X68000 Compact RedZone VRAM Issue

[costa]

Hi there.
Long time since I last visited this space, hope it still thrives.

Looking for some technical advice, I have a few X68000 and used to do basic repair (recap, bad connections) - onl simple fixes.

But My Compact RedZone has a VRAM issue, which is more complicated to diagnose and repair.

It looks mostly fine, but some graphics are corrupted in the backgrounds. Please have a look at the pictures below.









I am looking for some suggestions on how I can identify exactly where the problem is, since it might be a bad line, a bad ram chip, or maybe a capacitor. How can I test the parts such that I identify a failure?

Any ideas?
thanks

[costa]

Just retrieved this computer from storage, potentially to give it another look.
Last time I checked, the video issues got worse that seen in the pictures above.

Anyone looked deep in the compact schematic, that would like to share some thoughts on this issue ?

[costa]

For future reference, will keep updating this post with any developments I find about this issue.

I was checked the components around gthe VRAMs, and controllers, including the oscillators.
I may have found the issue - the oscillator X4 50Mhz seems to be faulty - it's frequency keeps changing, all around the place to values like 15Mhz, 32, 65, etc...it never stabilises at 50Mhz.

For comparison, the other XTALs nearby lock at the specified frequencies (69Mhz for X7 and 38Mhz for X6).

Checking the tension also seems odd - output pin is stuck a 5v -it should not.

Because X4 provides a clock for Oscian (I/O) controller, I am betting on this component as the root cause for the sync issues and potentially even the memory errors. I ordered a similar part to replace it, and will update the results here (could not find the exact part, not even any documentation).

[costa]

Bad news.
I removed the oscillator, and it gives perfect 50.40Mhz in the test bench.
Something in the PCB is causing the issue, so back to the tests to see if I can find which component is causing the problem with the X4.

[incrediblehark]

Were you able to solve this issue?

[costa]

No, I have not sorted this.

I closed the computer, got it back to status it was before.

Completely usable, only the graphics gets messed up.
It may be the VRAM memory, or even a decoupling capacitor nearby, but I will have to come back to it at other time.

[buffi]

Did you run memtest68k?

I did a vram repair on an older model recently, so ics will be different but maybe this is helpful

https://buffis.com/old-hardware/retro-computers/sharp-x68000-vram-repair/

[costa]

Did you run memtest68k?

I did a vram repair on an older model recently, so ics will be different but maybe this is helpful

https://buffis.com/old-hardware/retro-computers/sharp-x68000-vram-repair/

Yes, I did, the error is in the GVRAM.


Now I need to do the work you have done, find out which CI is defective, buy I was hoping it could be a problem with some decoupling capacitor

Edit: Actually, the error seems to be always in bit 1 for every address, which makes me think that the error may be a bad trace.

[costa]

Someone could confirm to me, which bank is the GVRAM in the PCB ?
Also, as per my screencap, I understand that the problem is in the access to the GVRAM, does it seem right ?