Portable contrast lines

[LaPorta]

Hi everyone,

Attached is the video of what I have been describing. Rolling lines upwards when the contrast is adjusted. I recall this happening over 17 years ago when I first got the machine, so it’s not due to anything I have replaced recently. I’m not sure why this occurs, and it is worse than it looks in the video. When the contrast is up, it disappears, and when it is all the way down, it is gone too. It seems like it’s some sort of interference. Good thing is i leave the contrast up so I dont see it...but I hate things not working properly.

any insight?



View attachment 25438

 

[Johnnya101]

I'd start with Techknights advice from theother thread first.

"Next to the right hand side keyboard/mouse connector, there are 2 small SOT23 tran sistors. That drives the contrast waveform on the LCD panel.

the LCD panel itself can do this, or nois e in the contast PWM signal will do this. I think there is a couple smooth ing cap s there that need checked. You can check those transistor s as well for opens or shorts. "

Clearly it's probably not a capacitor issue, if it's been happening this long, since I doubt that they were leaking that long ago.

 

[LaPorta]

What thread was this that he wrote all of that in? I must have missed it.

 

[Johnnya101]

I think your my portable is next. It’s one of the top few at the end.

 

[techknight]

Saw the video clip. This looks like interference between the PWM signal and the LCD scanning signal. 

Its probably the LCD panel itself, You need a scope for this one. probably scope out the PWM signal while your adjusting it to make sure there is no "jittering" in the drive (caused by bad transistors or a bad PMU IC)

 

[LaPorta]

It looks like this is long term and a large learning curve for me...I have no oscilloscope, nor have I ever used one! Thank you very much for walking through this with me.

 

[LaPorta]

I did a little fooling around last night while installing my SCSI2SD. I noticed that a distinct, low-level hum comes from the LCD itself, and the pitch of this hum changes when the contrast slider is adjusted. To me, that means one of two things: something on the main board is causing that, or there is a part behind the screen that is causing it. This is no different than what you said before, techknight, but I was hoping that it might indicate that the issue was indeed behind the screen so I could start there.

 

[LaPorta]

I’d appreciate it if you could walk me through this, where the components are, etc...if you have the time.

 

[techknight]

I have no further input. Could be anything, you need a scope and basic troubleshooting skills. 

 

[LaPorta]

Ok thanks that’s no problem, I appreciate all of your help this far.

 

[LaPorta]

So I checked those two tiny transistors. One shows a voltage drop of .75, the other of .92. Is that .92 one out of spec, should it read as the other? Where would you find replacements for this tiny thing?

 

[LaPorta]

Well, wouldn't ya know it...re-soldering the connections between those transistors and using flux/cleaning to remove the grit from the traces solved the issue! The contrast works perfectly now. Thank you, Techknight, for pointing me in that direction,

 

[Johnnya101]

Congratulations! Glad it was so simple.