Zenith Fix

The "Zenith Fix" or "Zenith Mod" was a workaround developed by Sega to allow the Genesis/Mega Drive to work properly with Zenith branded TV sets. Fundamentally, the Zenith mod removes the HSYNC pulses that are typically present during the VSYNC blanking period, improving compatibility with Zenith TV sets. There appear to be at least two iterations of the fix: one using prototyping board and one using a bespoke PCB.

Photos

Photos were taken from a Model 1 VA6 that was serviced by the factory to have the modification.

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  Zenith Mod - Component Side

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  Zenith Mod- Solder Side

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  Later revision of Zenith Mod in M1VA6 - Solder Side

Installation Details

The Zenith Mod interrupts the CSYNC connection to the video encoder, and instead injects the modified CSYNC signal. This requires first severing the connection to the encoder (pin 10), connecting VDP CSYNC and VSYNC to the mod inputs, and then routing the mod output back to pin 10 of the encoder.

1. Disconnect CSYNC from encoder pin 10

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  CSYNC disconnected from CXA1145, Pin 10

2. Connect CSYNC from VDP to CSYNC input on fix

2. Connect VSYNC to to VSYNC input on fix

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  VSYNC Example Solder point

4. Connect Corrected CSYNC to encoder pin 10

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  Fixed CSYNC connected to DIN, pin 7

Schematic

Zenith Fix Schematic.pdf