HP150 Touchscreen again

Rik Bos hp-fix at xs4all.nl
Mon Mar 21 05:45:14 CDT 2011

> -----Oorspronkelijk bericht-----
> Van: cctalk-bounces at classiccmp.org [mailto:cctalk-bounces at classiccmp.org]
> Namens Tony Duell
> Verzonden: zondag 20 maart 2011 21:36
> Aan: cctalk at classiccmp.org
> Onderwerp: HP150 Touchscreen again
> 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,
> 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
> couple of afternoons do design and build it. It starts with a 4MHz master
> 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
> 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
> PCB and an end-of-scan pulse to latch the received data and send it to the
> 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

Are those the same LED's used in the HP 16500 series touchscreens ?
As you may know I updated my 16500A to a 16500C version with a upgrade kit
from Agilent.
Upgrading included changing the touchscreen PCB, so I've an A-version which
I don't use or maybe one day will part out.
I think change is the LED's are the same, let me know If you want some of


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