper in the printer and a ribbon with a working printhead to see the result.

Get the LA100 user manual as the serial interface does use CTS/ and RTS/ unless
disabled.  there are a bunch of DIP switched hidden under a flap in the printing
area that set up some of the features.

Allison