ALLPCB
Touchscreen Repair Cases
Repair Case 1
Siemens touchscreen TP270. Fault symptoms: powered with DC 24V, screen backlight off, current about 230 mA. Diagnosis process: touching the screen produced a beep response, indicating the program was running. Measured the high-voltage strip: 12 V DC was normal and the control signal ENABLE was a valid low level, indicating the fault was in the high-voltage generation circuit or the lamp was damaged. On inspection, one end of the lamp was blackened. After replacing the lamp the screen still did not light; the high-voltage transformer was found to be damaged. A suitable transformer was not available, so an LCD high-voltage strip with two tubes was obtained, wired in and secured. After powering up, the backlight operated normally.
Repair Case 2
Hitech touchscreen PWS500. Fault symptoms: no display when powered, current about 100 mA. Diagnosis process: touching the screen produced no beep, indicating the program was not running. Multimeter measurements of several voltages (3.3 V, 5.1 V, 12 V) were normal. The user reported an attempted repair during which the unit worked once while powered, but failed after power cycling. Analysis found a cold solder joint on a chip pin causing a bus error and preventing the program from running. A hot-air gun was used to heat the fine-pitch chip pins while pressing the chip with tweezers to reflow the solder. After this procedure, the unit was powered again, current rose to 470 mA and normal operation was restored.
Repair Case 3
Fujitsu touchscreen 650. Compatible models: 660, 670, 253, 280, 1120, 2610, 2620, 2630. Fault symptoms: touch handwriting not recognized, keyboard works, pointing stick mouse does not work, external USB mouse works. Based on previous repair experience, the touch panel glass was judged to be fine and the pointing stick was faulty. Replacing the pointing stick immediately resolved the issue.
Repair Case 4
Touchscreen shows no response. First check whether all connector interfaces are loose, then check for serial port or resource conflicts such as baud rate or IRQ conflicts; adjust resources to avoid conflicts. Inspect the touchscreen surface for cracks and replace if cracked. Check for surface dirt and clean with a soft cloth if necessary. Verify the control box indicator LED is operating normally; when normal the LED is green and flashing.
Repair Case 5
Touch becomes unresponsive after use. In some installations, inadequate grounding can cause the control box enclosure to accumulate static electricity, disturbing the internal operating field and causing touch failure. In this case, connect a wire to earth the control box enclosure and restart the device.
The cases above illustrate common touchscreen repair methods. If you encounter similar problems you can perform initial checks yourself; if the issue is not resolved, consult a qualified repair technician.
Common Touchscreen Faults and Remedies
This section summarizes common problems encountered during touchscreen repair: inaccurate touch, inability to calibrate, no response, and long response time, and provides common remedies.
1. Why is the touchscreen inaccurate?
a. For surface acoustic wave (SAW) screens, after a period of use the reflective stripes around the screen can become covered with dust. Wipe them with a dry soft cloth, then power down and restart the computer and recalibrate.
b. The reflective stripes of a SAW screen may be slightly damaged and cannot be fully restored.
2. Why cannot the touchscreen be calibrated?
a. The touchscreen controller may receive input signals before the host has loaded the touchscreen driver. Power down and restart the computer and recalibrate.
b. For SAW screens, the reflective stripes around the screen may be heavily covered with dust. Although some areas respond, calibration may fail. Wipe with a dry soft cloth, then power down, restart the computer and recalibrate.
3. Why does the touchscreen not respond?
a. One of the cables connecting the touchscreen to the host keyboard or interface port may be disconnected. Check the cabling.
b. The touchscreen driver may not be installed for the serial port number actually used by the touchscreen. Reinstall the driver and select the correct COM port.
c. The host may be a locally supplied OEM machine whose operating system image has been altered, causing nonstandard serial communication that is incompatible with the touchscreen driver. If feasible, reinstall a standard operating system and then install the touchscreen driver.
d. Check whether the host has frozen. Inspect the reflective stripes on the four edges of the touchscreen for scraping. Check whether the touchscreen control cable connection to the host or controller card is loose. For SAW screens, inspect the sensors at the corners for damage, cracks, or foreign contamination. Check whether the control cable is damaged, especially the cables around the screen and at the connections to the sensors for open circuits. Uninstall and reinstall the touchscreen driver. Check for virus infection on the host.
4. Why is touchscreen response time long?
a. Water droplets or mobile moisture on the touchscreen can trigger continuous input; wipe with a dry soft cloth.
b. The host computer may be low performance, with a low clock frequency; replace the host if necessary.
5. Why is part of the touchscreen unresponsive?
a. A reflective stripe in that area may be covered; wipe with a dry soft cloth.
b. A reflective stripe may have been scratched off by a hard object and cannot be repaired.
6. Why does the touchscreen appear fine but cannot operate the computer?
a. The touchscreen controller may be receiving signals before the host loads the touchscreen driver. Power down and restart the computer.
