I recently obtained a 99/4a. After purchasing a power supply and video cable (thanks arcade shopper) I attempted to boot the device. The HDMI monitor detects a signal, but the screen is completely black (there are perhaps some evenly spaced but very faint vertical bars?). fwiw the video cable does work with my C64. the output voltages on the internal PSU appear to be correct
based on what I read here on atariage, I purchased a replacement 9918 on ebay. this did not solve the issue. based on another post, I then removed the sound chip (SN94624) and tried both 9918s. this did not help. my next attempt was going to be to replace the ROM CD2156NL based solely on a blog post (at nightfallcrew.com).
does this make sense as the next step? are there better ways to identify the source of the issue? thanks!