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

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By misterspin

Resident (56)

misterspin's picture

07-12-2011, 03:18

@ Manuel: Can you post a higher resolution one, so I can have a look at the chips and read the texts on them?
Okay, will do shortly.
<...> +5v exists<...> That's measured on the main board itself, not just @ the power supply?
Yes, it was measured on several points throughout the board.
With "get" I hope you mean "use/borrow", not "buy for this occasion" ?
Wink My son said he will try to borrow one from his college friends. Or else, as a last refuge, our local electronics store has 30 days 'no-questions-asked' return policy... Wink
...what's going on at various Z80 control signals. Especially the Z80 clock input, but also signals like /MREQ, /RD, /WR, chipselects at the ROMs (pins 20 / 22), and RAMs (/RAS, /CAS).
Okay, will figure it when I get an oscilloscope (hopefully, in a day or two)...
feel with a finger if any IC's on the main board become suspiciously hot
Just did... no, nothing too hot. normal warm temperature.
Z80: dead ones do happen, but it's quite rare (usually results from something inserted into cartridge connector while powered on).
Ah-ha! As far as I recall now I was plugging in (or un-plugging) an external floppy, and I think, I was dumb enough not to turn the machine off.
So if I'd had to take a gamble, I'd put my money on the VDP @ this point. Should that turn out to be the culprit: do you have a replacement for that?
No, I don't have a replacement VDP Sad I think, I've seen Z80s on eBay, not sure about VDPs Sad But, since Z80 now is prime suspect because of my cartridge screwup, I will probably check signals on it first, and then try to replace it, if it doesn't look good.
what's that 40-pin chip between Z80 (Sharp LH0080) and 8255?
It is M60222, could be seen on photo #3 in the album below:

More photos in hi-res. ( There is a way to zoom in in Picasa AFAIK ):

https://picasaweb.google.com/111314133182937932580/HiResHitachiH2?authuser=0&authkey=Gv1sRgCM-GwJvd-tvt8AE&feat=directlink

By misterspin

Resident (56)

misterspin's picture

07-12-2011, 03:31

Misterspin,
http://www.ebay.com/itm/Hitachi-MB-H2-MSX-Vintage-Computer-New-Boxed-Japan-H2-/290586266589:)

Grts Frits9164

Yeah, I've seen it. The listing is almost 6 month old. But thanks anyways... :)

By Eugeny_Brychkov

Hero (593)

Eugeny_Brychkov's picture

07-12-2011, 08:54

Ah-ha! As far as I recall now I was plugging in (or un-plugging) an external floppy, and I think, I was dumb enough not to turn the machine off.

What else preceded the appearance of the problem?
If all the stuff - machine and floppy drive - were grounded properly, then there should be no component damage. And there's less probability to "fry" machine if you remove than when you insert - because you remove just upwards completely off the slot, but you may insert it with one side lower than another thus in skewed manner and have theoretically shorten power pins with signal pins - but it's again hard to believe as you may shorten power pins this way (thus in worst case killing power supply). In your case it seems PSU works ok...

By misterspin

Resident (56)

misterspin's picture

07-12-2011, 15:58

What else preceded the appearance of the problem?.
Nothing unusual: I was loading some games, formatting / writing to floppies - normal stuff, no tinkering with the guts or anything.
<...>hard to believe as you may shorten power pins this way (thus in worst case killing power supply). In your case it seems PSU works ok...
OK, so summarizing all the responses, this is the plan, I guess:
It seems that first I will have to check if the Z80 is not fried. They are sold @ about $3-$4 on eBay, so this would be an easier fix.
Then, if the CPU is fine, I'll start looking at the VDP.

Either tonight or tomorrow I'll get an oscilloscope, which brings these questions:
What do I have to look at?
What is the best way to see iv the Z80 is functioning
, using the oscilloscope?
Which pin should show what?
Unfortunately, I don't have any other Z80 - based machines, otherwise I'd just compare them.

I will dig the 'net to find the answers, but since we are already discussing this, I thought, I might as well ask here.

Thanks, folks, for sharing your knowledge and being patient with a noob like myself.

By Daemos

Paladin (891)

Daemos's picture

07-12-2011, 18:00

I believe that the microcontroller should at least try to read some memory and data. and thus 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.

You should then also be able to see 5 volts on pin 11

a 3,58 mhz ugly square wave at pin 6

If you see absolutely no square waves at all the MCU may have not started or is fried for some reason.

By misterspin

Resident (56)

misterspin's picture

07-12-2011, 19:51

I believe that the microcontroller should at least try to read some memory and data. and thus 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.

You should then also be able to see 5 volts on pin 11

a 3,58 mhz ugly square wave at pin 6

If you see absolutely no square waves at all the MCU may have not started or is fried for some reason.

Will check that and report the results tonight (maybe even with pictures)

By misterspin

Resident (56)

misterspin's picture

07-12-2011, 20:05

@ RetroTechie: I myself have a homebrew logic tester I'd use for that, which has a small piezo buzzer that gives an acoustic indication of frequency.
I guess, something like this:
http://www.ebay.com/itm/2-PCS-Piezo-Piazo-Buzzer-Alarm-Speaker-Transducer-/280552095158?pt=LH_DefaultDomain_0&hash=item4152353db6#ht_985wt_1165
should work, right?

By RetroTechie

Paladin (1008)

RetroTechie's picture

07-12-2011, 21:26

Not unless your ears can detect sound in the MHz range... Wink

But that's indeed the kind of piezo buzzer that's on my logic tester. IIRC, I knicked it out of a supposed-to-repel-mosquitos thingie. Further it's got a 74HCT04 (6x inverter, used as buffer & for driving 2 LEDs), and a 74HC4040 (counter, used as frequency divider to bring MHz signals into audible range) + a few jumpers to select divisor. Built 15 year ago on the cheap, somehow still works... oO

By hit9918

Paragon (1370)

hit9918's picture

08-12-2011, 01:15

About cpu vs VDP: I guess testing the 8255 slot select pins is easy?

Interrupt works on FD9A etc system area, that got to be initialized first,
so likely the BIOS will select RAM in page 3 before getting stuck by a dead VDP.

So all slot select pins same = cpu dead, different pattern = vdp dead?

Except there are exotic things like cpu polling VDP before system area init
or some 32k MSX with RAM in slot 0.

By hit9918

Paragon (1370)

hit9918's picture

08-12-2011, 01:52

When turning on HB75, cassette motor relais does click! Same on Reset.
One gets so used to it that one thinks it is a sound of those keys.
Maybe it is a cpu test hint? It happens quick after reset, long before the screen turns blue.

The funny thing is, it makes a motor-off sound, without ever motor-on sound appearing.
Like it makes on/off so quick that the on does not hit the other side.
Maybe other relais make no sound, need to know your machine.

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