Scrolling screen with Composite and vLA81

[Rabs]

Hi, I am new to the Zeddy and need a bit of help.

Originally I had a very dark screen, so replaced the RF module with a composite replacement. The screen was more stable but did not fix the issue, which I suspected would be the case.

So replaced the ULA with vLA81 and almost there but can't seem to stop the screen scrolling. I must be doing something wrong.

Update

Yes I was doing something wrong. See later post.

[Rabs]

This is the display I get with the original ULA (2C148E)

[Rabs]

At one point the display stopped scrolling, so I switch it off, had a cup of tea and then checked again and it started scrolling again.

[1024MAK]

So this looks like a lack of vertical synchronisation. The board that replaces the modulator, what do the instructions say? What do the DIP switches do (near the top right corner as shown in your photo)?

With a vLA81, you don’t need any additional back porch (and if this board that replaces the modulator had a NE555 in a socket, I would telling you to remove it).

You will almost certainly need to adjust the two variable resistors (presets) labelled TR1 and TR2 on this board. Have you tried to adjust these?

Mark

[Rabs]

The DIP switches on the modulator replacement control back porch.

1on / 2off = back porch
1off / 2on = no back porch

So I have back porch off at the moment on the modulator. 1 OFF, 2 ON

But on the VLA81, I have back porch on. BP OFF.

The 555 is not socketed.

I did try to adjust TR1 and TR2 but no change, will try again,

[Rabs]

The trim pots seemed to make no difference. Note at one point the display did stop scrolling but I don't know why and all I did was switch it off and on again and it started scrolling again.

Note I have to set the VLA81 MOD switch ON but when I set this OFF I get a stable display but dark.

[Rabs]

This is the video signal I see, when the display is scrolling. Vsnch pulses are at 50Hz.

[1024MAK]

So on the vLA81 you need the back porch ON. It also needs to be set to video output, not modulator output.

On the board that replaces the modulator, it needs to be set to back porch OFF. With it set like this, I don’t think the settings of the variable (preset) resistors is important.

However, the combination of the two boards may be producing an output level that the TV / monitor is not happy with. Most want a peak to peak signal not exceeding 1V.

Mark

[Rabs]

Thanks

Modulator board has back porch off, vLA81 has back porch on but also DIP switch 1 on the VLA81 is ON, extended such level. With it OFF the scrolling stops but display is dark.But pk-pk is much lower.

[1024MAK]

What are the levels with respect to 0V/GND?

Have you tried another TV / monitor?

Mark