[SiriusHardware]

This is an interesting contrast between what you had before with the apparently blank MROM in and what you have now - essentially what you had when you didn't have the Z80 fitted (your post #59) so it looks like:-

-The video circuit is working as it is independently displaying the content of the video RAMs

-The Z80 is not executing code, or it is, but not getting as far as the code which clears the screen, or it is, but the screen clear is failing due to some hardware problem.

Before we dive in too deep, let's look at a few basics which will also act as a kind of 'trainer' for your scope. From the sort of things you've already done I think we can assume you know how IC pins are numbered. I'm also assuming you have had a bit of a tinker with your scope by now and know roughly how to use it, but if you need any specific help please don't hesitate to ask.

As you may or may not know the ground of the scope needs to be connected to the 0V / ground of the system under investigation. You can do this either by connecting the flying earth lead attached to the scope probe to a ground / 0V point near to where you will be taking the measurement from, or the scope may have a front-panel earth socket intended to accept a 4mm 'banana' plug - if so you can take a lead from there to your system ground / 0V and that will provide the scope's ground connection to the MZ-80.

Set the AC/DC/GND switch for channel 1 to 'GND', then adjust the vertical position of the channel 1 trace so that it is sitting on the lowest line on the scope 'graticule', the 'grid' overlaid on the screen.

If the scope probe has a 'x1 / x10' switch on it set it to x1 for now, and turn the Volts / Div knob for channel 1 to the '1' position, not to be confused with '0.1 or .1' further around the dial.

Rotate the small grey knob in the centre of the larger Volts/Div knob fully clockwise until it goes 'click' at the position marked 'Cal' - and leave it in that position.

Now, set the channel 1 AC/DC/GND switch to 'DC'. Touch the channel 1 scope probe tip to a point which is known to have +5V on it and you should see the line jump five squares upwards, each square representing one Volt. If that's all working, then carry on.

Let's look first at the reset input to the Z80, on IC46 (The Z80) pin 26. You can do this check with a meter, with your logic probe or with your scope, but however you do it check that pin 26 normally has a high / logic 1 voltage on it, but goes low / logic 0 when you press and hold the reset button. When you let go of the reset button it will stay low for a little bit longer, then return high. If this is not happening, report back.

Using only channel 1 on the scope for now, look at the signal on pin 8 of IC7 and also at the signal on pin 8 of IC9. You should see square-waves on those two pins. If necessary, wind the Time / Division knob setting upwards or downwards in order to display several cycles of the waveform across the screen.

The service manual has pictures of what the waveforms on these two pins should look like, they are the first two waveforms shown on PDF page 13 / Manual page 11. Let us know if those two waveforms are present. I think they probably are, but they will be good waveforms for you to look at while you are getting to know your scope.

You may find that the waveforms don't display as a steady 'picture' but instead wander from right to left or left to right across the screen. We'll explain how to sort that out in a bit.