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Questions about an LCD

arlaneenalra

arlaneenalra

Experienced Member
Joined
Mar 26, 2024
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59
I have a couple of machines that exhibit this behavior and I'm trying to figure out if it's old and degrading components of if I just don't remember it. I've noticed that the LCDs seem to fade on one edge and become more solid on the other. It's pretty dramatic when the display is showing solid colors.
lcd-fade.jpg

If you adjust the contrast, the display seems to fade in and out from the darker edge.


I've seen this on two different super-twist LCDs now and I really don't remember this behavior when they were new. The contrast was never very good of course, but I always remember it being consistent across the whole screen. Any ideas as to what's going on? Maybe how to fix it?

Chris S.
 
It's worth check for bad or leaking capacitors on the LCD driver board inside the screen, but that's probably caused by the LCD panel itself. At least it hasn't developed LCD vinegar syndrome. I'm starting to see that a lot on older LCDs, and even some that aren't all that old.
 
No vinegar syndrome on these so far. Just the weirdness with contrast. I have two others that seem to be missing random pixels in the middle. I’m not sure if it’s damage to the LCD itself or the polarizer. It kind of looks like something might have impacted/pressed against the display on those, though no cracking. This one is in the best shape except for the fade out. I’ll have to take a look at the caps. I don’t remember seeing anything leaking in there, but they could still be failing. The PSU on the Tandy 1400HD/FD series seems to universally be full of bad, leaky caps, it wouldn’t surprise me to find more in other parts of the system.
 
arlaneenalra said:
No vinegar syndrome on these so far. Just the weirdness with contrast. I have two others that seem to be missing random pixels in the middle. I’m not sure if it’s damage to the LCD itself or the polarizer. It kind of looks like something might have impacted/pressed against the display on those, though no cracking. This one is in the best shape except for the fade out. I’ll have to take a look at the caps. I don’t remember seeing anything leaking in there, but they could still be failing. The PSU on the Tandy 1400HD/FD series seems to universally be full of bad, leaky caps, it wouldn’t surprise me to find more in other parts of the system.
Click to expand...

Sometime lost pixels can be the result of bad FFC connections. It can be the solder connections or the cables themselves can develop gaps in their traces. Usually when I see that it's whole lines that are lost though and not just random pixels, but might be worth going over the cables to see what's up, or reflowing solder if you're capable of doing that on an FFC cable. Depending on the kind of cable used, it can be easy to burn through when soldering and so you would need to control your heat (and time) very carefully and use good flux.
 
The replacement caps I ordered arrived and I've gone in and recapped the display. The ones I pulled out were all 10uF 25v electrolytics and had an ESR in the range of 5~6ohms ... which seems a bit high to me. I tried putting in 10uf 25v Tantulum Poly capacitors as a replacement since they fit the footprint on the board and were small enough for the display to fit back into the case. Unfortunately, that gives me all black display. I'm not sure why, the caps measure correctly, they circuit just doesn't like them. Thankfully, it seems I didn't damage anything permanently. I had some 22uF 35v electrolytics that are a little larger than the originals physically (and value wise) but seem to work ok in the display.

I think it's a bit better, but I'll have to play with it for a few days before I can tell if it actually is.
 
vol.2 said:
Sometime lost pixels can be the result of bad FFC connections. It can be the solder connections or the cables themselves can develop gaps in their traces. Usually when I see that it's whole lines that are lost though and not just random pixels, but might be worth going over the cables to see what's up, or reflowing solder if you're capable of doing that on an FFC cable. Depending on the kind of cable used, it can be easy to burn through when soldering and so you would need to control your heat (and time) very carefully and use good flux.
Click to expand...
FFC wouldn't be a bad idea, if this display actually had one :( Instead it has a 15pin ribbon connector that I've reseated a couple of times already on both ends.

20241125_104324.jpg
 
arlaneenalra said:
The replacement caps I ordered arrived and I've gone in and recapped the display. The ones I pulled out were all 10uF 25v electrolytics and had an ESR in the range of 5~6ohms ... which seems a bit high to me. I tried putting in 10uf 25v Tantulum Poly capacitors as a replacement since they fit the footprint on the board and were small enough for the display to fit back into the case. Unfortunately, that gives me all black display. I'm not sure why, the caps measure correctly, they circuit just doesn't like them.
Click to expand...

Assuming that you correctly oriented the polarity of the tantalums (they are polarized, and not bipolar like film and ceramics), the only reason I can think of is the extremely low ESR of the tantalums. ESR actually helps dampen oscillations and voltage spikes in circuits, and many designers at the time would have been specifically doing so with the expectation of the ESR in the circuit.

Sorry the ribbon cable idea didn't work out, but tbh you should probably be glad it's got a more robust cable.
 
vol.2 said:
Assuming that you correctly oriented the polarity of the tantalums (they are polarized, and not bipolar like film and ceramics), the only reason I can think of is the extremely low ESR of the tantalums. ESR actually helps dampen oscillations and voltage spikes in circuits, and many designers at the time would have been specifically doing so with the expectation of the ESR in the circuit.
Click to expand...
That actually makes sense. I'm pretty sure I oriented them correctly, unless the band on a SMD tantalum doesn't have the same meaning as the strip on the electrolytic. The LCD is using an LA5316 that all of the caps are attached to and I do think it's much better with the new caps. Contrast is more consistent across the screen and I have to move the contrast adjustment very very carefully now to see any of the gradation I was seeing before. So, it at least helped some.
vol.2 said:
Sorry the ribbon cable idea didn't work out, but tbh you should probably be glad it's got a more robust cable.
Click to expand...
Oh I am, I also have a Tandy 1110HD and the dang FFC cable for the floppy drive is a pain in the rear, thankfully I'm not working on that one as much right now.
 
MInor update, I have since aquired another FD unit and decided to recap it with tantulums, in the correct orientation this time around. These worked much much better. Now for the backlight!
 
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