Severe MD1 VA4 issues, what could be the cause?

Hiya!

My PAL VA4 megadrive developed some issues after being stored for a couple of years. There are some games that play fine , others with small gfx glitches and others are completely broken.

Here is Ghouls 'n Ghosts with some static glitches on the screen (after the 28th second the camera focuses and it's easier to see them)



and here is Gunstar Heroes completely broken:



Sonic3 also has glitches like G'nG when underwater - it even slows down with vertical stripes of glitches but it does not freeze, once out of the water all is OK.

and this is the ram test screen, no errors found



So I poked around for broken traces and other visible damage and found nothing. I replaced the power rail caps with new quality ones but it made no difference. Measured the 7805s and they give a nice 5.04v each , also tried different PSUs.
I reflowed cart slot, 68000, ram ICs - no change

ATM I am out of ideas so if anyone has any suggestions please respond This is one of the good revisions so I'd like to fix it , I am suspecting it has bad ram tbh. Mega-Everdrive that is used for the testing works fine , Sonic also works fine so I guess it's a bad region near the end or something.

TIA for any help/tips!

 

VRAM is bad, likely the read back through the serial side is failing in one of the two chips (seems like only one bit is failing).

Mega Drive use a special kind of DRAM which can be read through a serial port (sequentially, for raster drawing) and a parallel (regular DRAM port, for CPU), which is called "VRAM". Created by a clever guy at Texas Instruments (the same guy who developed the TMS9918 videochip) that DRAM made things like fast VGA cards on PCs possible as early as 1987. Later on as ASIC complexity advanced the VRAM became less desirable.

Since the testing program reads the VRAM through the parallel port, there's no way it can catch problems with the sequential port. Before trying to replace VRAMs make sure no traces are broken, as its possible one of the traces might have corroded due to leaky capacitors.

If you had a problem affecting the whole memory all games would be broken.

 

Thanks for the info l_oliveira - I'll replace the ram and see what happens. It's the 2 TC51832SPL ones right? I have checked for broken traces, didn't find anything.

 

That's the CPU work ram. The video memory is the two weird ZIP (zig zag pin package) ICs between the videochip and the CXA1145.

 

ah, got it - I know what you are referring to
thanks again for the help!

 

Did the vram replacement today but the issue remains the same. I can't see any damaged tracks/components so maybe some IC is busted... no idea what to do next other than store it

 

Do yourself a favor and download the mega drive/sega genesis model 1 service manual at console5 wiki then check the traces between the 315-5413 chip (VDP) and the video rams as per the schematics using a continuity meter (beeper). If no fault is found and the RAMs are known to be good, then it's very likely the VDP developed a internal fault and that's the source of the glitches.

 

good idea - can't find a manual though for my md1 VA4 , all I can find are md2 ones ?!

 

https://console5.com/wiki/Genesis

For your purposes, VA0, VA1, VA2, VA3, VA4 all use the same VDP so the same wiring. Starting with VA3 they switched to a newer bus controller chip (315-5364) from the original 315-5308 found on VA0-VA2 and it contains the logic from the small patch board they had on VA0 which fix a issue with the VDP mode H40.

This page has extensive information on how a programmer see the Mega Drive VDP including some extra features that on the Mega Drive actually go unused (pixel and pallete buses, 128KB VRAM):

http://md.squee.co/VDP

At the bottom of the page there's a rom which is designed to show effects and features of the video chip. You could try running that and see what happens, too.