HP150 Touchscreen again

[Tony Duell]

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 [1] 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.

[1] 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 
was right. 

Now all I need to do is find a replacement.

-tony