MSX1 ROM/RAM problem?

Master (166)

29-05-2014, 13:47

Yesterday after 2 months, received a long awaited National CF-3000 from Japan.
I used a step down converter and I turned on the computer.
When it go to basic interpreter, even without keyboard connected, random characters are appeared on the screen, most of them are the same like a "random command" again and again. The keyboard works and I can type some letters, spaces, "return" or function keys commands, but all of this, between garbage characters.
I loaded some cartridge games from slot "A" and "B" and works fine. Very strange situation.
I suspect some bad RAM chips, or maybe corrupted ROM. I need some advices to find the problem.
If I know the problem, I will ask from a local electronic shop, to repair or replace the faulty parts.
If someone knows, please help.

By l_oliveira

Hero (534)

29-05-2014, 17:00

if games work, there's nothing wrong with the computer memory ...

I would focus my attention on the keyboard cable ...

What happens when the keyboard is disconnected ?

By bakoulis

Master (166)

29-05-2014, 18:59

Same things happens, with or without keyboard connected.
Is not a keyboard fault. I am sure the problem is from computer, not from keyboard.

By bakoulis

Master (166)

29-05-2014, 19:17

Every 2-3 seconds, computer produce 10-15 pseudo-random, letters and graph-chars mostly same every time, also without keyboard connected.
This thing, fills the screen after 20-30 seconds.
The computer have 64K RAM. Maybe this games use only 32K RAM.

By l_oliveira

Hero (534)

29-05-2014, 19:30

The problem is on the keyboard circuit.

Could be the resistor array which pull up the "sense" lines on the PPI ports being broken. That would cause "false" read of keys pressed. Keep in mind that the circuit I am talking about is inside of the computer.

By bakoulis

Master (166)

29-05-2014, 20:06

That sounds very possible.
Have you any extra advice or idea?

By psodas

Supporter (4)

09-07-2014, 00:21

Finally the problem was on the keyboard unit, and specific on "honda" connector. One grey cable was cut, and one more red which used as jumper between 2 pins, was worn resulting ground on metallic body.

Except this, the specific MSX computer need to power on with keyboard connected. In other case an array of characters repeated on the screen.