Z88 LCD panel

[Jbizzel]

One issue I have with either screen is the screen seems to have uneven contrast. This is true with both screens, so it must be some issue with the board?



With the new screen it's only visible when the contrast is very low (pictured).

Any thoughts about what might be causing this?

[bwinkel67]

The old me would have cavalierly opened it up to see if cleaning the contacts to the electromeric strips would help. However, they are so compacted down, that unless moisture got into them, that shouldn't be an issue. Plus, knowing how the LCD is constructed now, even being careful to open it up would still make me feel like I'd risk breaking something. The main circuit board (on the metal bezel one, at least) is made up of surface mounted IC's so there is little that can be done other than replace a defective one if it can be determined that's the cause (i.e. didn't really see any capacitors, or the like, that could be replaced).

There is a second variable resistor (VR2) on the motherboard. VR1 is the contrast and attached to a knob on the side. What does VR2 do? I will say that the plastic bezel screen (the one completely enclosed) has a slightly lower range or contrast. It goes fully blue but when dimming, it doesn't completely disappear, so I wonder if VR2 adds something to that...haven't decided to play with it since my screen is pretty good now.

Btw, I did a video of the repair, though it's mostly talking over the pictures I took:

https://www.youtube.com/watch?v=AMgFMgNfDEQ

[bwinkel67]

Btw, I have the pleather case so I keep it in that always....so it's never been in sunlight when stored.

[Jbizzel]

I think the screen is fine, I think the issue I'm having is at the z88 end.

I think this because I have 2 screens and both display this behaviour in this particular z88.

Could it be a bad capacitor or something like that?

[bwinkel67]

I think the screen is fine, I think the issue I'm having is at the z88 end.

I think this because I have 2 screens and both display this behaviour in this particular z88.

Oh, I didn't realize it was across screens.

Could it be a bad capacitor or something like that?

It doesn't seem to use those leaky 90's surface-mount capacitors. You could try replacing the electrolytic ones but on my 1984 QL, I haven't touched them and things work well. I follow Adrian Black, from Adrian's Digital Basement, who advises to not replace 70's and early 80's ones if they look good. Seeing as how cheap Clive manufactured things, I would suspect his 1986/87 creation may fall under that.

Have you tried adjusting the VR2 variable resistor on your circuit board. If VR1 is contrast, VR2 could be brightness or saturation which could equate to how much each pixel differentiates from the background (i.e. darkens the each pixel that's on as opposed to darkening every pixel).

[g7mmj]

The service manual is available on line at https://cambridgez88.jira.com/wiki/disp ... bridge+Z88 (it's in there somewhere, although I get an annoying "Browser not supported" error since they changed something a while ago )

Anyway, the relevant paragraph says:

<quote>
1.2 Adjustments

There is one adjustment only on the z88 main board. With the index
page displayed and potentiometer RV1 set to minimum the menu topics
should just be visible on the screen. If the topics are not visible adjust
padding potentiometer RV2 until the topics are just visible. Padding
capacitors C17 and C18 are factory set and must not be adjusted under
any circumstances.
</quote>

Hope this is of some help.

Steve

[Jbizzel]

Hi guys, I gave that a go, and tbh it was already set perfectly.

There isn't really much of a problem, because at normal contrast the z88 looks great. Just curious why the contrast is so unbalanced at lower contrast levels. Something that has present on 2 separate screens!

We may never know!

[bwinkel67]

The service manual is available on line at https://cambridgez88.jira.com/wiki/disp ... bridge+Z88 (it's in there somewhere, although I get an annoying "Browser not supported" error since they changed something a while ago )

Anyway, the relevant paragraph says:

<quote>
1.2 Adjustments

There is one adjustment only on the z88 main board. With the index
page displayed and potentiometer RV1 set to minimum the menu topics
should just be visible on the screen. If the topics are not visible adjust
padding potentiometer RV2 until the topics are just visible. Padding
capacitors C17 and C18 are factory set and must not be adjusted under
any circumstances.
</quote>

Hope this is of some help.

Steve

Thank you for finding that!

[okto]

https://hackaday.com/2019/03/03/designi ... computers/

Would something similar be possible for the z88?

I think the main limitation is money. You can absolutely do that, but would it be worth $300 apiece?

The Model 100 community flirts with this every decade or so, and it always comes down to: there’s no way to get the unit price reasonable unless you have them make you 5,000 of them, and there might not be 5,000 Model T or Z88 users left on the planet.

[Jbizzel]

Fair point.

I think the future is in a FPGA device that can connect to a modern LCD option. Possibly with a bigger resolution using OZ5 - bigger resolution is one of the aims of that project.