MV-1A problem

Hi,
The board powers on, but i only get some glitches.
The board has been modded for 5v only.
There are no visible broken traces.
The screen flashes like this:
http://www.youtube.com/watch?v=ebmKuGZOsWM
Bios tested on another board and it's good.
I've checked the data and adress lines between bios and 68k and thery are ok. But i couldn"t test CE. Where CE should be connected?
I've tested working ram and back ram and they are ok.

With the system stuck in watchdoq like that, how were you able to verify that the Work RAM and Backup RAM are okay? Did you remove them from the board and test them with a programmer or something? Check the data and address lines between the 68k and the RAM as well. If that does not help any then check the states of those lines and also the BIOS lines with a logic probe.

Hi,
Yes, I removed both work ram and backup ram and they are good (tested on another board). Sadly, i dont' have a logic probe. But i did a liitle experiment. On a working board i removed work and backup ram and i get a similar problem. But the screen it's not flashing.

The screen typically looks like that whenever the system cannot start up at all. In fact, if you remove the BIOS entirely and power up the board with no BIOS at all, the screen typically looks like that. Try it to see if removing the BIOS makes any difference at all in what you see when you power it on (on a MV-1A the BIOS should be socketed so it is easy to remove). If all the traces between the 68k, BIOS, and RAM are good and you know that the Work RAM is good then I think it will be very difficult to diagnose the problem without a logic probe (or oscilloscope). It is possible to build a simple logic probe from common, easy to find parts if you do not want to buy one. The only other thing I can suggest is check the bottom of the board and make sure none of the pins are touching each other and none are bent over and piercing through the overcoat and touching traces underneath.

Thank you for your suggestions.
I noticed 2 things:
1. Bios CE wasn't conencted to GND
2. I did a stupid mistake when i did the 5v mod. I've cut the trace in the wrong placed.
I corrected both, but no changes.
If i remove the bios, i get a simillar thing, but the screen is more red.
Also noticed that sometimes when i power the board i only get a pink, blue or gray screen.

I am not that familiar with MV-1A boards, but are you sure the BIOS /CE is supposed to be connected to GND? If the data lines of the BIOS are shared with anything else then /CE and /OE cannot both be GND because that would give the BIOS exclusive use of the data lines all of the time. What is the logic state of /CE and /OE on the BIOS ROM?

OE goes to a 74hc32.
CE - i couldn't find a connection.
In FZ CE is connnected to gnd.
I don't know if i remember correctly, but i think the seller mentioned the fact that he switched the 5v and 12v lines.

gorgyrip said:

OE goes to a 74hc32.
CE - i couldn't find a connection.
In FZ CE is connnected to gnd.
I don't know if i remember correctly, but i think the seller mentioned the fact that he switched the 5v and 12v lines.

Click to expand...

If /OE goes to a 74hc32 then /CE probably is connected to GND if it has no trace connected to it. The board has two embedded layers you can't see which connect to GND and +5v. These layers make it easy to connect any point to GND or +5v.

If the seller switched the 5v and 12v lines that is very bad. 12v is enough to fry all the ICs that were meant to be powered by 5v (and that is most of them though the YM2610 should be okay). Whether the board has many fried ICs or just a few is not clear (a logic probe could help to figure it out) but if you look at the datasheets they will all tell you that immediate damage can occur if more than about 6.2v or so is applied to them. You may want to consider whether this board is worth trying to fix as the damage could be extensive (though it may not be too bad since you tested the RAM ICs and they were okay). It may be better to use for spare parts (though a lot of the ICs may not be possible to salvage).