Benq XL2410T, Can't handle 15KHz as the specs say

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Many monitors seems to refuse 15 kHz on VGA port. Thus use CVBS / SCART / Component etc..

Another nasty possible modification can be to replace crystals on the monitor controller board by half such that recognise a 15 kHz vertical sync as a 31 kHz vertical sync..

(if no suitable crystal is found, a D-flip-flop can be used)

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DVI/HDMI offers way more options for frequencies and resolutions.