HP150 Touchscreen again
ard at p850ug1.demon.co.uk
Sun Mar 20 15:36:02 CDT 2011
A couple of months ago I asked about diagnosing problems with the
TOuchscreen board in an HP150.
Well, I took some time off that project  but finally got back to it. I
decided in the end to make a text unit that would plug into the
touchscreen PCB, produce the clock signal, accept the sync and data
signals and display the status of the 35 beams on LEDs.
 Sort-of classic computer related. I've been writing a set of articles
for HPCC on how to fix HP9800 machines.
Being me, I built it from TTL (actually HC and HCT parts). It only took
me a couple of afternoons do design and build it. It starts with a 4MHz
master clock, divided down with a couple of '393 counters. A '30 adn '138
produce a paair of spaced clcok pulses from this, a '02 combines one
ofthes with the sync signal from the tocuhscreen. These are all latched
in a '175 to procude 3 clocks -- the clock to the touchscreen PCB, a
clock to sample the data from the touchscreen PCB and an end-of-scan
pulse to latch the received data and send it to the LEDs. The data is
shifted into '4094 shift registers/latches whihc feed a couple of rows of
LEDs. The powrr supply stats as 12V from my bench supply. That feedsthe
+ve input of the touchscreen. A 7805 powes the logic and LEds inthe test
box. And a 7660A provides the -12V supply for the tocuhscreen PCB.
Anyway, after connecting it to the defective tocuhscreen PCB, I found
that one of the beams appeared to be blocked all the time. Blocking other
beams got the appropriate response from the test box, so I was pretty
sure the logic was all working properly.
Since I knew which beam was malfunctioning, I tested its IR LED in-circuit
with an ohmmeter (system powered down, of course, It read differnetly
from the others either side of it, so I desoldered it and tested it out
of circuit. It's open.
So the guy who told me that the emitter (LED) was the most likely failure
Now all I need to do is find a replacement.
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