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Thread: XRGB-3 lost vertical sync on VGA out - fix

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    miisalo's Avatar
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    XRGB-3 lost vertical sync on VGA out - fix

    I have been using XRGB-3 with my Naomi Universal to display 15KHz picture from Jamma boards, mainly neo-geo, in the 31KHz monitor. Two days ago when I started the system, all I got was rolling picture. I first thought that the monitor chassis had problem but with little testing, I realized that problem was with the XRGB-3.

    After checking that it wasn't any settings, I took the xrgb apart and traced the components used for the sync. I measured the resistors etc and traced back to 74HC04 hex inverter that is used to buffer the sync and also invert the pulses. I measured the pulses on input and output and chip was cooked. From 6 units in the IC, only the first one worked as it should, the second one used for v-sync, produced pulse having only tiny drop on voltage. Rest of the units are not used so I tested them if I could use them instead using jumper wires. But all were cooked.

    I didn't have any spare 74HC04 ICs in SO14 package but I did find 74LS04 which in most cases can be used as replacement but in PDIP package, so I made temporary fix with few wires and the good old dip package and tested the XRGB. And it was working as it should.

    As I didn't have patience to wait until I get SO14 chips, I quickly checked that did I have any hardware that I could use to scavenge the chip and found that Gotek USB floppy emulators had the chip and I had plenty of those. So I took the chip from one of them replaced my temporary fix with it. Now I can later pick up chips from component shop and repair the gotek.

    So if you lose syncs (either or both vsync or hsync) on RGB out connector, it's probably the buffer/inverter chip and its quite trivial to replace.

    I made quick video about the scavenging and the replacement:

    https://www.instagram.com/p/BznjkX0H239/
    Last edited by miisalo; 07-08-2019 at 11:37 AM.
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