Need help repairing Philips MSX2 NMS 8250

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By llopis

Rookie (28)

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25-09-2018, 11:29

Hello everybody. I'm pretty new to the MSX (and MSX2!) world, but I'm pretty familiar with other systems and Z80 architectures. I was hoping that people could guide me a bit trying to diagnose and fix this Philips 8250 I got "untested".

When I turn on the computer, it powers up, but I get no video at all (using SCART cable).
I opened it up and I seem to have 5V everywhere I checked (CPU, RAM, etc).

This is the actual board (in case there are multiple types):

I first checked activity around the Z80 and everything seemed fine (clock, addresses, refresh, M1, etc).

Then I noticed that the left column of RAM chips (102-105 I'm assuming those are the video memory from the diagram I saw) get quite hot. I can keep my finger on them, but barely. On the other hand, the right column are normal temperature.

When I check the signals on those, they look pretty odd.
This is the data bus on the RAM chips. I guess it might make sense if it's all black with some sync or something? Still a bit weird.

The address signals look even more strange to me:

They seem to be always high except for occasional spikes down? That's consistent across all the address pins.

Then I looked at /CAS. On chips 102 and 104, it looks exactly like the address memory pins. On chips 103 and 105, it looks more like a normal pulsing pattern:

/RAS was more or less the same.

Then moving on to the non-video RAM:
/CAS is always high (is this normal??)
/RAS is a normal pulsating pattern.
Addresses look fine.
Data looks a bit weird. It's not clean square signals and instead I get ramps and weird values. I don't know if it's because the bus goes to high-impedance at times and so the reading isn't really valid. I'd be curious to hear from people with experience with other similar models:

Any thoughts on what might be going on? Or something I should test next?

Thanks.

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By hamlet

Scribe (2409)

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25-09-2018, 11:47

Welcome Ilopis.
Nice avatar! What version of Gunfright is this?

By CASDuino

Expert (91)

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25-09-2018, 12:51

Amstrad CPC version by the looks of it.

By llopis

Rookie (28)

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25-09-2018, 14:34

hamlet wrote:

Welcome Ilopis.
Nice avatar! What version of Gunfright is this?

Hahaha yes, it's the not-relevant-to-this-forum Amstrad CPC version. I usually have this one

but the forum didn't seem to like animated GIFs.

By luppie

Paladin (854)

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25-09-2018, 17:05

By Wild_Penguin

Hero (541)

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25-09-2018, 17:54

Welcome, llopis!

OT: I noticed the forum does not like optimized GIFs (i.e. they need to repeat every frame in totality, not just the changing part; I may misremember the actual term used in the GIF specification). Anyhow, while making my avatar GIF, I noticed I need to save it with right options from GIMP (without this optimization), otherwise something on this site (resizing script / imagemagick?) will screw it up during upload. [/OT]

Nice machine, I own one, too! Don't know how to help with troubleshooting, since oscilloscopes / signals and their interpretation are not part of my skillset.

By llopis

Rookie (28)

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25-09-2018, 17:52

Yeah, I got those. The Repair Manual seems to rely on a special cartridge I don't have. Is it publicly available somewhere?

By Meits

Scribe (5458)

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25-09-2018, 18:06

There's a button for test purposes. On your first photo it's the white button on top. When that button is pressed the computer doesn't do much.

By JohanMSX

Rookie (31)

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25-09-2018, 21:08

Good remark from Meits. I 'repaired' a 8280 once by just pressing this button. Easy thing to try.

By llopis

Rookie (28)

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25-09-2018, 21:40

Meits wrote:

There's a button for test purposes. On your first photo it's the white button on top. When that button is pressed the computer doesn't do much.

Is there an explanation somewhere of what pressing that test button does? The repair manual mixes that with the test cartridge, so it's hard to know what's what.

Edit: Oh! You're saying maybe it was pressed to start with :-) Haha... I doubt it, but I'll check!

Edit2: No such luck: Pressing the button does nothing visible. I might look at it with the oscilloscope tomorrow to see if any of the patterns change.

By luppie

Paladin (854)

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25-09-2018, 21:43

When powering up the machine, do you hear a mechanical click (relais)?
Does the drive spin up when powering the machine?

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