X68000 Repair: Need help with video issue

[wolpi]

Hello everybody,

I have a problem with my newly acquired X68000. I recapped the machine completely (except the floppy drives), changed the power supply and got it running. However, the (VGA-) video output is corrupted. It usually starts like this:


(Noisy white vertical lines, static blue vertical lines)

After a while (or with some fiddling of the video-output board) it changes to this image:


(White noisy vertical lines on the left, static blue vertical lines)

Unfortunately I am no expert in electronics. I ordered an oscilloscope (but I don't have it yet - and I'm also not sure if my knowledge is enough to make any reasonable readings) and would appreciate any help diagnosing the cause of this fault.
What I found so far:

1.) The intensity of the white noise changes when changing the intensity of the green signal (there are three potentiometers for the three colors)
2.) The intensity of the blue lines changes when changing the intensity of the blue signal
3.) Changing the intensity of the red signal doesn't really change anything

When looking at the schematics of the mainboard:



The issue COULD be the video processor (IC50), the DAC (IC14), the VRAM or one of the transistors or resistors.

When looking at the schematics of the video output board:



The issue could be a transistor, resistor or one of the caps (which I replaced with 16V 470uF models instead of 10V 470uF). The issue could of course also be my amateurish designed VGA-Adapter which I realized through a custom PCB.

Could anybody help me with a more experienced perspective?
Thank you SO MUCH already!

[Edit/Update]
I just noticed that inbetween all the OKI M51C262-10 memory modules there is one slower single MN47464L-12 module, which seemed to be soldered in by hand. However, when I opened the X68000 everything seemed pretty much in its original condition. Only the PSU was defective due to leaking capacitors. Could this module be the problem - or could it be a VRAM problem in general? I don't want to perform a shot in the dark and order new parts which aren't the culprit.

[wolpi]

Just a short update:

I now removed the non-fitting VRAM chip and the problem persisted. I removed another VRAM chip and the problem got worse. So I did the hard work and desoldered the 16 OKI DRAM chips, added sockets (the shield bulges a bit now) and inserted brand new DRAM chips - and voila - the problem is gone.

However, my X68000 doesn't really like me

I now got everything running (including the X68000 booting from a SCSI2SD device) but following problems do occur:
- After some minutes the machine randomly freezes. Afterwards, a reset doesn't work anymore and it usually takes multiple attempts/power cycles to get the X68000 running again. The freezes occur basically everywhere - in the basic OS / command prompt or in a game (the graphic was shifted and the game totally frozen).
The memory test software MTEST is running fine. Could the freezing issue still be a memory issue?
- The speaker is extremely silent. When I maximize the volume knob, the speaker outputs a relatively loud noise/hissing. However, when I start a game afterwards, the game sound has a VERY low volume (however, it is there). I haven't tested the headphone output yet - but I will do that, soon.

Does anybody have an idea what could be wrong with my machine - and how to fix these two remaining issues?

[kamiboy]

My condolences, the X68000 is a complicated beast and random freezes are of the most cryptically difficult classes of computer faults.

The cause could be anything really. CPU, ram, buffer IC’s, traces, custom IC’s.

The audio issues are usually easier to diagnose though. I would have a look at the op amps, otherwise a google search should net you a bunch of threads by people with similar audio issues who managed to solve it.

Of your machine is one of the ones once equipped with a barrel battery, which no doubt leaked at some point, I’d go through all traces and vias around the battery and make sure they are ok. Same with any components around there.

[ShootTheCore]

You mention that you changed the power supply. What did you change it to?

[Hoagtech]

The first thing I would do is check your capacitor legend and check off each location looking for orientation, bridging, cap values etc..

I did a recap on an ms826 and had one cap reversed. I found out by checking continuity.