486 PC Ground Bounce Problem
I have a Ground bounce problem with my 486 pc.
The issues I have:
- PC freezes suddenly and when the freeze happens weird things (bit floats) are happening: for example ome time when I type the command: "dir" the PC writes "¿dir" and freezes at the exact moment. Also the freezes depends on which cpu frequency I choose. 50mhz is more stable than 33mhz. 25mhz even worse (already freezes at the bios self check) but 16mhz is more stable.
Also when I use an isa graphic card instead of the onboard s3 graphic it also gets a bit more stable. Using a sound card however make the freezes even more worse. So it feelg like the more the bus activity the board is doing the worse it gets.
Visual issues:
Graphical Noise at the same time when activity on the board happens (for example when a memory test is running).
Blinking pixels (wrong pixel colors) when I use true color mode or high color mode also changing a higher resolution makes the issue worse. In 256 colors the issue doesnt appear. Also I can change this issue when I put my finger on some areas the board (make it worse or less worse). But its not the pressure of my fimger that cause the change its the capacity of my finger that works like a antenna.
Horizontal Lines that stays
Diagonal Lines that moves. I can change the direvtion of
When I use a isa card all of those issues are gone except the horizontal lines and 2. idk bc the ramdac of the isa card isnt good enough to display true color.
Sometimes the hdd controler doesnt work. It just dont boot into the system and also didnt display any error messages. So I think the hdd controller just freeze
sometimes the pc also freezes when the floppy controller is doing something
Data gets corrupted or file names get unamed. For example "edit.exe" suddenly have the name "edti.ex e"
I checked all the tantal capacitors all of them are fine but there is one weird thing: Almost all tantals are connected with 5v and gnd. However 1 capacitor is only connected with gnd and not with 5v. Also this capacitor have always 0v when I turn on the mainboard all others receives 5v from the power supply. First I thought this is normal but all 5v pins on the board have a mark. So seems like the trace or via maybe is broken here. The problem is the 5v traces are inside the board so I cant see them. So I tried conneced the capacitor to the 5v line. Now it recieves 5v but unfortionally it doesnt change the issue. Maybe its important where at the 5v line it is connected?
Ram, cpu, power supply and hdd is not the problem bc I already changed those and the problem doesnt go away.
What else could be the problem here? Maybe a bus-logic chip that doesnt work anymore? Or a problem with the gnd traces or maybe the clk generator for the cpu?
Also when I put the mainboard outside the pc case the freezes changes so seems like it is a gnd issue but I dont know what cause this.