My MSX machine just died. Is it final, or can it be fixed?

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By misterspin

Resident (56)

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08-12-2011, 19:50

Okay, I guess, then I will start looking for a RAM cart, and postpone further tinkering with the scope, unless someone else has some other ideas.

By misterspin

Resident (56)

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14-12-2011, 21:51

Just a thought: Is there anyone who would be willing ( and able ) to fix it at a sane price?

By misterspin

Resident (56)

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16-12-2011, 01:53

@RetroTechie et all
Someone is sending me a RAM cart, might take a week or two during holiday season.
Meanwhile I have calibrated the scope and re-checked the pins:

Pin 6 (clock) is on the first shot, for comparison.
Scale is 2 volts per square.
All shots are in the same scale (volts and timing).
https://sites.google.com/a/vassenkov.com/hitachi-mb-h2/oscilloscope-check-z80

Pin 16 has just 5 volts, nothing else.
Does that mean the VDP is dead?

Pins 19 and 20 have some garbage.

What gives?
Shall I replace the VDP first?

By RetroTechie

Paladin (1008)

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16-12-2011, 03:17

That looks fine as clock signal, so leave VDP alone for the moment...
Pin 16 is /INT, this should be high most of the time with very short low pulses, unlikely you'll see that on a scope easily.
Pin 19 is /MREQ, and shows regular memory request + refresh cycles (no garbage! Wink ). Same for pin 21 (/RD).
Pin 20 is /IORQ, it's normal if that's high >99% of the time (most program code involves very few I/O instructions).
And since it's probably just hanging in a loop or something, it's likely not writing anything either. So pin 22 (/WR) is also what you would expect.

It's too early to say for sure, but (if this machine comes back to life) I think you'll find that Z80 is working fine (have you pulled it out & re-socketed yet Question ). Looks like it's just reading & executing instructions, as normal. @ This point I'd suspect the RAM, and thus wait for that RAM cartridge to arrive. RAM failure is common for old machines like this, and if just 1 RAM chip is dead, the MSX will get stuck very early in the boot process. Other things you could check:

  • Signals on /CS (pin 20) and /OE (pin 22) of the BASIC ROM (IC12). One of these might be hardwired to +5V, but at least one should look like /MREQ or /RD pattern.
  • Signals on port A pins of the 8255 (PA0/PA1 probably low, PA2/PA3 possibly too, PA4-PA7 might be varying depending on what the Z80 is doing).

By misterspin

Resident (56)

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16-12-2011, 05:18

oops. my bad, sorry. I meant to say - 20 and 22 had strange shape... And - yes, I have reinserted the z80, as you've suggested. Smile

By misterspin

Resident (56)

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20-12-2011, 02:45

@ RetroTechie:
BTW: In case I will need to replace the Z80, does it have to be by the same manufacturer, or any will do?

 

By Daemos

Paragon (1161)

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20-12-2011, 09:22

Any Z80 will go. Clones will go as long as they are the compatible ones. I believe that it is even possible to use another clock graded Z80. The zilogs are very cheap though on ebay.

By flyguille

Prophet (2286)

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20-12-2011, 20:16

Please, post here, a full resolution pic of the mainboard, but the soldering side.... FULL FULL resolution... take pictures fron at 45º left side point of view & 45º right side point of view.

That way, a technician's eye can examine the solders.

 

By flyguille

Prophet (2286)

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20-12-2011, 20:17

you already tried to push in its sockets the ROMs chips (if any socketed)?

By Sky_hawk

Champion (265)

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20-12-2011, 20:24

Dunno if you tried already, but does inserting a game cartridge have any effect?
if it does boot a game cartridge it is most likely a ROM problem. 
check any socketed IC that starts with the number 27..... (like 27C512 or alike).
You might have luck just removing+reinserting them once or twice. (that usually solves small oxidation problems on IC pins in sockets)

Gr,

Jan

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