HP 7090A LCD Exam

A quick look at the LCD panel of the HP 7090A Measurement System Plotter. I took the LCD panel off, cleaned the zebra strips, and reassembled it. I'm a bit conflicted but I'm leaning toward replacing the strips. Though I will need to measure the voltage on the output IC just to be sure.

[@tgsoapbox]

Thinking about this some more, I wonder why you can see good contrast at say the 0:08 mark but at the 0:10 mark you see the top and top left segments in low contrast - All of them should be using the same common - Maybe it is panel damage and this is the best I'm going to get...

[@PixelSchnitzel]

Awww. I have a digital scale that exhibits the same behavior. I've cleaned the zebra strips on it too which helped, but didn't fix it. It re-degraded anyway. I think the rubber compounds degrade over time. So now you've inspired me to find new strips. I don't know if new strips would help you, but it seems logical that rubber degradation could be the culprit here.

[@tgsoapbox]

That is my plan - Get some new strips and try those - As well as check the driver IC output voltages to make sure that the segments are being driven properly.

[@PapasDino]

I've never had much luck rehabbing zebra strips, maybe I'm just unlucky! ;-)

[@tgsoapbox]

These certainly didn't rehab so I'm in the same boat - I think I need to get new ones and see if that makes a difference.

[@robbieaussievic]

..... I thought it looked like uneven pressure on the LCD, either from the polycarbonate front or the rear.
Or is pressure uniform on the zebra strip itself ?

[@tgsoapbox]

The pressure is pretty even - The plastic enclosure that the screen and PCB slide into is very well designed and pushing on it didn't seem to make any difference, which is why I'm going to change the strips just to see what goes on.

[@TheDefpom]

looks like the driver to me, as the same segments are missing across the digits.

[@tgsoapbox]

Thanks for the comment - Hope things are well with you (which I assume is the case because you're pumping out content like a boss).
I thought that might be the case but flipped in that you'd expect the drivers for each segment to be individual but the commons to be the same - In this case, there seems to be a similar segment issue but they still "work".
I'm really wondering if maybe there is an issue with either the panel itself or the connectors on the panel - It might even be a combination of them all together causing the issue - Would appreciate any thoughts you have.

[@TheDefpom]

@@tgsoapbox I am fairly sure it would be the driver, it could possibly be some corrosion (or solder whiskers) between some pins on the driver.
LCD's are driven with AC not DC, so you should be able to measure the drive voltages for each segment/row, or even better scope them to check the signals are consistent with each other.
It still could be the display though, it depends on how it is configured, a trick I have used is to heat up the edge of the display with a hot air station where the connections are, you might find it brings them back again until it cools down, this would confirm if it is a display / connection strip fault or a driver problem (if it doesn't change with heat).

[@TheDefpom]

@@tgsoapbox You could also swap the zebra strips from top to bottom of the LCD, if the fault doesn't move, then the strips aren't the problem.

[@tgsoapbox]

@@TheDefpom Thanks - I took the strips out, rubbed the connections with some IPA and a kimwipe, and then tried shifting the strips (the actual contacts are only a small section of the stip that was used). Didn't make any difference - The thing that is weird, which is leading to my vacillation on whether it is a strip, driver, or panel issue is that it happens to the same segments on different parts of the display. I'm just not sure where to focus my next steps.