NMS8280 problems

Master (198)

22-02-2014, 19:22

Hi all,

I received a NMS8280 via post.
The package took a beating during transport because the front casing is cracked.

When I start the computer up with a cartridge (ex. nemesis2) everything works fine.
When I remove the cartridge and start up then I get the blue MSX screen with the logo and the light of the FDD flashes but then the computer keeps resetting over and over by itself.

So my question is: is it just the FDD that are broken or is their an another possible reason?

Hope you can help me out.

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Por Manuel

Ascended (19225)

22-02-2014, 20:31

I guess there's no disk in the drive then? (If indeed no disk in drive: I expect some serious issue, but I have no idea what exactly... perhaps a broken connection somewhere...)

Por bakoulis

Master (166)

22-02-2014, 21:57

Maybe the a button from the front broken panel have stuck. Unscrew the casing and remove the front panel. Check all the front panel buttons and cables from motherboard. Also check the disk drive cables, if are have full inserted.
Try again with the computer uncovered and then put all back with the screws and try again.
Also try with floppy disks in both drives. Post again with the results.

Por AxelF

Champion (395)

22-02-2014, 21:58

It is known that the plastic front bezel becomes very brittle over time.
Would be nice if there was a design to create a new front with a 3D printer... HINT

Because of the rough handling, it is possible that IC chips are become loose.
Try to take off the cap, and push on all the chips on the board that could be sitting loose?

It is worth trying.

Por Gregory

Master (198)

23-02-2014, 11:47

I started taking the computer apart.
It seems to me that the computer had had some work.

There is a small circuit board right next to the diskdrives, that has been stuck to the chassic with tape.
On that separate PCB their are 7 IC's, I can't tell you what they are because there are no numbers on top of them (probably erased). If I have to take a guess I would say probably a memory expansion.
I didn't do anything so far just wiggle some wires here an there.
I started the computer up and made it all the way through the basic screen. FDD A seems to be working allright but if I give the command Files "B:" I get the message 'Disk offline'. So I guess FDD B has a problem (it's a mitsumi).

I haven't done anything really so far, just wiggling the wires of the expansion to get the computer to properly boot up.
So probably there's a bad connection somewhere.

How do I know if the memory expansion still works correctly?
On start uo I get Video Ram 128 kbytes en user Ram 128 kbytes.
Shouldn't it say 256 kbytes for the user ram?

Por Gregory

Master (198)

23-02-2014, 17:13

Oh and there is also a switch on the front of the casing, i think it was a push button because it isn't complete anymore.
It connects to pin 14, 27 and 28 of IC117.

Por Daemos

Paragon (2027)

23-02-2014, 17:50

Do you see a print soldered on top of the CPU?

Maybe you have 7Mhz upgrade installed. That could cause problems when reading FDD.

Quote:

How do I know if the memory expansion still works correctly?
On start uo I get Video Ram 128 kbytes en user Ram 128 kbytes.
Shouldn't it say 256 kbytes for the user ram?

nope that is hardcoded in the bios. If you want to know the real amount of ram use testram.com or something else that can test your RAM.

Por Jipe

Paragon (1586)

23-02-2014, 17:56

IC117 is the disk rom
14 is GND
28 is +5V
27 is /PGM on 27128 but A14 on 27256
the normal size of this rom is 16K , 27128
check if your rom is a 32k , 27256
the pin 27 invert 2 bank of 16k memory

Por Gregory

Master (198)

23-02-2014, 18:25

It reads 27256. It looks like an eprom to me, their is a piece of black tape over de window.

Por Daemos

Paragon (2027)

23-02-2014, 19:02

thats an eprom

Por Gregory

Master (198)

23-02-2014, 21:36

I just managed to run testram and got the following results:

Base Ram: slot 3,2 : 64 k size
Video Ram: 128 Kb
Megaram: not found
Memory Mapper: slot 3,2; 256k size
No failures

Funny thing:
the computer starts up perfectly when I hold the FDD's vertical. As soon as I lower them in a horizontal position everythings goes bad, the computer locks up and keeps resetting...