Mac II to VGA to HDMI (StarTech Troubleshooting)

[Iesca]

I just acquired a used StarTech VGA2HD2 Component / VGA to HDMI Converter. I am using it with a Macintosh II with a 4•8 card (upgraded to 8•24 with additional VRAM). Between the card and the StarTech I am using the (normally robust) TTL Macintosh Desktop Adapter for converting between Mac HD15 and VGA.

The Mac-to-VGA adapter works beautifully when used with a VGA input monitor, so I know it functions normally. Previously, when using a cheap VGA to HDMI adapter powered by USB, I get a picture using that adapter, but it's a washed out green and the phase is out of whack, resulting in a legible, but otherwise terrible picture when sent to an HDMI monitor.

I figured that upgrading to a StarTech would improve the situation, but the best I can get is a black raster, and only on a couple of the settings (3-4-5, 8, VGA 640x480, for example). Any other (valid) settings for the adapter, including the various MultiSync settings, results in no picture at all, just a "No Signal" from the monitor.

The StarTech is being powered by a universal power adapter set to 4.5 V, which should be enough for a 5V device, but I also set it to 6V just in case and the results are the same.

 

[nyef]

The "washed out green" effect specifically (not necessarily the phase being out of whack) is because Mac cards produce a "Sync on Green" signal, where sync signals are transmitted on the green color channel either instead of or in addition to any separate sync channels, and quite a few systems that accept VGA input treat the sync levels (which are normally a slightly negative voltage) as black levels (normally zero volts). Most sync-stripping circuits and ICs do nothing to improve the situation.