Another black screen topic

Champion (376)

23-11-2016, 16:47

Almost more interesting as the msx quiz... I have a nms8250 which gives black screen.
So that's a nice opportunity to learn something about hardware.
+5v, +12v and -12v voltages are ok.
I've bought a logic probe (BK DP21) and started probing.
The probe blinks on pin 6 of the z80 and pin 8 of vdp, so it seems to be getting a clock signal.
However the probe does not blink on pin 25 (int) Does this mean the vdp is dead ?
That would be a nice opportunity to attempt an msx2+ upgrade ;-)
Also it does not blink on the address and data pins of the z80 but I don't know if it should without int.

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By Philip

Champion (376)

23-11-2016, 18:12

Also measured the reset voltage:
4,87v when reset button not pressed, 0,15 when pressed.

And I found an external memory mapper, which doesn't help.

By RetroTechie

Paragon (1563)

23-11-2016, 19:30

* Caps Lock response? Any sound on pressing CTRL-G (beep) ?
* Do you have any game cartridges that make sound without keyboard input? If so: try with & without external mapper, and see if you hear anything.
* What are you using for video output? Composite? RGB? RF?
* Any activity on the floppy drive(s) ?
* VDP probably doesn't generate interrupts when reset, needs to be initialized by first. Seems like Z80 doesn't get to that point. So no activity on /INT pin doesn't say much.

By Philip

Champion (376)

23-11-2016, 22:59

The caps lock light does not go on and there is no sound on pressing ctrl-g.
I tested Rollerbal and Konami tennis, both have a demo with sound effects but there is no sound. Both cardridges work on my hb-75p.
Also tested with an external mapper, which also works on my hb-75p.
Video output is RGB, the same cable and screen work with the hb-75p
There is no activity on the floppy drive.

I also replaced the roms but that didn't help.
And the sw101 switch is also not pressed ;-)

By NYYRIKKI

Enlighted (5669)

24-11-2016, 14:00

Philip wrote:

However the probe does not blink on pin 25 (int) Does this mean the vdp is dead ?

No, not really. It is likely that the boot newer goes so far that the VDP gets request to enable interrupts.

Quote:

Also it does not blink on the address and data pins of the z80 but I don't know if it should without int.

Yes it should... MSX BIOS does not use HALT, so Z80 should be running all the time.

By l_oliveira

Hero (531)

24-11-2016, 15:32

Z80 is actually a fragile IC and is easy to blow/damage. Hence why most MSX computers have buffers between the Z80 and the rest of the computer. That decreases ten fold the chance of the Z80 failing.

You should use a solder pump and remove it from the board. Install a socket and have it tested at another computer.

By Philip

Champion (376)

24-11-2016, 16:56

Thanks for helping so far everybody !
What if I replaced the z80 with a Z84C0020PEG such as this one ? Would that work ? I would put it in a socket ofcourse.

By RetroTechie

Paragon (1563)

24-11-2016, 19:33

If supply voltages, clock signals & reset line(s) are okay, and external RAM doesn't help, then either ROM (more likely) or Z80 (less likely) would be next suspects imho. So if

Philip wrote:

I also replaced the roms but that didn't help.
And the sw101 switch is also not pressed ;-)

And

Quote:

Also it does not blink on the address and data pins of the z80 but I don't know if it should without int.

Then replacing the Z80 would be in order. And if possible, test the old Z80 in another system.

I'd probably go for a CMOS Z80 (4, 6, 8 or 10 MHz version) from a respected distributor. They're not hard to find or expensive. Z80's from these cheapo Chinese sites have a significant chance of being fake. Like lower-speed version with markings changed. And even if you get a 20 MHz version, well... that's not the best match to replace 4 MHz Z80 on a large pcb like in the NMS8250. Short version of long technical story: faster isn't always better.

By Philip

Champion (376)

25-11-2016, 11:20

My only other msx is a hb75p which does not have it's z80 in a socket, so I would rather not mess with it.

So I ordered a Z84C0010PEG (8mhz version was not in stock) from a local dutch shop. Hopefully more reliable as the cheapo Chinese shops.

By AxelStone

Prophet (2794)

25-11-2016, 13:48

Good luck, there are really bad news when you turn on your computer and only a black screen is shown :-(. Recently my VG8235 has that fault, but was voltage regulator that was damage (12v line not working). Simply replacing it and working again.

Good luck and tell us if you reach the solution.

By Alexey

Guardian (2950)

26-11-2016, 12:07

With MSX the repairs are more or less easy. I am now fixing the Apple 2 clone board that has around 100 chips. Well, 6 ROM chips are goners, 1 chip in the clock circuit is a toast, 2 RAM chips had holes all over the memory space, video bank switching chip stuck at logical 1. Video mode and character inverse/blink select chip is glitchy. Besides, the board doesn't have a mask, so any tiny loose piece of tin can screw anything on it. So I am glad when I have to fix an MSX.