So I have a Lear Siegler ADM-5 terminal that doesn't work. Voltage
regulators are either bad or not there - and the electrolytics have
probably long dried out. RAM might be bad, no telling if the resistors
are out of spec. The caps lock light doesn't even change. Basically it
would take a heckton of money and time to get the thing working as it
originally did (and to me, it's not worth it).
However, the display board didn't seem to have any issues. The screen
just showed up as a bunch of block characters, but they were being shown
on the CRT without a problem. This made me really curious as to whether
or not I could feed another video signal through the connector coming
off the display board. I made a forum post (somewhere) about it and
asked if possibly VGA would be a compatible signal. I didn't know much
about this at the time and didn't know if it was a simple plug-and-go or
if it was much more complicated than that. Thanks to some forum users, I
avoided that and moved on to try and find a compatible signal.
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I tried finding a PDF of the service manual for the terminal online to
no avail (this was two years ago, so I don't know if it's there now or
not) and could only find manuals for the ADM-3A, which had a bit
different of a board. As a result, I bought one on eBay and crossed my
fingers hoping it would have the monitor specs. Well, lo and behold, it
did! I even added a sticky note into the book to say which color wire
went to which pin.
The monitor can handle basically any horizontal sync from 15khz to
19.2khz. As a result, I believed that I could get it to work with
composite video as long as I extracted the sync signal.
Well, the problem with composite is that it's loaded up with too much
crap. You've got RGB, Intensity, and Composite Sync all on one signal
that NTSC and PAL Televisions do an incredible job of decoding. However,
I can't just feed that directly into the terminal - which is meant to
display basically one particular thing that it was built for. (edit: for
clarification, composite also isn't the correct voltage.) Still, I didn't
know that at the time and tried anyway.
I built a sync extractor using an LM1881 IC and used the color chart I
wrote to hook it up to the driver board, using the 15V from the terminal
board.
And... it *kind of* worked. I was attempting to play a video from my VCR
(with it also showing on a small television so that I knew what I was
looking for), and every so often, I could at least tell that the picture
I was seeing was what the picture was supposed to be. It wasn't perfect,
but it was a start. I continued on messing around with this until I
finally decided that it just wasn't going to work the way I wanted it
to. I continued to research video signals and types to see what I could
possibly get to work on the monitor.
And then I came across MDA. Monochrome Display Adapter (format).
I knew that this had to have been the correct video format, but I didn't
have any device that could output it, and such devices would be really
expensive (an IBM 5150 with a Hercules card would run you a heckton). I
looked even further, but the only other compatible format would have
been CGA, but again, I had no such device.
Well, last year I bought a Tandy 1400FD, which has CGA output, and this
year I finally decided to try it.
And damnit, it worked.
For the first time ever, I got crystal clear video on that monitor. It
was beautiful, and I loved it. It was really nice considering the
Tandy's LCD didn't even work because it was also the first time I had
seen video output from the computer. I was in love.
.. Until 3 hours later when the capacitors finally croaked, the monitor
drew too much current, and the fuse inside the computer blew. Pfft.
Annoyed the hell out of me but y'know, it's fixable. At least now I
finally know what will work.
