View Sharp 28HW-53 (serv.man22) Technical Bulletin online
Most vertical faults can be traced to either software corruption or output IC failure. If the NVM corrupts, then it can
cause severe vertical distortion, no drive or fold-over at the top of the screen. An NVM problem can be eliminated from
the fault-finding procedure by reflashing.
+5V. At any voltage less than 2.7V, the output of the IC will be attenuated by 70dB, therefore if C511 or R509 fail, the
above symptoms with be noted.
(100nF). Replace it with Sharp part number VCKYTV1HF104Z.
Sometimes red, green and blue lines can be seen at the top of the screen. This normally means that the automatic
grey-scaling lines (generated during the vertical fly-back period) are not being blanked correctly. Adjustment of the G2
setting on the horizontal output transformer will normally cure this problem. However, it is possible for this fault to be
generated by mis-operation of the vertical fly-back circuit. First check that the +25V supply generated by D510/C520 is
not low (below +18V), and there is no excessive ripple. If there is a problem in this area, the fly-back circuit is unable to
generate enough of a pulse to send the spot back to the top of the screen. It is not unusual to find that either C520 or
R530 (feed resistor) to be faulty in some way.
location controls the vertical blanking point and if set incorrectly can cause red, blue and green lines to flash
intermittently at the top of the screen. This location can be incremented to 0E, 0F or 10 if necessary. 10 is the
maximum value allowable, as any setting greater than this will cause the blanking level to become unstable. If the fault
persists with a setting of 10, there is a fault elsewhere on the chassis.
You may experience a blank picture fault symptom that has been caused by a fault in the vertical stage. To alleviate
any problems should there be a vertical collapse, i.e. a line burnt onto the CRT face, IC801 monitors the vertical fly-
back pulse. If this is missing, IC801 will blank the picture by shutting down its RGB output. When fault finding this fault
condition, it is advisable to check the PROT pin of IC801 (pin 11). This should have a 50Hz pulse at 5V amplitude
applied to it. If there is a problem in the fly-back circuit, this pulse is not generated, or may be corrupted, and IC801
cuts off the RGB drive to the CRT base panel. Normally Q502 and/or Q503 being leaky or short circuit causes this
fault. In most cases the +25V line is also low.
The vertical output IC is the same as that used in the audio stage, so substituting the IC will prove if it is faulty.