diagnosing an Intel Series II MDS monitor failure

Eric Smith spacewar at gmail.com
Thu Jul 23 20:56:28 CDT 2015

A few weeks ago, while I was testing a spare IPB-80 CPU card in an
Intel Series II MDS, the monitor stopped working, with the raster
collapsing to a very bright dot in the center of the screen. I hit the
power switch and pulled the line plug immediately, but the dot
persisted for several minutes, only gradually dimming.

I just started investigating it. With the monitor cable unplugged from
the IOC board, the +15V DC at the IOC connector reads 0.74V. Of the
two DC power supplies described in the service manual, this MDS uses
the Power One. The +15V rail uses a separate transformer winding, a
723C regulator, a house-marked (12500-4) NPN pass transistor, and a
zener and SCR crowbar. The 0.74V makes me think the crowbar has
tripped and the 723 regulator is current limiting. Nothing else in the
MDS uses the +15V supply.

I was a bit lucky that this particular MDS used "method A" of
installing the monitor, which makes it easier to remove. Once the
metal shield is removed, the label was visible. It is a Ball
Electronic Displays TV-120 monitor, which is a common enough model
that it was easy to find the service manual online. I'll test the
monitor on a lab power supply to see if it's drawing too much current.
I may kludge up Rich Ottosen's PIC-based TV test pattern generator on
a breadboard.

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