MSX AX-200 - Black video output

By Shinobi

Expert (84)

Shinobi's picture

06-07-2020, 00:42

Hi ..

I was given a rusty, smelly AX-200. When turning it on, it was:
-Black Screen
-No Beep even when pressing CTRl + G
-CAPS Lock always ON and won't change when pressing it.

Only one time when turning it on (I was just trying after several months of leaving it out):
-The MSX copyright screen appeard
-A long Beep
Then all of a sudden it went to black again

After cleaning it and get rid of rust and examining the board:
-It's clean, no rust or corosions.
-Voltages ok 12,-12, 5
-still black screen
-still CAPS lock ON and won't respond when pressing it.
-no beep even when pressing CTRL+G

What should I do??

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By Alexey

Guardian (2792)

Alexey's picture

06-07-2020, 15:55

Does it work with an external RAM expansion cartridge? If yes, then the on-board RAM is busted.

By Shinobi

Expert (84)

Shinobi's picture

08-07-2020, 02:55

/RAS pin on Fujitsu/MB81416/ two chips (VRAM) is reading 50+ V DC, and I am not kidding, I don't know from where it's coming.

By Shinobi

Expert (84)

Shinobi's picture

08-07-2020, 03:15

Now I figured out that RAS pin of the VDP of my MSX is giving 50+ VDC... is my vdp damaged??

By Alexey

Guardian (2792)

Alexey's picture

08-07-2020, 12:55

That is simply impossible if you measured the main voltages (+5, +12 and -12) and they were ok. If you would have +50V on board, then that board would be a toast in less than a second. Doublecheck your readings and main voltages.

By Shinobi

Expert (84)

Shinobi's picture

10-07-2020, 01:02

is it normal that the two upper pins (the 1st on your right and 1st on your left) of the eeprom nec d27256d be shorted??
A continuity test shows that they are shorted and I couldn't find the pinout for this chip

By Alexey

Guardian (2792)

Alexey's picture

10-07-2020, 12:07

If the ROM inside 27C256 EPROM is 16kb, it's no wonder if pins 27 and 28 are shorted.

By Shinobi

Expert (84)

Shinobi's picture

11-07-2020, 00:24

No, VPP and VCC are shorted, later I understood it because I found a pinout.
Also, for keyboard socket on the main board, seems that last two pins are both shorted and connected to 5V+ and later I knew after searching that this is normal.
With only a multimeter for now, how I check that chips are working??
Am I doing it right, set the multimeter to DC and check for example A4 of Z80, if it changes, from 5 to 2 for example then to 0 then to 5 , means it's working.
Also, check voltage on the IC VCC to GND, and check if there is any change in output pins.
Any suggestion on how to check ICs using multimeter??

Also, my PSU voltages are White -12, Orange 12, Brown GND, Red 5, two blue wires are shorted on the mainboard so that the PSU could work.

Thanks for your help, any suggestion will be welcomed.

By Shinobi

Expert (84)

Shinobi's picture

11-07-2020, 03:12

I am checking Z80 voltages, when putting multimeter in DC mode, sometimes all the Address pins are 5V (with no change)
sometimes some of them are 5V, others are 0V even after repeating checking that for more than 5 minutes. The Z80 chip gets hot after 5 minutes of turning the machine ON.