Hi.
First time here, but long time EEVBlog Youtube watcher : )
I have an MSX computer that I cannot make boot up properly.
The machine is a Sony HB-T7. Some technical specs here:
https://www.msx.org/wiki/Sony_HB-T7The main problem is that when I turn on the computer using the ON/OFF switch, the machine starts up, but only shows a black screen.
But if I press the "reset" button, the machine restarts and reaches the boot up screen (blue background with MSX logo). This only works after the machine is ON for some time and then I press "reset". If I press it right after turning it on, it doesn't go to boot up screen.
I measured a few voltages and I found that in the CPU, a Z80, the voltage in the CLOCK pin (pin 6) against ground when I start the computer starts at 1.8volts. Then in the next 2 or 3 seconds, it decreases to 1.6volts, and finally, it jumps to 1.84 ~ 1.85volts and stays there.
Is this a normal behaviour? I measured the same in another working MSX and the voltage of CLK is always stable at 2.5volts from the very moment I press ON/OFF switch.
Another thing that I found is that the boot up screen, besides the MSX logo on blue background, only shows the available VRAM (128Kb), but doesn't show the USER RAM at all, but it should. So I am not sure if this is a RAM problem that makes the whole system to go nuts or a CPU problem that doesn't reach further than testing VRAM and then stops working. Or it could be a problem from a faulty crystal.
Anyway, my first question now is if these voltages and behaviour for a Z80 CPU CLOCK pin are normal.
Thanks a lot.