LG OK75, Service Manual

Page: 30 / 87

3-5 Copyright © 2018 LG Electronics Inc. All rights reserved.
Only for training and service purposes.
ONE POINT REPAIR GUIDE
3. VFD IS NOT DISPLAYED WHEN POWER ON THE SET
When power on the set, any icons or characters on VFD are not displayed.
3-1. VFD (VFD801)
3-1-1. Solution
Please check and replace VFD301 on TOP FRONT board.
3-1-2. How to troubleshoot (Countermeasure)
1) Check if VFD_12V, DVCC_3.3V and VCC_5V are output from SMPS to VFD via the main board.
2) Check if the IC501 outputs VFD_CLK, VFD_STB, and VFD_DAT to the top board.
3) Check if the VFD grid current amplifier circuit on the top board.
Check the drive signal to the transistor’s(Q801,2) base.

If the control signals from VFD (DGND, VDD) isn’t output, replace VFD with a new one.
3-1-3. Service hint (Any picture/ Remark)
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R810(TP841)
Q801 Emitter
about
30 Vp-p
< TOP FRONT circuit >
< Waveform of the grid current driver>