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, 03:29

Okay, I've borrowed an oscilloscope for several days, here are the first results / observations:

at first glance:
- most chips show some sort of activity (square waves) on some of the pins.
- some chips have either saw-shaped or some weird shape waves
- one or two chips seem to have no activity, at least on the first attempt

Now, a little more details:
I have checked the Z80, and most of the pins mentioned above by Daemos, have squares:
you should see square waves at least at pin 30 to 40 pin 12 to 15 pin 7 to 10 and pin 1 to 5.
https://sites.google.com/a/vassenkov.com/hitachi-mb-h2/oscilloscope-check-z80
(Of course, with my luck, camera battery died on third shot, so more pics to be uploaded soon.)

So, I've made the following conclusions / assumptions so far:
- the clock is working
- the Z80 is not fried completely
Not much, I understand. Someone with more H/W knowledge would figure much more, I guess.

By misterspin

Resident (56)

misterspin's picture

08-12-2011, 04:00

By Daemos

Paladin (987)

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08-12-2011, 11:59

Ok I see however that the readings on all the pins are the same.

Try some of the following: press some keys on the keyboard. Do you see the patterns change on the scope image?
Or when you start up the machine do you see the pattern change and then go into constant form as they do now?

I am not sure how to check the VDP. Don't have the pinout here right now.

By misterspin

Resident (56)

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08-12-2011, 15:42

press some keys on the keyboard. Do you see the patterns change on the scope image?
Nope. Actually, I didn't expect it to react to keyboard strokes, since the caps lock didn't light up.
Or when you start up the machine do you see the pattern change and then go into constant form as they do now?
Yes, whenever I reset the machine, it does exactly that.
I am not sure how to check the VDP.
Someone told me to check pin 16(INT/) on VDP (9928). If VDP works, there should be a typical sawtooth-shaped frame scan shape.
And there is one, I'll post pictures tonight when I'm home.

By RetroTechie

Paladin (1008)

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08-12-2011, 16:01

Thx for the detailed pix, misterspin!

Ok I see however that the readings on all the pins are the same.
That looks like it, but what you're really seeing is that there's same pattern on these pins. Pins 1,2,3 are A11-A13 so it's not unlikely they change at same time(s) when Z80 goes from one address to another.

At least it looks like the Z80 is doing something, but could you confirm directly it has a good clock signal (pin 6) ? This should be 'clean' 3.58 MHz square wave. Also I'd be interested if you can detect any activity on Z80 /INT pin (16). If VDP is dead enough that it still gives a clock but doesn't produce interrupts, it might be that Z80 hangs in a loop somewhere, and doesn't try to run a cartridge or boot into BASIC.

I spotted that the Z80 is socketed! Therefore, with machine switched off, pull it out, and re-insert into socket (be careful not to reverse position in socket!). Then, with an external RAM cartridge inserted, re-try Caps Lock response & CTRL-G tests. If unsuccessful, re-insert Z80 into socket once more & repeat tests. If again unsuccessful, replace Z80 (because it's socketed, cheap, and therefore dead simple to replace) & re-try the above.

If still unsuccessful, have a look at waveforms on pin 20 and 22 of the 32 KB ROM (IC12).

Personally I would just unsolder & swap the prime suspects @ this point (VDP, ROM, Z80), but since you don't have spares you'd have to investigate some more I'm afraid... (great that you have a scope though Big smile ).

By misterspin

Resident (56)

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08-12-2011, 16:33

RetroTechie, I've just realized that I mis-read 'RAM cartridge' in one of your earlier posts... Sad
Unfortunately I don't have one. I have regular (ROM) game cartridges, and that's what I ment when I said I tried to insert them and re-boot the machine.
My bad. Sorry for the misunderstanding. Sad

By misterspin

Resident (56)

misterspin's picture

08-12-2011, 16:33

Tried to look for RAM cartridges on auctions...
Ouch. The least expensive one is $70 on eBay.
But there are some @ around $20 at Japanese auctions... Will try to get one, but for now will nave to do without it.

By RetroTechie

Paladin (1008)

RetroTechie's picture

08-12-2011, 17:10

Tried to look for RAM cartridges on auctions...
Ouch. The least expensive one is $70 on eBay.
But there are some @ around $20 at Japanese auctions... Will try to get one, but for now will nave to do without it.

Forget those - try to borrow an external RAM cartridge from someone. Risk for using it is minimal if power supply is okay, and even the most simple 16KB or 32KB RAM cartridge is useful for diagnostics. Because if RAM has failed, the Z80 can't:

  • Call subroutines (needs a stack).
  • Respond to interrupts.
  • Initialize system variables.
  • Run ROM cartridges.
  • Start BASIC.
  • etc, etc.

(So you might see similar things as what we're seeing now, even if everything else is okay). Inserting an external RAM cartridge is a quick & easy way to detect a RAM failure, so that really should be tried before wasting time to investigate other causes. And memory (especially RAM!) is often the cause of failure in old machines like this Hitachi.

By Daemos

Paladin (987)

Daemos's picture

08-12-2011, 18:17

Just a quick hunge: isn't it possible to take out all the RAM chips and try them one by one in a socket on the first slot? You only need one page right?

By RetroTechie

Paladin (1008)

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08-12-2011, 18:45

Those are 64K*1 chips, each chip provides 1 bit in all 64K memory locations. So main RAM fails if any of the 8 is bad/gone (=you need them in multiples of 8 to put any size RAM together).

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