19 Jul
2011
19 Jul
'11
11:41 a.m.
I managed to scavenge a Tandy 102 earlier today (1988 build, 32K memory).
Physically it's in good condition, and powers up, but the display's a bit
unwell, having an entire row of dead pixels 4 up from the bottom, and a
32-pixel high column of dead pixels 22 pixels in from the left (on just the
lower half of the screen, the upper 32 in that column are OK).
I am going to assuem that the display is similar (if not identical) to
that used in the M100. I don;t have any technical information om the
M102, but I have the M100 Techref to hand, and it includes the LCD schematic
The LCD is a 240*64 pixel unit. Electircally it's better considered as a
pair of 240*32 pixel untis one above the other. There are separate column
lines ot the display cclass for each column in the the top and bottom
halves of the display and 32 backpalne connections, for each of the 32
row pairs (row 0 in each half, reow 1 in each half, etc)
The HD44103 ICs drive the backplanes. It's not shown on the schematic,
but from the PCB layout, it may well be that each output of these ICs
goes to 2 pins on the display, one for each half.
If so, your single misisng row is probably a bad connection from a
backplane contact on the LCD glass to the PCB.
The HD44102 ICs drive the columns, each output driving half a vertical
column of pixels (remember my comment that it's 2 displays one above the
other :-)). So your mising half-column is either a bad output on a 44102
or a abd conenction from that to the display,
The fault seems a little bit more subtle than that, however; sometimes
(depending on which app I'm in, and the angle of the screen, and what I
type) I can see some garbage in the first two characters (12 pixels) of the
dead row - usually much fainter than normal "on" pixels (barely even
visible), and occasionally a fully-on pixel, but at the wrong x-offset for
whatever I've typed.
Welcome ot th world oF LCDs...
Each pixel on an LCD is electrically a cpaacitor, and unlike LED
displays, it's not polarised. So if you make a matrix of them, sneak
paths must ecist (they are blocked in LED matricies by virtue of the fact
that some LEds would be reverse biased on such a sneak path). If an
electrode is flating rather than bing clampled to the right voltage, you
will get ghosting, etc.
Somebody else mentioend cleanign the zebrastrips betwee nthe display and
the PCB (pull the whold thing apart and clean the PCB, the connector
strips, and the flanges o nth eglass where th estrips connect with
propan-2-ol). You might also check the sodlerign of the ICs on the LCB
board, in other machines I've had dry joiuts on the driver pins casue
similar problems.
-tony