so I got hold of a French CV (board model 91209 rev B), thinking that given its built-in RGB support it would be a good replacement for my current av-modded PAL model.
Upon receiving it I turned it on, and despite being in bad shape (extremely dirty) it started correctly, and I immediately saw a sign of bad vram chips (there was character corruption exactly like described in the technical manual). Apart from this the unit looked fine. So I took it upon myself to install replacement vrams (I bought a bunch of MB8118-10 chips) doing the 5V mod at the same time. I removed coils L2 and L3, and the left pin of L3 to a 5V source nearby.
Then I turned on the CV (without cart) and got a black screen. I turned it off again, inserted the Q*Bert cart, and back on. Now instead of a black screen I get a blue screen, no chars or anything else, just blue. I can hear the initial jingle and if I press 1 on the controller the game appears to start, but hangs shortly after. the screen is now black again and in the middle of the screen there is a single yellow artifact. For all that I know it might be a sprite. But at the point the CV is frozen, no sound, and no way to do anything else save resetting and going back to the blue screen.
At this point I also socketed the VDP and two ram chips, and tried installing replacements that I have sitting around, but to no avail.
I checked and rechecked all the solderings, and did continuity tests between all vram chips and the vdp, and everythin appears in order. Successively I also decided to deep-clean the board by washing it in a dishwasher and drying it in the oven at 65C. But nothing changed, still same blue screen plus freeze.
The next step I would try is to reflow the whole board by placing it into an oven at 250-300C, that might maybe fix any soldering issues.
Does anyone have any other suggestion on what is going on and what I might try before throwing the board into the oven? I have also ordered 2 replacement Z80A and will try to install that and see if it makes a difference. At this point almost the whole board is socketed save the bios, the sound chip and the various 74LS... chips. I do not have an eeprom programmer so I have no way of replacing the bios chip.
Side question: on the board there is a trimpot. I have not seen any of these on ntsc or pal boards. Any ideas what it is for?