Need help repairing Philips MSX2 NMS 8250

Page 5/5
1 | 2 | 3 | 4 |

By llopis

Rookie (25)

llopis's picture

31-01-2019, 22:28

Oh wow, that's awesome. I'd love to give it a try, but I might need a bit of help.

Which of the ROMs should I replace with that? The 32K one I take it (IC119, it was labeled as Basic and Bios).

Any chance you could post a binary for that? It would remove another source of failure if I use the wrong options on a compiler or something like that.

By NYYRIKKI

Enlighted (5300)

NYYRIKKI's picture

31-01-2019, 23:29

llopis wrote:

Which of the ROMs should I replace with that? The 32K one I take it (IC119, it was labeled as Basic and Bios).

Yes, that sounds right.

Quote:

Any chance you could post a binary for that? It would remove another source of failure if I use the wrong options on a compiler or something like that.

Ah, I would so much hope that this forum would have attachment button, but it does not... Here the file is anyway...

(If you want to compile it some time later your self, try ie. Sjasm... Only needed parameter is input filename... On last line there was a typo though... The line should be DS #8000-$,255 to make the file correct size and reduce burning time by using "1" bits.)

By NYYRIKKI

Enlighted (5300)

NYYRIKKI's picture

31-01-2019, 23:56

Oh, stupid me... I just realized that you could not see INT going on & off since It was not acknowledged... Just in case you have VRAM problems I also changed the colors to Yellow on red... Now even if your VRAM is completely out you should see at least red color. Download link is still same.

By Wlcracks

Champion (317)

Wlcracks's picture

01-02-2019, 17:53

Do Yamaha VDPs go bad? More likely RAMs/ROMs..?

By llopis

Rookie (25)

llopis's picture

01-02-2019, 19:31

NYYRIKKI wrote:

Ah, I would so much hope that this forum would have attachment button, but it does not... Here the file is anyway...

(If you want to compile it some time later your self, try ie. Sjasm... Only needed parameter is input filename... On last line there was a typo though... The line should be DS #8000-$,255 to make the file correct size and reduce burning time by using "1" bits.)

Sweet! I'll try it as soon as I get a chance (might be late next week) and report back here. Thank you so much!

Wlcracks wrote:

Do Yamaha VDPs go bad? More likely RAMs/ROMs..?

That's the weird thing: I've already ruled out system RAM and the ROM. I'm thinking there may be one of those logic ICs that is messing up a data bus line. But I'll keep investigating...

By Alexey

Guardian (2238)

Alexey's picture

01-02-2019, 19:58

Wlcracks wrote:

Do Yamaha VDPs go bad? More likely RAMs/ROMs..?

The bad VDP happened to me only one time in my 30+ years of MSX experience.

By Wlcracks

Champion (317)

Wlcracks's picture

01-02-2019, 20:23

I think this guy repair (z80) board had some good tips of troubleshooting boards.

https://www.youtube.com/user/OneCircuit

Sorry for the spelling but somehow i cannot edit this on a ipad.

By llopis

Rookie (25)

llopis's picture

08-07-2019, 22:44

All right, I finally had some time to sit down and work on this machine again. The results are not good, but I learned a few things and I have several new questions.

The first thing I learned is that the top two ROMs (IC119 and IC118) did not have D1 connected to the bus!!! That's really shocking because I thought this machine was untouched. The ROMs come in sockets, but I figured that was from the factory, and the D1 trace runs under those sockets (which are solid). So that was really weird! That explained why I was getting really odd readings on D1!

I didn't find any other missing connections. I bridged D1 to the lower ROM (which was connected correctly) and... still a black screen! Although now D1 looks fine in the oscilloscope.

So after doing a lot of checks, I decided to burn the HelloWorld ROM that NYYRIKKI so kindly created. And... still seeing a black screen. At this point I'm thinking it's the VDP. Except that I start looking at the signals around the Z80, and I'm not sure they make sense. Maybe someone can check this for me?

The HelloWorld ROM seems to disable interrupts, initialize the VDP, send some characters, and HALT the Z80. Some things make sense: /CAS on system RAM is always high, because that ROM doesn't access memory and DRAM refresh happens just by strobing /RAS and the right addresses on the bus. That was fine.

The first odd thing I noticed is that the /HALT line was always high. Shouldn't it go low and stay low after the program is done executing? I'm not even seeing a down edge.

Also, /IORQ should only go low whenever the Z80 performs an OUT instruction, right? So we should see a few dozen /IORQ pulses when the computer starts up and then nothing. But instead we're seeing periodic /IORQ pulses every 14 us. Does the VDP or the MSX Engine or something else generate those pulses?

I even went as far as creating a ROM that's just this:

DI
JP 0000

And that's still generating /IORQ pulses.

Any ideas what might be going on?

By Angelo Sanna

Rookie (23)

Angelo Sanna's picture

11-07-2019, 01:02

llopis wrote:
Meits wrote:

There's a button for test purposes. On your first photo it's the white button on top. When that button is pressed the computer doesn't do much.

Is there an explanation somewhere of what pressing that test button does? The repair manual mixes that with the test cartridge, so it's hard to know what's what.

Edit: Oh! You're saying maybe it was pressed to start with :-) Haha... I doubt it, but I'll check!

Edit2: No such luck: Pressing the button does nothing visible. I might look at it with the oscilloscope tomorrow to see if any of the patterns change.

That "button" is to set the MSX in the service mode (see the service manual). You need the test cartridge from Philips, to access the service menu. But there are ROM dumps of this test cartridge, it was on the MSX Hans Otten website. But I don't know if it is still there in the backup of the website, since Hans quit his page. But if you can get these ROMS, it would be much easier for you to find the problem with this motherboard. You must place the test cartridge in one of the cartridge slots, I believe this must be slot #1. Good luck!

By Angelo Sanna

Rookie (23)

Angelo Sanna's picture

11-07-2019, 01:09

llopis wrote:

Any ideas what might be going on?

I found the direct link with the test cartridge for you. You can find it here: https://hansotten.file-hunter.com/special-msx-hardware/phili... "The service test cartridge for the Phililips MSX 2 takes complete control of the computer.
On the VG8230 it requires setting of jumpers, on the NMS8250/55/80 computer pushing the service switch on the mainboard."

Page 5/5
1 | 2 | 3 | 4 |