Mega Drive 1 only outputs PSG-Sound, SMS games don't boot

I have a faulty Mega Drive which I'd like to get working again, maybe someone here can help. It's a PAL VA4 unit.
Many MD games boot fine but have no sound, some just show a black screen and others are missing sound effects.
Here's Sonic 1 for example:

Master System games don't boot at all.

I've used ComradeOjs Multi-Test-ROM (http://www.sega-16.com/forum/showthread.php?30044-Genesis-SEGA-CD-multi-tool-homebrew) to further diagnose the issue. The tests "FM" and "DAC" produce no sound output, "PSG" works fine.
The 68k RAM test and VDP RAM test are ok, the Z80 RAM test however fails on the very first address.

What I've done so far:
First I replaced all capacitors. No change.
Then I've gotten a second, working Mega Drive and swapped the following ICs:
- Sony CXA1034 (IC12)
- Yamaha YM2612 (IC11)
- Zilog Z80 (IC6)
- Z80 RAM (IC7)
The other MD continues to work, so those ICs were definitely ok.

Any suggestions are much appreciated

 

If SMS doesn't work, there is likely issue with Z80. Have you checked over for bad trace or bad via? If you can find schematic for your model, try checking it.

 

Yes, it all points to the Z80. But I've spent a good amount of time probing around and checking connections to the relevant ICs (data, control bus) and couldn't find any bad connections.
Is it reasonable to rule out the sound circuitry (CXA1034, YM2612)?

 

Still haven't made any progress on this one unfortunately.
Maybe someone can chime in.

 

I am actually experiencing a similar problem on a JP VA4 console. Only sound out of the right channel works. Tried swapping ICs similar to you to no effect. (Confirmed good ICs by swapping into a US Genesis console). Also tried touching up all solder points on all surface mount resistors and capacitors to no effect. I'm stumped on it, so I shelved it.