I recently acquired what seems to be a real unicorn of a device, a Sharp PT-9000 tablet computer, circa 1994. It's a chunky tablet computer with a 640x400 B&W LCD paired with a resistive touch panel, that runs DOS and GeoWorks (PEN/GEOS, I guess?). The hardware is organized around the Vadem VG230 chip, which was also used in the HP OmniGo 100 and the IBM Simon. Main storage is with a battery-backed RAM disk.
When I first got it, installed fresh batteries, and turned it on, it took me many attempts to get past the pen calibration routine - it seems unskippable, and involves touching four points near the corners of the screen, and then touching within a very small box at the center of the screen (perhaps 1cm x 0.75cm). If you fail, the process starts over. (Maybe if I had an XT keyboard, I could plug it in and try to interrupt a cold boot
I did eventually get past calibration, but it was obvious that there was something wrong - there was a definite offset between where I was touching and where it was registering the touch, as well as more slowness than I expected (though I have no idea how performant this system actually was).
However, I had to remove the batteries again which cleared the RAM disk, and now I am stuck at pen calibration, and have tried dozens and dozens of times, to no avail. The touch panel obviously responds in some way, as I can tap on the screen to proceed through each step, but tapping in the confirmation box always fails.
I've never had a screen fail in this way before
I have opened the back cover and tried re-seating the touch screen's ribbon cable - it's a standard 4-wire setup - but there was no effect. But the connection and the cable seem sound, since it does respond to taps (so there's some change in resistance happening, that's being read) and there are no moving parts involved here that could have damaged the cable.
My theories now are:
1. classic resistive screen degradation, where the separator degrades and the upper and lower sheets make contact on their own
2. a bad ADC or other component involved in reading the touchscreen position.
I've read about applying heat to screens to fix the first issue, though how much heat and for how long seems like a total guess based on what I've read so far. I really don't want to damage this machine, and I don't have the service manual either (despite there being thousands of Sharp service manuals online, no one has this one). I also don't have the power adapter for this machine, and it has a non-standard plug, so powering it up while it's partially disassembled is a challenge.
Has anyone had a problem like this before? Should I try some heat, or check for bad components? My idea would be to heat up a hot/cold gel pack and put it on the display for a little while. I've read about people holding screens over gas stovetops or putting screens in the oven which seems a bit nuts to me. I also have access to a hair dryer, heat gun (eg. for removing wallpaper), and a hot-air reflow station.
When I first got it, installed fresh batteries, and turned it on, it took me many attempts to get past the pen calibration routine - it seems unskippable, and involves touching four points near the corners of the screen, and then touching within a very small box at the center of the screen (perhaps 1cm x 0.75cm). If you fail, the process starts over. (Maybe if I had an XT keyboard, I could plug it in and try to interrupt a cold boot
I did eventually get past calibration, but it was obvious that there was something wrong - there was a definite offset between where I was touching and where it was registering the touch, as well as more slowness than I expected (though I have no idea how performant this system actually was).
However, I had to remove the batteries again which cleared the RAM disk, and now I am stuck at pen calibration, and have tried dozens and dozens of times, to no avail. The touch panel obviously responds in some way, as I can tap on the screen to proceed through each step, but tapping in the confirmation box always fails.
I've never had a screen fail in this way before
I have opened the back cover and tried re-seating the touch screen's ribbon cable - it's a standard 4-wire setup - but there was no effect. But the connection and the cable seem sound, since it does respond to taps (so there's some change in resistance happening, that's being read) and there are no moving parts involved here that could have damaged the cable.
My theories now are:
1. classic resistive screen degradation, where the separator degrades and the upper and lower sheets make contact on their own
2. a bad ADC or other component involved in reading the touchscreen position.
I've read about applying heat to screens to fix the first issue, though how much heat and for how long seems like a total guess based on what I've read so far. I really don't want to damage this machine, and I don't have the service manual either (despite there being thousands of Sharp service manuals online, no one has this one). I also don't have the power adapter for this machine, and it has a non-standard plug, so powering it up while it's partially disassembled is a challenge.
Has anyone had a problem like this before? Should I try some heat, or check for bad components? My idea would be to heat up a hot/cold gel pack and put it on the display for a little while. I've read about people holding screens over gas stovetops or putting screens in the oven which seems a bit nuts to me. I also have access to a hair dryer, heat gun (eg. for removing wallpaper), and a hot-air reflow station.
