MSX2 stuck on MSX Logo screen

Por Sodius

Supporter (1)

Imagen del Sodius

26-03-2020, 08:22

Hello.

I have an MSX 2 that I cannot make it to properly boot up.

When I turn on the machine, the screen stays just in a black screen.
If I press the RESET button, most of the time reaches the MSX logo screen, but stops there. No keyboard sounds, caps lock key not lighting up. Also, pressing RESET doesn't always work. Something it just stays at the black screen, but I think it always works when I let the machine ON for some time and then I press RESET (but not sure about this, maybe it is just a coincidence).
The RESET button is in the keyboard, as a normal keyboard key (not connected directly to the machine like a separate button) that communicates with the machine through a ribbon cable, so I believe that the keyboard input is being read by the machine.

The machine had 2 batteries that were removed, so I'm not sure if that could be the cause. I think it's unlikely, but who knows...

I tested voltages in the voltage regulators and they are fine. Also I replaced a polyester capacitor that was death with another with the exactly the same specs (volts and capacitance exactly the same).
I tested all the PWR voltages in the RAM ICs, Z80, Video chip, ROM, etc.. and all of them are fine and stable.
Also, voltages in cartridge pins are fine.
Also, the MSX logo screen is sharp and clear, no video glitch of any sort that I can see.

Then, after reading the forum, I found a thread about something similar:

https://www.msx.org/forum/msx-talk/hardware/diagnostic-dead-...

The reply by Eugeny Brychkov says to measure the voltages in CLK and RST pins from the z80. I did so, and I obtained, voltage in CLK pin = varies from 1.70v to 1.84v.
Voltage in RST = stable ~5volts.

I noticed when I turn the machine up, the CLK voltage in the Z80 starts at 1.80 something volts, then it lowers little by little down to 1.65v for the next 2 or 3 secs, and finally jumps up to 1.84v ~ 1.85v the rest of the time and stays there.

In comparison, I measured voltage in another working MSX2 machine and its Z80 CLK is 2.5v stable from the very beginning.

What could it be the problem? I was considering either a bad Z80 or some problem with a crystal. But I would like some opinion before I start replacing parts.
Also, I don't have access to oscilloscope.

I tried several game cartridges as well as a memory mapper (SD512) and I obtain the same results.

Thanks.

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Por Dolphin101546015

Master (254)

Imagen del Dolphin101546015

05-04-2020, 21:36

If you see logo, z80 working nice (as VDP too)
Last time one my friend have same problem.
He bough mother board without anything and not able to start it first time.
Then I told him reflashing bios to cutted out FDC ROM, and it start with any cartridges, but when he trying start to Basic, it hungup again.
After it he change SRAM chips to new, and machine working with Basic also.

Por Meits

Scribe (5804)

Imagen del Meits

05-04-2020, 23:19

What's your MSX model? In the rare case you have your internal ram in slot 0 a memory mapper cartridge won't be the first in line.

First things I'd try would be changing ram and recapping the whole machine. Even if the latter isn't the solution, it won't be the reason for problems in the next decades either.